SOLAR OBSERVING FAQ
(Version 1.1)

Warning: observation of the sun should never be undertaken without proper safety precautions! PERMANENT AND INSTANT BLINDNESS WILL RESULT IF OBSERVATIONS ARE NOT MADE IN A SAFE MANNER! Safety information contained in this FAQ is not warranted to be accurate, free of error (typographic or otherwise), or universally applicable. Independent evaluations of safety claims should always be made. Prospective observers are instructed to review the references cited in question 1.07 for reliable safety information.

SECTION ONE: SAFETY

1.01	Why is safety a concern in solar observing?
1.02	Is eye damage or injury from improper solar observing permanent?
1.03	What are the mechanisms that cause eye injury during unsafe solar observing?
1.04	How are safe exposure levels to solar radiation calculated?
1.05	What does it mean that a solar filter is coated to density four, or density five?
1.06	Are aphakic observers at additional risk from observation of the sun through a safe solar filter?
1.07	What are some references concerning solar safety that have been consulted in the preparation of this FAQ, or that contain comments about solar safety?

SECTION TWO: GENERAL OBSERVING EQUIPMENT

2.01	Are there any unsafe solar filters?
2.02	What are the characteristics of different commercial filters?
2.03	How do I get in touch with companies that make completed solar filters?
2.04	Aren’t there other sources of solar filters that you have not talked about?
2.05	Have you received reports of any otherwise safe filters, that have experienced unsafe failures while being properly used?
2.06	Have you received reports of safe solar filters that were optically bad?
2.07	So which should I buy, a glass or mylar filter?
2.08	What solar filters are recommended?
2.09	What are some characteristics of mylar filters?
2.10	What are some characteristics of glass solar filters?
2.11	Are there any advantages to the various colors that these filters offer?
2.12	What about color filters, like for the planets? Are there any applications for those in solar observing?
2.13	What do you consider to be the ideal white-light filtering system?
2.14	How should I care for my solar filter?
2.15	I have a glass solar filter with some pinholes in the coating. How do I repair this?
2.16	Can I repair damaged coatings on a mylar filter?
2.17	Can I make my own solar filter?
2.18	Where can I get solar observing mylar to make my own solar filters?
2.19	What grades of mylar are available for use in solar observing?
2.20	I have a large telescope. Should I get a full-aperture solar filter for it?
2.21	I am going to build or buy a dedicated solar telescope. What size should it be?
2.22	Why is a Huygenian eyepiece preferred for solar projection?
2.23	Where can I buy a Huygenian eyepiece?
2.24	What eyepieces use cemented elements?
2.25	I am forced to use cemented-element oculars for projection. What should I do?
2.26	What is a Herschel Wedge?
2.27	What is a Dobson Solar Telescope?
2.28	What is a Vacuum Solar Telescope?
2.29	What is a heliostat?
2.30	What is a coelostat?
2.31	Where can I get plans or design ideas for heliostats and coelostats?
2.32	Can I observe the sun spectroscopically?
2.33	Where can I get plans or design advice for making a spectroscope or spectrograph?
2.34	Should I use a grating or a prism in my solar spectroscope?
2.35	What does "first order spectrum", and second order, etc, mean?
2.36	Is there any use for "narrowband" filters, of the sort that deep sky observers use, in solar astronomy?

SECTION THREE: OBSERVING METHODS AND CONSIDERATIONS

3.01	How can I safely observe the sun with my unaided eye?
3.02	How do I safely observe the sun with my telescope?
3.03	How do I use a pre-telescopic filter for solar observing?
3.04	What is solar projection?
3.05	My solar projection images aren’t very good. How can I improve them?
3.06	Can I use my finder to locate the sun?
3.07	Can I use my finder in projection mode to find the sun?
3.08	If I can’t use my finder, how do I locate the sun?
3.09	What is white-light or integrated light solar observing?
3.10	What is monochromatic solar observing?
3.11	What is "seeing"?
3.12	What is dispersion?
3.13	Is there any published data about daytime seeing conditions?
3.14	What can I do to improve seeing conditions at my site?
3.15	What is a white-light flare?
3.16	What are some guidelines to use when hunting for solar flares?
3.17	How do I best see white light flares?
3.18	How can I get automatic notification of solar flares in progress?

SECTION FOUR: NARROWBAND OBSERVING EQUIPMENT

4.01	What does it mean that a filter is a Hydrogen-Alpha solar filter?
4.02	What is so special about Hydrogen-alpha filters?
4.03	Are there any other narrowband lines of interest on the sun?
4.04	What kind of a filter is needed to see the sun in hydrogen-alpha light?
4.05	What is a Fabry-Perot etalon?
4.06	What does a hydrogen alpha solar filter cost?
4.07	Why are they so expensive?
4.08	What does Full-Width, Half-Maximum mean in relation to solar narrowband filters?
4.09	What FWHM must a hydrogen-alpha filter have to show acceptable views of solar details?
4.10	What factors degrade the performance of a Fabry-Perot etalon?
4.11	What is an instrument angle, and why does it result in the bandpass of the filter widening?
4.12	If a .6 A filter really performs at 1.4 A, isn’t that false advertising?
4.13	What is a field angle, and why does it result in the bandpass of the filter widening?
4.14	Can I put a .6 A hydrogen-alpha filter on my Schmidt Cassegrain telescope, which has a focal ratio of f/10, and stop it down to an aperture that will give me f/30? Or, could I use my f/10 refractor, and put in a University Optics Klee barlow, to give f/28?
4.15	Can I use my very large reflector, and use an off-axis mask that gets me to a very long focal ratio to get the bandpass desired?
4.16	Is a narrowpass filter in an SCT, stopped down off axis to a couple inches, and with a focal-ratio increasing barlow in front of it a bad configuration?
4.17	How does one get around these pernicious and seemingly insurmountable problems then?
4.18	Does the telecentric position have any substantial problems?
4.19	What are some sources of hydrogen-alpha solar filters?
4.20	Which filter is better?
4.21	But hasn’t Del Woods been around a lot longer than these Coronado people? They are reliably shipping a good product, after all.
4.22	How do Coronado filters mounted in telecentric positions differ from any other filter mounted in a standard commercial telecentric configuration?

SECTION FIVE: PHOTOSPHERIC OBSERVATION AND SUNSPOTS

5.01	What is the photosphere?
5.02	What features are present on the photosphere?
5.03	What is the R number?
5.04	What is the Wolf Number?
5.05	What is the "International Sunspot Number"?
5.06	What is the "NOAA Sunspot Number"?
5.07	What is the Zurich Sunspot Classification system?
5.08	What is the McIntosh Classification system, and what is the Modified Zurich Class?
5.09	Why was the McIntosh Classification system devised, and why is it used now?
5.10	What system or systems does the ALPO Solar Section use?
5.11	Why should casual observers care about the McIntosh class of the sunspots being observed?
5.12	What is the McIntosh Classification system exactly, then?
5.13	What is a Stonyhurst Disk?
5.14	How are Stonyhurst disks used?
5.15	What do the ephemeris abbreviations Bo, Po, and Lo mean?
5.16	What is the Mt. Wilson Magnetic Sunspot Classification System?
5.17	Do sunspots seem to prefer a particular hemisphere of the sun?
5.18	How do sunspots develop?
5.19	What is the Wilson Effect?
5.20	What is "sunspot area"?
5.21	What is the "sunspot deficit"?
5.22	What is a "bright ring" in reference to sunspots?

SECTION SIX: CHROMOSPHERIC OBSERVATION

6.01	What is the chromosphere?
6.02	What are some definitions for terms that are commonly encountered, and for details of the chromosphere commonly seen, during hydrogen-alpha observing?
6.03	What are some characteristics of quiescent filaments/prominences?
6.04	What are some characteristics of active region filaments/prominences?
6.05	What causes a solar flare?
6.06	How are solar flares classified?

SECTION SEVEN: SOLAR ACTIVITY CYCLES

7.01	What is the Solar Cycle?
7.02	How does the distribution of sunspots change as the solar cycle progresses?
7.03	Who discovered the 11-year solar cycle?
7.04	What is the Maunder Minimum?
7.05	What is the Sporer Minimum?
7.06	What is the Grand Maximum, and what is the Medieval Climatic Optimum?
7.07	What are the "Waldmeir Laws"?
7.08	What is the long sunspot cycle?

SECTION EIGHT: MISCELLANY

8.01	What is the solar core?
8.02	What is the radiation zone?
8.03	What is the convection zone?
8.04	What is the transition region?
8.05	What is the corona?
8.06	What is the Polar Crown?
8.07	What is the rotation period of the sun?
8.08	Why are sunspots black?
8.09	What is Joy’s Law?
8.10	Where can I go to see animations or movies of solar phenomena?
8.11	What is "facular area"?
8.12	What is "facular excess"?
8.13	What is "K-line excess"?
8.14	Where can I find glossaries of terms used in solar astronomy?

SECTION NINETY-NINE: ADMINISTRIVIA

99.01	What references are cited in this FAQ?
99.02	What is the copyright status of this FAQ?
99.03	Who in addition to the maintainer and copyright owner has permission to mirror or independently host Solar Observing FAQ, or post it or sections of it to newsgroups or mailing lists?
99.04	Who is the maintainer of this FAQ?
99.05	Did the maintainer of this FAQ write the FAQ?
99.06	Who has contributed non-cited information to ?
99.07	What is the revision history of this FAQ?

SECTION ONE: SAFETY

1.01 Why is safety a concern in solar observing?

The sun is very bright, and radiates strongly in infrared and ultraviolet as well as in visible light. The effects of this strong solar radiation will cause permanent eye damage instantly if the sun is observed with optical aid in the absence of safety precautions. The sun will even injure eyes during naked-eye observation after only a second or less. Other than the possibility of uncomfortably bright light, no pain will be experienced during eye injury due to the nerve population of the retina. For this reason, it is imperative that the sun only be observed using safe methods that prevent the damaging infrared and ultraviolet light from reaching the eye, as well as cutting down on the visible light and bringing it to comfortable levels.

1.02 Is eye damage or injury from improper solar observing permanent?

Yes.

1.03 What are the mechanisms that cause eye injury during unsafe solar observing?

The eye will transmit most of the radiation between 3800 and 14,000 angstroms to the retina. This will result in retinal burns.

Exposure to high intensity visible light triggers a complex chemical reaction within the rod and cone cells of the retina. These reactions impair the ability of the eye to respond to light and in extreme cases can destroy the cells. The injured observer experiences a temporary or permanent blindness. Light of blue or green color is most likely to cause these injuries.

When red and infrared light enters the eye, it is absorbed by the dark pigmented epithelium just below the retina. The light is converted into heat which burns the exposed tissue. A process called photocoagulation destroys the rods and cones, leaving a permanent blind are on the retina. This kind of damage can also occur as a result of extended exposure to green and blue light.

BOTH TYPES OF INJURIES OCCUR WITHOUT THE KNOWLEDGE OF THE VICTIM. There are no pain receptors on the retina to alert the observer to retinal burns, and on both cases the visual effects take several hours to manifest themselves.

Exposure to ultraviolet light contributes to accelerated aging of the outer layers of the eye and skin and the development of cataracts.

(Chou 1998, Espenak et al)

1.04 How are safe exposure levels to solar radiation calculated?

Dr. B. Ralph Chou laid out this method in Sky & Telescope, February 1998, p.36-40, which is recast in the FAQ maintainer's own words:

The damage levels for each wavelength of light are well known. This allows the ‘safe level’ to be calculated for a filter, using as a starting point the ratio between the maximum solar intensity and the damage threshold. Just to be extra safe the maximum allowable transmittance is then set to between 1% and 0.1% of this ratio. For the bandpass between 3800 and 14,000 angstroms (blue through near infrared), a filter that transmits .0032% is safe. This corresponds to a shade number of 12 (welders glass is rated by shade number). For visual comfort, a darker filter of .0003% transmittance (equivalent to shade 14, or density ~4.5) is recommended. (Chou 1998)

1.05 What does it mean that a solar filter is coated to density four, or density five?

Pre-telescopic filters are described by their density, their transmission, and their extinction in astronomical magnitudes. Transmission is simply the proportion of transmitted to incoming light and is usually expressed as a percentage. Optical Density is calculated from the transmission by:

Density = log₁₀ ( 100 / Transmission )

The commonest amateur filters can thus be described in this table:

Density	Transmission	Extinction in Magnitudes	Filter Purpose
3	0.1	7.5	Photographic
3.5	0.03	8.75	Photographic
4	0.01	10	Photographic/Visual
4.5	0.003	11.25	Photographic/Visual
5	0.001	12.5	Photographic/Visual

Notice should be given that in the cases of Photographic/Visual grade filters, this assumes that the extinction in infrared and ultraviolet is similar to that in the visual band. If this is not the case, these filters can still be dangerous. If you purchase a density 4 filter from a company that is advertising the filter as photographic use only, it is probably due to the transmission out of the visual band. The filter maker is always to be believed in such cases. Assuming that the IR and ultraviolet bands are subject to proper extinction, a filter of density four is safe for visual use - though the image will probably be found to be too bright without the use of additional filtration.

The very similar table in Beck et al, Solar Astronomy Handbook, Willmann-Bell 1995 on page 26 contains a typo in the transmission of a density 4.5 filter. The figure above is correct.

(Beck, et al, 1995)

1.06 Are aphakic observers at additional risk from observation of the sun through a safe solar filter?

No. Safe solar filters reduce the UVA transmission to well below safe occupational exposure levels. (Espenak, et al.)

(Aphakia is the removal of the crystalline lens of the eye which normally blocks UVA.)

1.07 What are some references concerning solar safety that have been consulted in the preparation of this FAQ, or that contain comments about solar safety?

Chou, B. R., Safe Solar Filters, Sky and Telescope, August 1981, p. 119.
Chou, B. R., ________, Sky & Telescope, February 1998, p.36-40
Chou, B. R., NASA RP 1383 Total Solar Eclipse of 1999 August 11, April 1997, p. xx
Espenak, F., NASA RP 1383 Total Solar Eclipse of 1998 February 26, April 1996, p. 17
Marsh, J. C. D., Observing the Sun in Safety, Journal of the British Astronomical Association, 1982, 92, 6.
Pasachoff, J. M., and Covington, M., Cambridge Guide to Eclipse Photography, Cambridge University Press 1993.
Pasachoff, J. M., and Menzel, D. H., Field Guide to the Stars and Planets, 3rd edition, Houghton Mifflin 1992.
Sherrod, P. C., A Complete Manual of Amateur Astronomy, Prentice-Hall, 1981.
American Conference of Governmental Industrial Hygienists, Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices, ACGIH, 1996, p.100
Chou, B. R., Safe Solar Filters, Sky and Telescope, August 1981, p. 119
Chou, B. R., Eye safety during solar eclipses - myths and realities, in Z. Madourian & M. Stavinschi (eds.) Theoretical and Observational Problems Related to Solar Eclipses, Proceedings of a NATO Advanced Research Workshop, Kluwer Academic Publishers, 1996
Chou, B. R. and Krailo M. D., Eye injuries in Canada following the total solar eclipse of 26 February 1979, Canadian Journal of Optometry, 1981, 43(1):40.
Del Priore, L. V., Eye damage from a solar eclipse in M. Littman and K. Willcox, Totality: Eclipses of the Sun, University of Hawaii Press, Honolulu, 1991, p. 130.
Pasachoff, J. M., Solar Eclipses and Public Education, International Astronomical Union Colloquium #162: New Trends in Teaching Astronomy, D. McNally, ed., London 1997
Penner, R. and McNair, J. N., Eclipse blindness - Report of an epidemic in the military population of Hawaii, Am. J. Ophthalmology, 1966, 61:1452.
Pitts D. G., Ocular effects of radiant energy, in D. G. Pitts & R. N. Kleinstein (eds.) Environmental Vision: Interactions of the Eye, Vision and the Environment, Butterworth-Heinemann, Toronto, 1993, p. 151.
Reynolds, M. D. and Sweetsir, R. A., Observe Eclipses, Astronomical League, Washington, DC, 1995.

TOP

SECTION TWO: GENERAL OBSERVING EQUIPMENT

2.01 Are there any unsafe solar filters?

Yes. Any filter that screws into an eyepiece and is used alone is unsafe for solar observation. These filters are called "post-telescopic filters" and all such filters are dangerous. Because these filters are exposed to the focused, intense solar heat, these filters can heat up to incredible temperatures very quickly. Because the heating is not even, it is very common for these filters to unexpectedly crack with great force (many people describe this using the word "explode"). The resulting burst of light through the filter will instantly and permanently blind anyone looking through the telescope at the time. If you have any of these filters, destroy them and dispose of the remains to be certain they do not fall into the hands of the uninitiated.

Other unsafe solar filters might include pre-telescopic filters from less than reputable sources.

2.02 What are the characteristics of different commercial filters?

Currently or Recently Made White-Light Telescopic Filter Models

White Light Telescopic Filter Models Thought To Be Discontinued:

Filters Designated Photographic Use Only – POSSIBLY UNSAFE FOR VISUAL OBSERVING

Narrowband and Ultranarrowband Solar Filters and Equipment (all of this equipment is safe when properly used)

Other Solar Observing Apparatus

Popularly Used Solar Observing Apparatus that is Always or Sometimes Unsafe:

Apparatus Sometimes Mistaken For Solar Observing Equipment (Unsafe!)

Notes to the tables, above:

Inconel is a nickel-chromium alloy.

n-e is an abbreviation for naked-eye.

h-a is an abbreviation for hydrogen-alpha.

c-k is an abbreviation for calcium K.

In the narrowband equipment table, "varies by mounting" refers to the etalon's mounting position near the eyepiece end of the telescope. In this position, instrument angles and other factors will vary the effective bandpass of the filter despite its bandpass rating.

Determinations of "safe" or "not safe" are made by a review of published or advertised specifications, and assumes that the specifications are met both in the products’ manufacture and during use. The FAQ maintainers (and, where applicable, contributors) warn that the conclusions based on these evaluations may be in error, and admonish consumers and users to investigate all such matters for themselves.

Comments to the tables, above:

1 Coronado Energy Rejection Filters are made of crown optical glass, are polished to ¼ wave or better on the wavefront, and to a parallelism less than 5 arcseconds, according to advertising. The FAQ maintainer has tested one of these and found it to conform to these advertised specifications with room to spare. The pre-telescopic filter is coated to a density of 4 (this density is high enough that using this filter alone is visually safe) so that imagers and photographers will have generously lit conditions to work with; a second filter is placed in the eyepiece for visual use and provides a comfortable 5 ND transmission for the system.

2 The JMB "Class B" filter model is advertised as offering a white solar image, but most observers perceive the image as somewhat yellow.

3 The JMB "Class A" model filter offers a blue solar image and apparently has an additional steel alloy coating component that the "Class B" model lacks.

5 The Tuthill filter’s substrate is DuPont optical grade mylar, the same material used for large optical windows in telescopes, observatories, and other optical equipment. The aluminum coating is relatively low-density and two layers of this mylar are required for safe solar viewing. Optically this has been shown to be preferable to using a single dense coating and the necessarily thick mylar substrate such a coating requires. Some aspect of this double-layer, double coating technology is patented by Tuthill. In the past, "free color correcting" filters were shipped with Tuthill mylar filters – it is not known what this was.

6 While it is believed the orange mylar is of the same double layer technology as the traditional Tuthill product, the means of getting the orange image is not known. It could be that the coating is different, or it could be that a color filter is used in the system. This whole Tuthill orange mylar filtration system seems somewhat apocryphal or at least ephemeral – as of March 1999 there was no mention of it on the Tuthill website. It is included because of persistent reports at star parties and online about an orange image Tuthill mylar system.

7 Robert T. Little made a two-part mylar solar filter in which one layer of mylar went over the eyepiece and one layer went in the pre-aperture position. It was thought to be safe and of decent quality, if somewhat inconvenient. This is possibly the same filter that Little advertised in the 70’s, which was of the of the late-70’s popular three-vane obstructed, ventilated in the middle to "prevent heat build up" (a useless feature), magnetic mount design.

8 It is speculated that as of the beginning of 1999, Orion Telescope and Binocular Center glass solar filters are rebrandings of JMB "Class B" filters.

11 Claims made for this filter by its seller are that it uses BK7 glass, "1/6 wavefront transmission," "photovisual coatings," and has a substrate thickness less than 10mm. The two claims reproduced here in quotes are meaningless and not testable, unless "1/6 wavefront transmission" means 1/6 wave of wavefront aberration in some visual wavelength and in some accepted measuring method (RMS, p-v on the surface or wavefront, etc). None of these filters have been shipped as of the beginning of 1999. The less than one-centimeter thickness of this filter is questionable from a filter engineering standpoint– it will likely be subject to sagging and drooping in larger sizes, and will have to be supported very carefully around the edges.

12 The "Solarprism" is apparently an observing accessory of the Herschel wedge design. It comes with supplementary filtration and appears safe, at least for small telescopes. I examined and tested this device at RTMC 1999.

13 Welder’s glass is useful for naked-eye solar observing only.

14 Any ND eyepiece solar filter used alone is unsafe. Various configurations of eyepiece-end etalon and pre-telescopic energy rejection filter, or dual ND filters in pre-telescopic and eyepiece positions together, can be safe. If this latter is the case, follow the manufacturer's instructions.

15 These and other sunglasses-like filters cover both eyes. The filters are mylar of density no. 5.

16 Celestron Solar-Skreen and Tuthill Solar-Skreen are the same filter. Solar-Skreen filters are or were made by Tuthill for Celestron.

17 Questar filters come in two varieties or configurations. One is a full-aperture filter, the image from which is described as subjectively too bright and requiring the use of a neutral density or variable polarizing filter to bring to comfortable levels. The other configuration is 1.5" off axis, which provides comfortable light levels. This filter is apparently safe, though probably uncomfortable to use, in the full aperture configuration.

18 Televue made a solar telescope called the Solaris, which was mated to a Daystar hydrogen-alpha filter and was heavily advertised by the late Edwin Hirsch in the early 1990’s. The FAQ maintainer seeks additional information about this system, such as: was the Solaris available separately.

19 This filter was of the late-70’s popular three-vane obstructed, ventilated in the middle to "prevent heat build up" (a useless feature), magnetic mount design. It somehow converted to use as a lunar filter as well. This dual-mode operation might lead to unsafe operator error.

20 "New high-resolution, BK-7 crown element, flat to 1/10 wave, 2 layers of Inconel, neutral density guaranteed to resolve to theoretical limit of your telescope" claimed the late 70’s ads. If true, this would obviously be a premium filter. It apparently came in an attractive wooden box and was available in sizes between 2" and 20". They were slightly expensive – a 40mm (~1.6 inch) pre-aperture filter cost $41.95 US dollars in 1979!

21 Due to heating, these are not recommended by the manufacturer for apertures larger than 80mm.

22 This filter is tunable to various elemental bands (in addition to hydrogen-alpha) by swapping energy rejection filters.

23 The etalon mounts in the eyepiece end of telescopes with focal lengths greater than 1600 mm (for full disk viewing) and focal ratios of f/24 or slower.

24 Mounts in the pre-telescopic position, 40mm aperture.

25 Mounts in the pre-telescopic position, 60mm aperture.

26 These are 40mm and 60mm aperture, respectively.

27 This is a 70mm f/6 refractor with an integral filter mounted in supplementary optics to insure that no angles encountered by the filter are greater than +/- .29 degrees. See the section on narrowband observing to find out whether this is important to you.

28 Baader Planetarium made or marketed what are by most accounts decent solar filters. Claims made for them were smooth and plane parallel to 1/4 to 1/10 of a wave.

29 Baader Planetarium mylar is 12 micron, double-coated. It came in sheets of 100cm x 60cm. Two varieties were available, for visual and photographic use respectively. The photographic grade is not safe for visual use.

30 Baader Planetarium coronagraphs are of the eyepiece blocking variety, available in 2 and 10 angstrom bandwidths.

31 The Tele Vue Genesis Solar Kit attaches to any Tele Vue Genesis or Genesis-sdf telescope to allow mounting of a Daystar T-Scanner for h-a observing of the sun. The line of optimal bandpass frequently seen with this arrangement while tuning is not a problem with the Genesis Solar Kit, it is an inherent property of the filter. (See comment on T-Scanner models in the Narrowband table.)

32 Users of T-Scanners frequently report the visibility of a thin line across the field in which the bandpass characteristics of the filter are narrowest, with visibly degraded bandpass on either side. During such times the observer must scan the solar disk showing through this area by moving the telescope to build up a complete picture of the chromosphere. This is particularly so (though not limited to) those times when the filter is mounted in an off-axis optical system, or when the filter is being tuned off the centerline. The effect is a result of tipping the filter as the tuning mechanism (or of giving the filter an off-axis beam if applicable) which produces some unusual looking effects. It is not a defect of the filter, though it is a design consideration that users should be aware of.

33 The Type 8 Helioscope is an eyepiece-end accessory consisting of a Herschel wedge, double right-angle glass prisms, and polarization devices. The density of this combination is very great, approximately optical density 5. The Zeiss Jena version had an integral Barlow lens.

34 This product is currently and widely considered to be the best available white-light solar filter material, including by the FAQ maintainer.

2.03 How do I get in touch with companies that make completed solar filters?

Abel Express, 230 Main Street, Carnegie PA 15106 412-279-0672
Celestron Intl., Box 3578, Torrance CA 90510 310-328-9560
Coronado Instrument Group, http://www.coronadofilters.com/
Daystar Filters, PO Box 5110, Diamond Bar CA 91765 909-591-4673
Rainbow Symphony, 6860 Canby Ave #120, Reseda CA 91335 800-708-8400
Telescope and Binocular Center (Orion), PO Box 1815-A, Santa Cruz CA 95061 408-763-7000

2.04 Aren’t there other sources of solar filters that you have not talked about?

Its practically certain that there are. If they aren’t mentioned in this FAQ, I have either forgotten about them or not heard of them. Please contact the FAQ maintainer to report them. Also, if filters listed as 'discontinued' are still in production, or errors are found in the listings present, please contact the FAQ maintainer to report that, as well.

2.05 Have you received reports of any otherwise safe filters, that have experienced unsafe failures while being properly used?

No.

2.06 Have you received reports of safe solar filters that were optically bad?

Yes. Most inexpensive glass solar filters (tens to low hundreds of dollars depending on size) are either unworked plate or float, or at best inadequately worked plate or float glass. A quick check for flatness on any of the standard optical bench tests will show when this is the case. Industrial grade mylar filters have been reported to be optically bad which is no surprise. Visual grade mylar is fine for naked-eye or low magnification use but the image will break down at magnifications over ten or so. Optical grade mylar is generally of adequate optical quality, but it is known that over the years a few optically poor batches have made it past quality control and are in use as solar filters.

2.07 So which should I buy, a glass or mylar filter?

You should probably buy the Baader filter material and make a filter with that. Otherwise, it is up to you. If you have a personal preference concerning the color of the solar image that you get, by all means purchase a filter that offers that color image, no matter the filter substrate. If you demand sheer optical quality, it is easiest and cheapest to get it with Baader. If you demand an aesthetic experience, Baader is also the way to go, but inexpensive glass filters rendering an orange solar image frequently offer superficially sharp (though often optically horrible, low-resolution) and scatter-free images.

For those who don't want to make their own Baader cell, Larry Field makes some hardwood cells for certain telescopes.

2.08 What solar filters are recommended?

Any safe solar filter is recommended. Solar filters are primarily safety equipment, and secondarily optical equipment. BEWARE OF ANY SOLAR FILTER THAT DOES NOT EMPHASIZE SAFETY ABOVE ALL OTHER CONSIDERATIONS. Safety is never a foregone conclusion. Beware of any solar observing product that is only questionably safe - you want some margin to spare.

2.09 What are some characteristics of mylar filters?

Mylar filters made of DuPont Optical Grade mylar are optically adequate, usually rather better than the unworked float glass filters in popular use (according to independent optical tests). Other brands of optical grade mylar equivalents have been found to be almost as good. This kind of mylar is used in optically sensitive equipment, including telescopic optical windows, optical bench apparatus, observatory optical shades, and other applications. It is made extremely thin (much thinner than kitchen plastic wrap) and very evenly in density. It is a far cry from potato-chip bags!

The aluminum light rejection coatings traditionally found on mylar usually offer a blue solar image.

The aluminum light rejection coatings do scatter some light. The scattering is usually axial across the image plane; if the mylar is held up to a bright light source the scattered light will be seen to come principally from two sides of the light source. This scattering usually amounts to less than one percent of the total light transmitted by the filter (assuming the filter is a good one), but this can still be seen in the image. Fortunately, because of the unusually high contrast of solar detail compared to that of other celestial objects, and because most of the scattering is to distances beyond the solar limb, this is strictly an aesthetic consideration. Using a two-layer mylar system (e.g., Celestron, Tuthill), the mylar can be installed such that the scattering axes are set at right angles, to prevent the reinforcement of this scattering along one direction.

Mylar filters based on non-optical mylar – which includes most  single-layer mylar filters (all of them except Baader Planetarium mylar) – can have a badly degrading effect on image quality. Thickness and density variations in the mylar can contribute up to many waves of aberration if the mylar is of poor quality. Poor coatings on mylar can also contribute to image problems.

2.10 What are some characteristics of glass solar filters?

A glass filter becomes a telescope optical element which must be ground and polished flat on two exactly parallel sides in order to have equivalent optical quality to a good mylar filter (this is because glass must be thicker than the mylar, and because glass more easily refracts light than mylar). In essence, the glass filter substrate is first turned into an optical window, then coated. The manufacture of optical windows is not a simple process, and is labor intensive and time consuming. If the solar filter is not ground and polished flat, then it will contribute to optical aberrations in the telescope. If the surfaces are not parallel, the glass will act like a prism or, in some cases, like a lens, contributing color dispersion and aberration to the instrument. It is important, then, to be sure that you are getting a glass filter that has been carefully fabricated in this way. Price is an indicator – it is not cheap to grind and polish optical windows, and there are no shortcuts. Be advised that some (but not all!) advertisers which claim machine or hand polished glass filters are either lying, or aren’t polishing them to any standard whatever.

The coatings on most if not all glass solar filters are axially scattering. This axial scattering tends to be considerably less than that experienced using mylar filters. As with mylar filters, most of the scattered light lands in the image plane well beyond the solar disk.

Because the glass in a solar filter is an optical element which can change the optical characteristics of the telescope system much more easily than mylar, the filter must be supported in a cell that does not stress it in any way. Filters of even relatively small sizes will require a carefully constructed (or at least elegantly designed) cell, further adding to the expense of a filter. Considering the cost of even simple cells for achromatic doublet telescope objectives, and the fact that glass solar filters are generally quite thin, this is not likely to present easy or inexpensive resolution.

Most inexpensive (tens to low hundreds of dollars US currency, depending on size) glass solar filters are made of float glass or, sometimes, of plate glass. These inexpensive filters are rarely if ever optically worked at all, let alone to the required flat, smooth, and parallel specifications. Some idea of their optical quality may be had by holding up such a standard piece of glass to the telescope at night and observing how the image degrades. Typically, unworked float or plate glass from a well-annealed source contributes from one to twenty waves of aberration to the wavefront. Unfortunately, not all manufacturers have been found to consistently use well annealed glass, and their transmission characteristics can be even worse.

All of these problems suggest that, unless carefully made, a glass filter is rather likely to be optically inferior to a mylar filter, a fact which is born out in independent testing. Considering that mylar filters are optically not great either, this is generally depressing news. Choose carefully!

2.11 Are there any advantages to the various colors that these filters offer?

Yes. Bluish filters boost the contrast between the solar disk and the faculae. Orange filters boost the contrast between details in the sunspot penumbrae. Green filters (if of good optical quality) offer the best shot at seeing granulation.

Blue filters can suffer somewhat because blue light is more strongly scattered and absorbed by the atmosphere. This scattering is often mistaken by the uninitiated as an optical problem, but is really an atmospheric clarity problem. It is less noticeable in observing locations at higher altitude or with clear air. While the temptation is to go with an orange filter, keep in mind that in these filters the contrast of all but the strongest faculae is reduced to near invisibility on any part of the solar disk except the extreme limb, and for much of the solar cycle this is the most interesting thing to see in white light!

2.12 What about color filters, like for the planets? Are there any applications for those in solar observing?

WARNING: EYEPIECE COLOR FILTERS ARE NOT SAFE FOR USE ON THE SUN UNLESS USED IN CONJUNCTION WITH A SAFE SOLAR FILTER!

Yes. When used in behind a safe solar filter, the color filters can be used to boost the contrast between various solar features as described above.

One specific application is the use of an orange eyepiece filter with a blue-image mylar solar filter. This will render an orange solar image almost identical to the deeper orange glass filters. The opposite technique, of inserting a blue filter into the eyepiece while using an orange glass filter, does not seem to work nearly as well with the filters I have tested.

Specific useful colors for the sun include:

Red/Orange: Increased contrast of knots and radial streaks in sunspot penumbrae.

Green: Green light is practically necessary to see and photograph granulation clearly.

Blue: Blue light is practically necessary to see all but the brightest faculae, especially faculae not involved with sunspot groups or faculae far from the limb. Blue is also advantageous to seeing the relatively rare white light flares.

2.13 What do you consider to be the ideal white-light filtering system?

The Baader Astro-Solar material.

2.14 How should I care for my solar filter?

The same way you would any other optics. Don’t stretch or puncture the mylar, and don’t scratch the glass. Keep them clean. Store them in clean, dry locations. DIRT, EXCESSIVE HUMIDITY, CHEMICAL EXPOSURE, AND CONTAMINATION CAN EVENTUALLY RUIN THE COATINGS AND LEAD TO DANGEROUSLY POOR FILTRATION. It is a good idea to keep most of these filters away from household chemicals, including chlorinated swimming pools.

2.15 I have a glass solar filter with some pinholes in the coating. How do I repair this?

You don’t. Such a filter is garbage, and needs to be disposed of.

Pinholes come about through several causes. One is age and the degradation of the coating caused by the atmosphere and pollution. Another is through extreme roughness of the glass – rough glass will damage the coating as it thermally expands and contracts, and if it is rough enough, the coating won’t adhere properly to begin with. The third source of pinholes in coatings is the improper application of the coatings in the first place. Of course, mechanical damage such as during cleaning is another source of coating trouble.

In all cases, pinholes on coatings indicate that the coating is damaged or defective and is not redeemable. While the pinholes that can be seen could be covered up, the pinholes, cracks, and low coating density areas that are invisible or overlooked can allow dangerous leaks of harmful solar radiation, particularly UV. Covering up pinholes with a marker or paint is a fool’s errand and is likely to result in eye injury sooner or later.

2.16 Can I repair damaged coatings on a mylar filter?

No. Damage is usually caused by stretching of the mylar. Throw it out, its not worth the risk. Once you stretch the mylar, its optical properties have been degraded anyway.

2.17 Can I make my own solar filter?

Yes, you can. The easiest way is to get the required amount of Baader material or mylar and assemble the filter. If you are into optics, and can properly calculate extinction, you might also grind your own optical flat and send it off for the specified coating (an Inconel coating to ND 5 is generally considered safe). This latter method will not be covered in the FAQ.

2.18 Where can I get solar observing mylar to make my own solar filters?

You should probably get Baader Astro-Solar instead. But if you want mylar, before making a solar filter, make sure that the mylar you are getting is specifically designed for solar observing.

Abel Express, 230 Main Street, Carnegie PA 15106 412-279-0672
Astro-Physics (US supplier of Baader material)
Rainbow Symphony, 6860 Canby Ave #120, Reseda CA 91335 800-708-8400

2.19 What grades of mylar are available for use in solar observing?

Three grades of aluminized or otherwise coated mylar is available:

Industrial Grade, made by several companies, is optically inappropriate for even naked-eye solar observation. A four inch circular sample independently tested contributed tens of waves p-v of various aberrations to the wavefront. This material is better used as a diffuser than a solar filter, though it will form an image of sorts.

Visual Grade, made by several companies, is optically appropriate for observations made far from the diffraction limit. This includes naked eye observations and observations made with optical aid at a magnification of less than, say, seven to ten power. This mylar is often found in naked-eye eclipse viewers and the like.

Optical Grade, made by several companies, is mylar made very thin and is of extremely homogenous density. Since ripple in one surface is compensated for by an opposite ripple in the other surface, mylar begins to have a slight optical edge over unworked glass at this grade. Filters such as the Thousand Oaks telescopic mylar line are made with Optical Grade mylar.

DuPont Optical Grade is an enhancement on the standard optical grade mylar. It is made thinner than standard optical grade mylar, is more homogenous in density, and is made to tighter tolerances in thickness deviation. Celestron Solar-Skreen, Tuthill Solar-Skreen filters are the only ones known to be made with DuPont Optical Grade mylar.

2.20 I have a large telescope. Should I get a full-aperture solar filter for it?

If large means around 12-14 inches or more, you are approaching the point of diminishing returns for each step up in greater aperture. Getting a cheaper, smaller filter, and using it off-axis (it is assumed this is an obstructed telescope) might make more sense. The small filters are easier to make well and are more likely to be good optically, and they won’t cost as much to boot. Plus, if you can swing it, you will end up with an off-axis, unobstructed solar configuration that is likely to offer excellent images. Solar filters in off-axis configurations are available commercially, or you can make your own.

On the other hand, it makes a lot of sense to use as much aperture as you can afford to, especially when imaging. You don't need the light, but the increased resolution does make a difference.

2.21 I am going to build or buy a dedicated solar telescope. What size should it be?

Don’t bother to go over 12-14" unless you live on a small island in a lake, or at another site with similarly good seeing, unless you are contemplating imaging applications. This size range is getting to the point of diminishing returns considering the turbulent daytime atmosphere. Most of the time, from the vast majority of sites, views of the sun are going to be seeing limited with a telescope much larger than eight inches.

2.22 Why is a Huygenian eyepiece preferred for solar projection?

A Huygenian is an achromatic eyepiece having two lenses, which are air-spaced and not cemented together. When exposed to the intense, focused radiation of the sun, such an eyepiece will not be damaged by the heat (unless there are manufacturing defects in the glass, which would a priori ruin its optical characteristics anyway). By contrast, any eyepiece with cemented elements might be subject to extreme heating, melting, boiling, or burning of the cement, and consequent destruction of the eyepiece.

Ramsden eyepieces are also non-cemented oculars, equally safe. The primary difference is that the field of the Ramsden eyepiece is rather more strongly curved. On the other hand, spherical aberration is better controlled with the Ramsden than with the Huygenian.

On the whole, most solar projectionists prefer the Huygenian to the Ramsden.

2.23 Where can I buy a Huygenian eyepiece?

I don’t know. They are occasionally found on the used equipment shelves at scope shops, and are sold (sometimes in dangerously combustible plastic barrels!) with some department store telescopes.

2.24 What eyepieces use cemented elements?

Every eyepiece that I know of that is commercially available today uses one or more cemented elements.

2.25 I am forced to use cemented-element oculars for projection. What should I do?

Use a Kellner, with only one cemented element to fail. Use a short focal length one, so that the heating of the glass is more even (with a longer focal length eyepiece, the solar image heats the center of the lenses much more strongly than the edges). Take precautions so that if the cemented element does shatter, no-one will be injured.

Some Kellner eyepieces, like the Edmund RKE and the Meade MA’s, are modified Kellner eyepieces with a singlet eye lens and a doublet field lens. These eyepieces are better suited to solar projection than the traditional Kellner, as the eye lens protects bystanders from the possible shattering of the doublet field lens.

If you must use an eyepiece containing a cemented-element, be sure it is one you can afford to have destroyed. No guarantees are made that this will work.

2.26 What is a Herschel Wedge?

The Herschel Wedge is a shallow triangular piece of glass with optically flat surfaces. The surfaces are set at an angle of about 3 degrees. The wedge is used to reflect light from the forward surface into the eyepiece, while light from the rear surface is reflected out of the eyepiece field, preventing double images. HERSCHEL WEDGES ARE NOT SAFE WHEN USED ALONE. The 3% to 5% of the light reflected into the eyepiece is still intense enough to require filtration. According to various authorities, the use of a #12 welder’s glass as a secondary filter is safe. Those wanting to use Herschel wedges are advised to make independent measurements of what level of additional filtration is safe.

2.27 What is a Dobson Solar Telescope?

A Dobson solar telescope is a special telescope design invented by John Dobson. It is particularly well suited to public solar observing.

The Dobson solar telescope is a modification of the Newtonian optical design. An optical window is fitted to the front of the tube, which is coated on its back (primary-facing) surface with a 95% rejection coating. Some stock one-way mirror glass is of about this reflectivity, but is not optically good. This optical window is fitted at a 45 degree angle to the tube. The primary mirror is left UN-coated. The optical window serves as a secondary, and fitted to the bottom of the eyepiece drawtube is a welders filter, number 9 to 12 depending on the density of the window.

The construction of a Dobson Solar Telescope is described in:

Rik Hill, ed., The New Observe and Understand the Sun, Astronomical League 1990
?, ___________, Scientific American, May 1972
?, ___________, Sky and Telescope, August 1989

2.28 What is a Vacuum Solar Telescope?

This is a solar telescope that has been evacuated of all air, in order to eliminate thermal convection currents within the telescope that degrade the image. It is only worth evacuating a telescope at sites with very good seeing. The engineering difficulties of such a telescope are considerable but not at all insurmountable for the amateur.

Variations on the vacuum design in which the telescope is filled with helium (which has high thermal conductivity and hence convects less vigorously than air, and also has a lower index of refraction than air) exist and several have been made by amateurs. While hydrogen has similar thermal properties, its use is very dangerous as hydrogen easily explodes.

2.29 What is a heliostat?

A heliostat is a mirror-feed solar observation system that allows the observing instrument (telescope, spectroscope or spectrograph, etc) to be stationary, while the mirror feed system moves to track the diurnal motion of the sun. The usual configuration is a single optically flat mirror which directs the suns light into the instrument. The heliostat thus provides a stationary image of the sun, but the image is subject to field rotation as the day progresses.

2.30 What is a coelostat?

Like a heliostat, a coelostat is a mirror-feed system for a stationary solar observing instrument. The coelostat uses two or more mirrors, one of which is driven to follow the diurnal motion of the sun, in such a way that the solar image is both stationary and non-rotating.

2.31 Where can I get plans or design ideas for heliostats and coelostats?

Houston, Walter Scott and Maag, Russel C., A Horizontal Solar Telescope in The New Observe and Understand the Sun, Rik Hill ed., Astronomical League 1990

2.32 Can I observe the sun spectroscopically?

Yes! There is plenty of light to be had, and large instruments are not needed, so this activity is particularly well suited to amateurs.

2.33 Where can I get plans or design advice for making a spectroscope or spectrograph?

The following bibliography is taken from Rik E. Hill, Prisms, Gratings, and Spectroscopes, The New Observe and Understand the Sun, ed. Rik Hill, Astronomical League 1990, which should also be consulted:

GENERAL SPECTROSCOPY AND INSTRUMENTATION:

Cutting, T. A., Manual of Spectroscopy, Chemical Publishing Company Inc., NY 1949
Ingalls, A. G., Amateur Telescope Making, Book III, Scientific American Inc, 1953
Sawyer, R. A., Experimental Spectroscopy, Prentice-Hall, 1946 (reprints by Dover)
Strong, C. L., The Amateur Scientist, p.38, Simon & Schuster, 1960
Thackeray, A. D., Astronomical Spectroscopy, Macmillan Co, 1961
Veio, F., The Sun in Hydrogen-Alpha Light with a Spectrohelioscope, privately published 1978 with periodic updates

SOLAR SPECTROSCOPY:

Abetti, G., The Sun, Macmillan 1961
Menzel, D. H., Our Sun, Harvard University Press 1959
Nicolson, I., The Sun, Rand McNally, 1982
Zirin, H., The Solar Atmosphere, Blaisdell Pub. 1966
Zirin, H., Astrophysics of the Sun, Cambridge University Press 1988

Periodical articles:

Sky & Telescope: vol 77 p 585, vol 73 p 98, vol 59 p 157, vol 57 p 395, vol 54 p 65, vol 39 p 120, vol 37 p 45.
Scientific American: Jul 52 p 82, Sep 56 p 259, Apr 58 p 126, Sep 66 p 277, May 68 p 140, Oct 68 p 126, Mar 74 p 110, Jan 75 p 118, Apr 75 p 134.

(Hill 1990)

2.34 Should I use a grating or a prism in my solar spectroscope?

Either one you want is fine. Be aware though that a prism gives different dispersion at different wavelengths along a logarithmic curve. A grating, which produces multiple order spectra through diffraction, has a constant dispersion across all wavelengths. The spectra produced by gratings, however, are fainter than that produced by prisms.

2.35 What does "first order spectrum", and second order, etc, mean?

It refers to the different spectra produced by a grating. A grating will form a series of spectra on either side of a white light image. The white light image is referred to as the "zero order" image. The "first order" spectrum is the one closest to this white light image, and has small dispersion. The "second order" spectrum is the next one beyond the first order spectrum, and has somewhat greater dispersion. And so on. Orders higher than the first order can overlap to some degree, so keep this in mind when designing your spectroscope.

2.36 Is there any use for "narrowband" filters, of the sort that deep sky observers use, in solar astronomy?

Yes. Narrowband interference filters with bandpasses of tens of angstroms can be used to dramatically reduce dispersion, and if the interference filter has a bandpass near the hydrogen alpha line, some interesting contrast effects will be seen. A filter with about a 10nm bandpass at 520.2nm will bump up granular contrast considerably, as well.

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SECTION THREE: OBSERVING METHODS AND CONSIDERATIONS

3.01 How can I safely observe the sun with my unaided eye?

By holding up a #14 welder’s glass, or a mylar or glass solar filter, to your eyes and looking through it at the sun.

3.02 How do I safely observe the sun with my telescope?

There are several methods of safe telescopic solar observing. The most straightforward is to use a pre-telescopic filter made of glass or mylar and coated with an energy-rejection coating. Since you observe the solar image with an eyepiece, this is called the "direct view" method. Or, you could use a dedicated solar telescope that contains built-in safety provisions. Another safe method when done properly is solar projection.

3.03 How do I use a pre-telescopic filter for solar observing?

A pre-telescopic filter is one that fits onto the front, skyward-pointing end of the telescope, so that the light from the sun passes through the filter only once, and does so prior to encountering any of the telescope’s optical elements.

The main thing to consider with pre-telescopic filters is that you must make certain they are firmly attached to the front of the telescope. Under no circumstances should wind or an accidental bump be able to remove the filter from the telescope. If this happens while someone is looking through the telescope, permanent eye damage will result instantly!

Solar filters that attach to the front of the telescope by friction alone are not considered safe. Contaminants and thermal expansion can change the frictional properties of the filter cell and allow its unexpected removal. Mechanical methods of securing the filter are always desirable. In some cases, this can be as simple as using a couple pieces of duct tape to secure the filter to the telescope tube. In other cases, set screws might be used to secure the filter to the telescope.

The need to strongly attach the filter to the front end of the telescope should never be neglected, but it is PARTICULARLY IMPORTANT DURING PUBLIC SOLAR OBSERVING SESSIONS. Children and many adults, who are in ignorance of the consequences and of the function of a solar filter, have been known to attempt their removal as a practical joke or to ask ‘what this shiny thing is for’.

Once the solar filter is secured to the front of the telescope, you simply aim the telescope at the sun and observe using your preferred eyepiece.

3.04 What is solar projection?

Using an unfiltered telescope and a Huygenian eyepiece, the telescope is pointed at the sun. IT IS IMPERATIVE THAT YOU DO NOT OBSERVE THE SUN THROUGH A TELESCOPE BEING USED IN PROJECTION MODE. Instead of direct viewing, the solar image is projected through the eyepiece and focused onto a white card, or a wall, or other convenient flat surface.

It is important to take care that structural elements of the telescope or eyepiece are not burned using solar projection. Projection is best done by those who are familiar with their equipment and have an eye toward safety.

3.05 My solar projection images aren’t very good. How can I improve them?

Frequently, projected solar images are hampered by light from other sources than the eyepiece falling on the projection surface. For this reason, the projection screen should be shaded. For straight-through refractors, you might make a shade that fits around the eyepiece drawtube. For right-angle projection, the screen should at least have a lip that shades it from the sun. Any number of more elaborate shades have been designed, the most popular of which is the Hossfield Pyramid. At the extremes, some observers project the solar image into a building through an opening cut for the purpose.

Another source of degradation with projection results from the irregularities of the projection screen. If bumps or ridges render the projected image noisy, sand it down. Some of the best solar projection screens are made by very fine sanding of several coats of white paint, until it is smooth and matte. Many observers also contrive a rotating screen, so that irregularities are rendered invisible from the motion.

Finally, solar projection suffers from convection currents set up by solar radiation in the projection telescope. Fans might be used to control this, or a dedicated solar telescope might be painted white (preferably with an IR reflective paint such as Kool-Deck) on the inside to reduce this effect.

3.06 Can I use my finder to locate the sun?

Not unless your finder is also equipped with a solar filter.

3.07 Can I use my finder in projection mode to find the sun?

If you want to risk burning up your crosshairs, you can. But be sure to cover it when not actually looking for the sun – a hand or eye placed accidentally behind the finder could be burned.

3.08 If I can’t use my finder, how do I locate the sun?

The simplest way is to simply look on the ground at the shadow of the telescope, and move the scope around until the shadow reaches its smallest extent. Alternately, you could build a small "solar finder" of the sort that has an object (a pointer, a small ball, etc) at the front of the telescope which casts a shadow onto a surface at the back of the telescope; crosshair lines are drawn on the surface so that when the shadow is over the crosshairs the sun is centered.

Similarly, some observers like to center the shadow of the finder scope in the shadow of its mounting rings.

Also, you can remove the eyepiece and gaze down the focusing tube directly (with a filter in place obviously - not safe for projectionists!). This will allow you to zero in on the scattered light from the sun, allowing quick centering.

3.09 What is white-light or integrated light solar observing?

White light or integrated light solar observing is observing done using very broad-band transmissions in the visible spectrum. This means that a variety of wavelengths of light are passed by the filter, and though the solar image might take on a blue or orange cast, many wavelengths are actually passed and the solar image looks more or less like it would if it were viewed in its natural color. Solar projection onto a white surface is the only way to see the actual color of the sun. Glass and mylar filters are also considered white light or integrated light observing methods, even though they do exercise some color filtration over the image. Another technical term for the kinds of filters used in white light or integrated light observing is "continuum filter."

3.10 What is monochromatic solar observing?

This is observing the sun in a particular, narrow color of light. A white light, integrated light, or continuum solar filter will allow all sorts of wavelengths of light through, from the red end to the blue end of the spectrum (color variations are only seen because they allow one color through a bit more strongly than the others). Narrowband filters, however, allow only a very narrow bundle of wavelengths through, typically with bandpasses of a couple angstroms or less.

3.11 What is "seeing"?

It is the variable distortion of the image caused by the constantly changing atmosphere. It can be most easily seen as a waviness or shimmering at the solar limb. It dramatically degrades the solar image.

3.12 What is atmospheric dispersion?

It is the spreading of light into its component colors by the atmosphere. When observing the sun, it can be seen as one solar limb taking on an orange or red cast and the opposite limb appearing with a blue cast. It is nothing more than atmospheric chromatic aberration. Dispersion lowers resolution of the solar image. It can be controlled by the use of a color filter in conjunction with your usual solar filter. Or, a pair of thin prisms (also known as optical wedges) may be used face to face and rotated with respect to each other to cancel the dispersion.

3.13 Is there any published data about daytime seeing conditions?

Yes, lots of it. One of the most interesting published seeing reports is that at the best solar observatory sites, daytime seeing is better than one arcsecond less than 1% of the time (Bray, R. J., and Loughhead, R. E., Sunspots, Dover 1964).

3.14 What can I do to improve seeing conditions at my site?

Get away from any source of local thermal convection, such as blacktop or cement. Grassy ground is good, near a pond is better. On an island in a lake or at very high altitude is usually best. Among the best local seeing conditions are found just after the lawn is watered – set up the solar telescope where the sprinkler was.

Also, observe shortly after sunrise or shortly before sunset. How "shortly" is somewhat dependent on local conditions. The reason this is effective is that during these times, the earth's atmosphere tends to have very steady temperatures, which means that turbulence is greatly reduced. During the middle of the day, by contrast, solar heating causes the atmosphere to boil vigorously as it warms up. If you catch the sun near the horizon, you might need to use an eyepiece filter in conjunction with your solar filter to knock down the atmospheric dispersion present in the solar image (one limb will be slightly blue, the other red, if this effect is present).

3.15 What is a White-light Flare?

A white light flare is a bright flare seen interposed between Earth and a sunspot umbra or penumbra. They usually persist for only short periods, perhaps ten minutes or so. They are not actually rare, but are seen only fairly rarely due to the difficultie of seeing them and the trouble one must take to make the observation. Carrington and Hodgson observed the first white light flares in 1859 and several are generally reported near each maxima. They are best observed in blue light.

Photospheric bridges between two sunspot umbrae should not be confused for a white light flare. These bridges are simply an absence of sunspot, allowing the ‘normal’ photosphere to show; they can be distinguished from white light flares because they persist for much greater periods.

3.16 What are some guidelines to use when hunting for solar flares?

See also questions 5.08 through 5.12.
Flares tend to occur near the same location within a sunspot group.
Large flares are preceded by smaller flares and brightening of plages.
Small flares appear as one or more bright points of emission near sunspots on either side of a magnetically neutral line.
Large flares form parallel ribbons of emission on both sides of neutral lines.
When new sunspot groups grow and interact with older active regions, flare production increases.
More flares are seen when sunspots are growing, than when they are decaying.
Areas of high flare production can persist for several rotations or even years.

3.17 How do I best see white light flares?

It is important to increase the contrast between the photosphere and the flare. A good way to do this is to filter a transmission continuum around 4300 angstroms. This region of the solar spectrum is called the G-band, and it contains several absorption lines that go into emission during flares. The difference between the view during quiescent periods and flares is thus enhanced. Mylar filters that offer blue images have a transmission peak in this area.

Projection is not a good way to observe white light flares. The photospheric glare, passed at all wavelengths, overwhelms all but the brightest flares, which explains the lack of white light flare reports in the historical records.

3.18 How can I get automatic notification of solar flares in progress?

One way is to set up a radio receiver that listens to beacon stations in the 25 to 30 KHz region of the radio spectrum. Use a deviation alarm that sounds when the radio signal suddenly increases or decreases in strength.

You could subscribe to the Solnet List.

You could also subscribe to the Bear Alerts: http://sundog.caltech.edu/bear.html

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SECTION FOUR: NARROWBAND OBSERVING EQUIPMENT

4.01 What does it mean that a filter is a Hydrogen-Alpha solar filter?

A Hydrogen alpha solar filter is a filter that allows only a narrow band of wavelengths through, in this case those wavelengths that correspond to the center of the red hydrogen-alpha spectral line at 6562.8 angstroms. While white light observers see the solar photosphere, hydrogen-alpha observers see the solar chromosphere. For good chromospheric detail, a hydrogen-alpha solar filter must perform with a sub-angstrom bandpass at the eyepiece end.

4.02 Are there any other narrowband lines of interest on the sun?

Yes, hundreds of them! The Calcium K line at 3933.7 angstroms (deep blue) is one. The calcium K line is about ten times wider than the Hydrogen alpha lines, so the filters used for K-line observation can have larger FWHM’s than a hydrogen alpha filter, up to nearly a dozen angstroms. The H line of Calcium is also observed by some amateurs. The helium and iron lines are also of interest. The list goes on.

4.03 What is so special about Hydrogen-alpha filters?

These are the filters needed if you are to see the prominences, solar flares, filaments, plage, and other detail that is drowned out by the bright light of the rest of the sun. The view is simply amazing, with incredible detail and activity on the solar disk and extending well out into space from the limbs during times of solar activity. Hydrogen-alpha filters show the observer details from the chromosphere, while white light observers are limited to the photosphere.

4.04 What kind of filter is needed to see the sun in hydrogen-alpha light?

A Fabry-Perot etalon is the most common design for such a filter. A Lyot filter could also be used.

4.05 What is a Fabry-Perot etalon?

A Fabry-Perot etalon is composed of a transparent material on which special dielectric coatings are applied. In some etalons, the coatings are applied to either side of the same optical substrate; in others, two optical pieces are used, separated by a spacer. The spacing between the special coatings causes light to reflect in such a way that it is interfered with, setting up a condition in which a narrow slice of the spectrum is allowed through the filter. The refractive index of the optical substrate and the spacer have an effect on the frequency of the bandpass, but it is mainly the separation between the coatings that controls the behavior of the filter.

Because a Fabry-Perot etalon has multiple transmission peaks, a rather complicated arrangement of rejection filters, typically highpass and lowpass filters in conjunction with others, is used to limit the transmission to only the desired peak.

4.06 What does a hydrogen alpha solar filter cost?

Typically around $2000 US (1999) or more. This is likely an observing accessory for the enthusiastic or advanced observer only.

4.07 Why are they so expensive?

One reason that these filters are so expensive is that, to make a good one, the surfaces need to be flat and parallel to incredible precision. Also, additional interference filters must be constructed since, by itself, the Fabry-Perot etalon has a number of transmission peaks, which need to be quelled for quality and safety reasons. Further, the etalon is often housed in a temperature controlled ‘oven’ so that it can be tuned for slightly different bandpasses, and thus allow the user to hunt for strongly doppler-shifted features. If it is not in an oven, other tuning provisions are frequently made.

4.08 What does Full-Width, Half-Maximum mean in relation to solar narrowband filters?

Full-width, half-maximum (or FWHM for short) specifies the total bandpass (full-width) of a filter at 50% of its maximum transmission level (half-maximum). If a filter has a FWHM of one angstrom, it means that the bundle of wavelengths allowed through the filter at 50% efficiency is one angstrom wide; below the 50% level the bandpass is wider, and above it the bandpass is narrower. The FWHM is the standard method of specifying narrowpass filter characteristics.

4.09 What FWHM must a hydrogen-alpha filter have to show acceptable views of solar details?

That depends. If you only want to look at prominences and flares extending from the solar limb, then a bandpass of 3 to 10 angstroms is quite sufficient. However, such wide bandpass filters should be used with an occulting disk or other energy rejection method, since the solar disk at this bandpass is bright enough to be extremely uncomfortable and perhaps dangerous to vision.

To see chromospheric detail on the solar disk, the bandpass must be much narrower. A bandpass of two angstroms is considered very poor and offers a noticeably washed out image, with photospheric details seen clearly through the chromosphere. Passable contrast can be had at about 1.4 angstroms, with a .8 angstrom bandpass being considered very good and less than .5 angstroms excellent. From 1 to 1.4 angstroms is often considered a compromise bandpass, which allows the viewing of flares and prominences as well as chromospheric disk detail, but with neither seen well. In all cases, the bandpass is assumed to be centered on the hydrogen-alpha line. Most casual solar observers are happy with about 1.4 angstroms of actual bandpass, which is a good thing, since that is what they typically get, even with filter rated at a narrower bandpass.

When observing right on the hydrogen-alpha line, one is observing features high in the chromosphere and sees the highest contrast in the chromospheric disk detail. Observing slightly off the hydrogen-alpha line, in the wings of hydrogen-alpha as it is said, results in lower chromospheric regions coming into view. This will generally degrade the contrast of chromospheric features and render many of them invisible, but it will allow one to observe doppler-shifted features that would be dim or invisible right on the line.

Tuning a hydrogen-alpha filter one angstrom or more from the hydrogen-alpha line, or using a bandpass approaching 1.4 angstroms, will result in sunspots becoming conspicuous. This can be used as an informal test of the tuning calibration, or the bandpass being experienced at the eyepiece end.

4.10 What factors degrade the performance of a Fabry-Perot etalon?

Instrument angles and field angles are the two dominant effects that undesirably widen the bandpass. Other bandpass-degrading effects are caused by tilting the filter, and not keeping the etalon elements parallel.

4.11 What is an instrument angle, and why does it result in the bandpass of the filter widening?

When put in the business end of a telescope, the hydrogen-alpha filter must contend with several things. One of these things are "instrument angles", that is, the angles at which the light strikes the filter surfaces. The instrument angles are a result of the converging cone of light from the objective. An f/30 beam converges at .95 degrees. That means that light from the same point on the sun strikes the filter at normal incidence (=straight on), as well as at an angle of .95 degrees, and at all angles in between.

If light only struck the filter at normal incidence, straight on, then the bandpass of the light making it through the filter would be equal to the rated bandpass of the filter - for this and further examples we will assume that we are using a .6 A filter. Also, the light making it through would peak at exactly the frequency that the filter is rated to pass - the peak would occur right on the Hydrogen alpha line. So if the only light the filter encountered were of normal incidence, then the peak transmission would occur right on the Ha line. At .3 angstroms on either side of that line, the filter transmission would be 50%. So the filter would be said to have a .6 angstrom FWHM.

But normally incident light is a minority of the light present in a telescope. Our f/30 refractor, as we already stated, has light hitting the filter at up to an angle of .95 degrees (we will round to one degree). Light passing through the filter at an angle of 1 degree makes a longer trip between the etalon coatings than light passing through the filter straight on. This is physically the equivalent to changing the distance between the filter coatings. Changing the separation of the filter coatings changes the frequency of the peak bandpass of the filter. Thus, the light passing through the filter at an angle of 1 degree peaks a few tenths of an angstrom away from the H-alpha line (in the direction of shorter wavelengths), and drops to half intensity .3 angstroms from this unwanted peak. Since all angles from 0 to 1 degree are included, you can imagine an infinite number of peaks, all shifted just very slightly from the intended peak at the H-alpha line. This makes the transmission graph fatter; the new FWHM of the filter as it is actually experienced is now wider than intended. Although we have a .6 angstrom filter, we now see a 1.4 angstrom FWHM at the eyepiece of the telescope, much of that widening caused by instrument angles. (See also question 4.13.)

This is an example of an "instrument angle" - the angle of the converging cone of light means that light will strike the filter at various angles, thus making longer trips between the filter substrate. The longer trips result in a shift in the peak transmission and the widening of the FWHM.

4.12 If a .6 A filter really performs at 1.4 A, isn’t that false advertising?

No. Narrowpass filters are rated for their characteristics when encountering light straight on. This is an industry standard. So if you were sold a .6 A filter, it will pass .6 A of light if all that light is encountering the filter straight on. If you are offering it light at different angles, that is considered to be a problem in the way you have mounted the filter. After all, as we have seen, in different telescopes the filter will behave in different ways.

4.13 What is a field angle, and why does it result in the bandpass of the filter widening?

Let's suppose that you can afford to get a Fabry-Perot etalon big enough to put over the front end of your telescope. Because the light from the sun is coming from an essentially infinite distance, and since there is no optical element prior to the filter, there are no instrument angles for the filter to deal with. Now, all the light strikes our massively expensive filter at normal incidence, and now we should have an actual .6 angstrom bandpass at the eyepiece end. Right?

Well, it is actually wrong. The sun itself is not an infinitely small point; it has an apparent size of roughly half a degree. If you point the telescope at the center of the sun, then light from the center of the sun hits the filter at normal incidence, but light from the limbs hits it at angles of +/- 15 minutes. Again, these angles cause the light to take a longer trip between filter coatings, which causes the peak of the light passing through the filter to shift slightly from our intended peak. If we could afford the hundreds of thousands of dollars needed for a pre-aperture, full-aperture Fabry-Perot etalon, we would not really care about these field angles too much; the results would still be pretty good.

Why not, however, take a small etalon and put it on the eyepiece? After all, certain eyepieces (when focused for "normal" eyes) produce light of roughly normal incidence, and thus with no instrument angles. Well, at the eyepiece, the sun could take up 15, 20, 30 degrees or more; the "field angles" are now well outside the bounds of any filter handling them properly, and we would end up with an image of the sun that had a pretty good bandpass in the center of the field and really strange bandpasses at the edges. So that won't work either.

Field angles, then, are angles introduced by the apparent size of the sun. Like instrument angles, they degrade the effective bandpass of the filter by forcing light to make trips between the filter coatings that are of different distances for different angular locations in the image.

4.14 Can I put a .6 A hydrogen-alpha filter on my Schmidt Cassegrain telescope, which has a focal ratio of f/10, and stop it down to an aperture that will give me f/30? Or, could I use my f/10 refractor, and put in a University Optics Klee barlow to give f/28?

You can do that, and you can safely observe the sun that way, but you won’t get the .6 A bandpass you are expecting.

For one thing, both of those methods still result in an f/30 or greater instrument angle, and as we have already seen this results in widening the .6 A filter bandpass to a barely-acceptable 1.4 angstroms in actual use. But there are more problems beyond that.

Both barlow lenses and cassegrain secondaries are telenegative components that work the same way on the light. If we start with an f/30 telescope, and we toss in a 2x barlow, we have cut the instrument angles in half. However, we have almost tripled the field angles. If we put an eyepiece into the 2.8x barlow, the image looks 2.8x larger than without the barlow. An observer or filter after the barlow "sees" a solar image that is larger than without it. The apparent size of the sun also needs to be dealt with by the filter, or else we will have an H-alpha bandpass in the center of the image, and who knows what colors around the edges. No-one wants to have to get an H-alpha image of the sun by scanning around the entire disk and building up a progressive mental picture, right?

For those considering using an SCT (or a Mak-Cass, etc) with a filter, keep in mind that the amplification factors of the cassegrain secondaries therein are generally around five times.

These conditions apply when the filter is mounted as is traditional in the amateur community – near the eyepiece end of the telescope and without supplementary optics to control field and instrument angles. Some newer filters in the Coronado line are front-mounted etalons which do not have these specific problems.

4.15 Can I use my very large reflector, and use an off-axis mask that gets me to a very long focal ratio to get the bandpass desired?

Yes and no. You will get a narrow bandpass, but it won’t be centered on the hydrogen alpha line; sometimes this can be partially compensated for by the tuning latitude of the filter. Since the off-axis mask is offset from the optical axis, the converging light from the primary strikes the filter at artificially high angles, none of which are normally incident (straight on), as the filter is designed to expect. Suppose you stop the telescope down to f/30. Depending on how far from the optical axis the aperture mask is, your instrument angles could be between 3 degrees and 3.95 degrees, or worse (or not as bad – it all depends). This angle is a special case of instrument angle. With a .6 A filter, you can still experience a 1.4 A actual bandpass at the eyepiece that would be expected of an f/30 system, but the peak transmission will be shifted from the hydrogen alpha line toward shorter wavelengths. You will, in other words, still fall short.

4.16 Is a narrowpass filter in an SCT, stopped down off axis to a couple inches, and with a focal-ratio increasing barlow in front of it a bad configuration?

Yes – among the worst. The SCT secondary increases field angles which widens the bandpass; the off-axis mask shifts the center of the bandpass to shorter wavelengths than desired, and the barlow further increases field angles.

4.17 How does one get around these pernicious and seemingly insurmountable problems then?

One could build a horizontal solar telescope of tremendously long focal ratio, perhaps f/100 or longer. Design and construction advice for a horizontal solar telescope can be found in Rik Hill, ed., The New Observe and Understand the Sun, Astronomical League 1990.

One could build a very long refractor and place it in an optically folded configuration, using mirrors to control the size of the telescope tube or housing.

Alternately, one could mount the filter in what is referred to as a "telecentric" position. Such a design adds lenses to the telescope that serve to entirely eliminate instrument angles by taking the converging light from the primary and forcing it parallel; it then goes through the filter. After this, it encounters another lens that begins the light on a converging path again, so that an eyepiece or camera can be used at the focus of the telescope.

4.18 Does the telecentric position have any substantial problems?

Yes, it does. For one thing, the optics used to make the telecentric position are quite expensive. They must also be matched to the telescope. Very few telecentric kits are available for commercial telescopes, which means that in most cases an optician will have to be hired to custom-build the optical set. The mounting of telecentric optics is also difficult, particularly in standard Newtonian telescopes. Currently, the FAQ maintainer is aware that Astro-Physics and Tele Vue offer telecentric accessories for their telescopes.

But the principal failing of the telecentric position for hydrogen alpha solar filters is that telecentric optics increase field angles! The collimated beam of light coming from the first stage of a telecentric optical set is provided by a positive lens. An objective and a positive lens make up a Galilean telescope, which magnifies the image. This magnification could conceivably increase field angles tens of times. This, of course, will widen the bandpass of the filter substantially, most likely disastrously. There are ways to compensate for this, however, and Astro-Physics and Tele Vue both offer telecentric filter mounting units that include some means of compensation.

4.19 What are some sources of hydrogen-alpha solar filters?

Coronado Instrument Group, www.coronadofilters.com
Daystar Filters, PO Box 5110, Diamond Bar CA 91765 909-591-4673

The proprietor of Coronado is David Lunt. The proprietor of Daystar is Del Woods.

4.20 Which filter is better?

Both companies make good filters that perform to their specifications. Both companies have several models of filter, each with their own peculiar characteristics. You should pick the particular filter model that best suits your application.

Generally speaking, mounting a Daystar ATM, T-Scanner, or University filter in an optically advantageous manner that controls field and instrument angles to allow good bandpass characteristics at the eyepiece of a typical amateur telescope is more difficult than with certain Coronado filters; on the other hand, these Coronado filters tend to be more expensive. Coronado offers some of their filters in special mounting positions with supplementary optics that will result in good "eyepiece-end" bandpass performance. Other Coronado filters are essentially equivalent in mounting method to the Daystar filters mentioned and this advantage does not hold in those cases. Talk to your dealer to make sure you are getting what you expect.

4.21 But hasn’t Del Woods been around a lot longer than these Coronado people?

Del Woods (Daystar) has not been in the filter business as long as David Lunt (Coronado). Lunt made the world’s first Fabry-Perot etalon of the modern spaced design; Daystar still uses this filter design today which was first fabricated over thirty years ago. Lunt has been in the filter business for as long as Woods, but until recent years has been selling only to the professional, aerospace, and laser and synchrotron radiation optics markets. On the other hand, Woods has been selling to the amateur market for much longer. Basically, trying to figure out who has 'priority' here is a tossup.

Neither company has a stellar record for quick turnaround, simply because of the nature of the hand or batch fabrication of the etalons. Both companies have exhibited waiting periods of up to several months for their products. As of July 2001, Daystar's lead time for a T-Scanner (tilt-tuning etalon) is a bit over a year, and Coronado's lead time for a front-mounted etalon is almost exactly a year.

4.22 How do Coronado filters mounted in telecentric positions differ from any other filter mounted in a standard commercial telecentric configuration?

Several Coronado filters come packaged in, or are meant to be purchased with, additional optical elements that compensate for instrument angles and field angles. For example, the Helios I solar telescope is a 70mm refractor of f/6 focal ratio. We can already guess the instrument angles at f/6 are huge and would degrade the filter performance. To control this, prior to the filter, there is a positive lens (similar to the previously discussed telecentric position) that brings the light into nearly normal incidence to the filter.

"Nearly normal" incidence is done for a very special reason. An objective and a positive lens is a Galilean telescope, and that means the image is magnified, thus increasing field angles. But this problem can be controlled; by making the light coming out of the positive lens slightly divergent, the field angle can be balanced by a very minuscule and opposite instrument angle. Through this mechanism, both kinds of angles are controlled in such a way that the filter performs to its specification. After the filter, another lens provides an again-converging light cone so that eyepieces can be used. Similar configurations can be cobbled together for almost any filter position, given enough money to buy or make accessory optics.

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SECTION FIVE: PHOTOSPHERIC OBSERVATION AND SUNSPOTS

5.01 What is the photosphere?

The photosphere is the lowest layer of solar atmosphere observable. It is about 400km thick. White light solar filters show the photosphere, as do filtered naked-eye views.

5.02 What features are present on the photosphere?

Sunspots: These dark markings on the sun are easily seen. They are typically found in groups of several to a hundred spots, most of them small but often dominated by one or a couple large spots. Sunspots sometimes consist of two parts, a dark umbra and a lighter penumbra. The umbra is the darkest, inner portion of the spot, having a brightness of about .1 times that of the photosphere. The penumbra is a gray area usually surrounding an umbra, though they are sometimes seen in the absence of any umbrae; in these cases they are known as penumbral fragments. Penumbrae have a brightness of about .8 times that of the photosphere, but this ratio is strongly dependent on wavelength. Sunspots have strong magnetic fields which inhibit convection from below, making them at the center about 2500 degrees K cooler than the typical patch of photosphere. In the umbra, the magnetic fields are nearly vertical in orientation relative to the local photosphere, while in the penumbra, the magnetic fields are more horizontal.

Faculae: Faculae are irregular bright patches on the solar disk, usually more than an arc minute in extent. Often, sunspots will be seen to be embedded in faculae, though free-standing faculae are also commonly seen as well. Faculae are most easily seen near the solar limb, when their contrast is enhanced due to limb darkening; at the mean, they are about ten percent brighter than the bare photosphere. They are best seen in blue light. The singular of faculae is facula.

Pores: A pore is like a very small sunspot, usually only one or at most a few arcseconds in extent, but lacking the dark black color of a sunspot umbra. Instead, they are about the shade of a penumbra and are usually dark in green light; in integrated light they are from .2 and .4 times the brightness of the photosphere. Pores may appear and disappear in a matter of minutes. Those that persist might develop into full fledged sunspots. Pores often develop where several granule edges meet.

Granulation: Granulation is the fine-grain structure of the photosphere. Individual ‘grains’ are about one or two arcseconds across, and are rarely seen in telescopes less than 50mm aperture. The granulation is constantly changing, usually over time scales of minutes or less. Good seeing is needed to see the granulation; in poor seeing, clumps of granules and sometimes small faculae are seen as mottling. It is best not to confuse the two, as real granulation is a treat to see. Granulation is best seen in green light. Each granule is a convective cell which consists of a bright, roughly polygonal area of hot rising gas, and a cooler edge channel of descending gas.

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