Observation Techniques

Lunar Observing Techniques

Observing the moon can be done pleasurably with nearly any telescope. The moon's generous apparent size, relative to other solar system bodies, insures that a significant amount of detail can be observed even with the smallest telescopes.

In general, observations of the moon will be limited by three factors, given in order of their importance:

The seeing conditions
The quality of the telescope optics
The design of the telescope & accessories

This list is directly contrary to that of several self-styled amateur experts on lunar observing but is, I believe, borne out by my own long experience and by the behavior of lunar scientists. The quests for better and better seeing have been nearly pathological since the early part of the century, when French astronomers began using Pic du Midi as a station for making detailed planetary observations. This has been followed by the development of a number of sites noted for their excellent seeing, such as Mauna Kea, La Silla in the Atacama Desert of Chile, the Apache Point in New Mexico, and others. Not satisfied with these sites, which sport subarcsecond seeing on most nights, astronomers have spent millions developing adaptive optics technology to further beat the atmosphere.

Seeing:

All of this is the result of the fundamental limitations of the atmosphere. If the atmospheric seeing is two arcseconds, it doesn't much matter how good the telescope is - things will be a blurry mess at scales approaching that value. It is true that poor optics can make it worse, but even the best optics can't make it any better.

The lunar aficionado is probably best advised to simply observe from the backyard, no matter what the typical seeing conditions, for that is the site that will be most often used. Observing for twenty minutes on each clear night as the terminator marches across the moon's face is likely to allow you to see a lot more than a special trip once or twice a lunation.

If remote sites are on the menu, then it might be worth seeking out a site with good seeing. Strong possibilities include the windward side of hilltops (or mountaintops) and large grassy areas with few trees. Of course local factors come into strong play, and a site's seeing characteristics will probably best be discerned by going there several times and checking it out.

There is less that can be done about poor seeing induced by the weather. Observers stuck under the jet stream in particular can experience long durations of poor seeing. And observing during gusty winds is likely not to favor fine resolution views.

Instrument:

After seeing, the quality of the telescope comes into play. The primary mirror or objective of a refractor is the most important element in the system, because if you are stuck with a bad one, no amount of off-the-shelf gear will correct the problem. Buying the finest eyepieces for a mediocre telescope is casting pearls before swine.

Much is written about the relative merits of a particular design over that of some other design. In most cases, the discussion is of little merit. The rules for selecting a telescope design for lunar observing can be boiled down to three:

All other things being equal, a greater aperture is more desirable than a lesser one because it will provide greater resolution;
All other things being equal, the larger the central obstruction, the less desirable the instrument;
If an achromatic refractor is employed, all other things being equal, the longer the focal ratio at a given aperture, the better.

The trouble comes in when it is realized that all other things are almost never equal. The mass produced telescopes are notorious for being of variable quality from one to the next, sometimes with legendary (occasionally mythical) runs of excellent quality, but never with any actual testing to back up the claims.

This circumstance is complicated by the claims made for premium telescopes, especially by a few exaggeration artists that favor high-end refractors. While many of these claims simply are not credible, a few of them are, and picking the wheat from the chaff can be a hard job.

And when all is said and done, the telescope mounting is as important as the telescope optics. A sturdy mounting is essential, especially for any high power viewing you intend to do (the moon is known for inspiring its share of magnification inflation).

Two examples of good telescopes for lunar viewing are the two telescopes that I most frequently use for lunar observing. The first is an Astro-Physics AP130EDF Starfire, a very well made 5" f/6 refractor. It is mounted on a Losmandy GM-8 mounting. The second telescope is an 8" Cave Astrola, vintage 1956, on its original mounting.

The refractor is a fine telescope in a small package, and I have used it for most, but not all, of the observations made to support this web guide. It is limited by its small aperture, which limits both the amount of resolution available in the image, and the brightness of the view at high magnification. The maximum useful magnification of this telescope is about 230x for the moon. The telescope is solidly mounted on a nice German equatorial with slow-motion controls on both axes, a feature that I find very valuable.

The Cave Astrola is a classic instrument built in the early days of the legendary Cave Optical Company. Its primary mirror is made to tolerances similar to that of the refractor, but because of its significantly larger aperture, it is capable of resolving much finer detail on the moon and of bearing higher magnification. The secondary obstruction, which is about 17% in this scope, is not much of a factor.

The telescope mechanically leaves something to be desired. The mounting has a barely adequate drive, and no slow motion controls. The motions are not as smooth as those of a good dobsonian, so moving the telescope about can be a bit of a chore.

On balance, I believe that a Newtonian reflector, if well-made and if attention is paid to controlling thermal effects (principally by using a fan), is probably the best price-for-performance that can be had for observing the moon. (Unfortunately, a preponderance of extremely poorly-made Newtonians on the market tends to give the design a bad name overall.) A beginner with no knowledge or experience can, on the first try, make an 8" or 10" primary mirror of practically unsurpassed quality by hand, if needed. Otherwise, there are respected optical firms that will supply one. A secondary mirror, mounting, and spider by Protostar is the only option worth seriously considering. The rest of the material can be cobbled together for a couple hundred dollars and a world-class telescope can be had.

For a mounting I would specify mainly that it be stable and its motions be smooth. While tracking (at the lunar rate) is desirable, it is not completely necessary; I wouldn't hesitate to embark upon my lunar observing with a quality Dobsonian.

If you have the means and inclination to enter the realm of the high-end commercial instruments, by all means do so. For the most part, when it comes to telescopes, you get what you pay for, and the premium brands, especially Astro-Physics, are of the highest quality.

Accessories:

While most amateur stargazers concentrate on the quality of their objectives, the rest of the optics contribute to the final view. Some attention should be paid to things like diagonals, barlows, and eyepieces, in addition to the objective.

Lunar observers have traditionally looked to the Abbe Orthoscopic eyepiece for high-resolution, true-color views of the lunar surface. I am one of these old-timers, and since I do not wear eyeglasses, I do not have any trouble using a short Ortho. Some observers prefer using Plossls, but I find their even smaller eye relief compared to an Ortho to be distressing. Other observers will of course have other preferences.

Potentially very valuable to some observers are eyepieces designed specifically with long eye relief in mind. The Vixen Lanthanum was the first really good example of this genre and it is now considered the reliable classic of the long-eye relief crowd. Since they are significantly less expensive than competing eyepieces, and render views as good or better than competitors', a significant fraction of lunar observers favor them.

One class of eyepieces that are really not optimal for lunar observing are most of the ultra-wide field designs. These designs go to great length to correct the widest possible field of view, and in the process tend to compromise in important ways. In an effort to correct off-axis astigmatism, many of these eyepieces introduce chromatic aberration. Others leave astigmatism uncorrected and thus blur a significant outer portion of the field. On balance, anything more than about a 60° is of dubious application while lunar gazing, unless you are looking for an eye-filling view for the sheer joy of it.

Controlling Glare:

No more frequent or compelling complaint about lunar observing is heard than that looking in the eyepiece at the moon hurts the eyes, because of the moon's brightness. Fortunately this problem is easily solved.

To control glare, I favor the traditional method of observing the moon from a lighted observatory, patio, driveway, or what have you. The rationale for this method is simple. The moon is a sunlit surface, just as your backyard is during the middle of the day. However, we are most often observing it at night. Most people become at least partially dark adapted fairly quickly, and putting the moon in their telescope and having a look is the functional equivalent of exiting a photographic darkroom directly to the out-of-doors. Of course this hurts the eyes!

The traditional solution is simply to leave the lights on at the observing location, as illustrated at my own observing site to the right (note the grill lights; also turned on but less apparent are the sunroom lights behind the telescope and to the right, and the swimming pool light in the bottom left of the photo). Normal room lighting in an observatory or typical outdoor patio lighting in the backyard is adequate to quell your dark adaptation, and the brightness of the lunar view is quite comfortable even in large apertures. This solution leaves your eyes in a state of adaptation that is optimal for the lunar illumination, so the eyes do not have to overcome any adaptation effects and therefore will give you the best view possible on any given night. In addition, simple chores like reading a lunar chart are rendered trivially simple - you do not need to be carrying around a flashlight to do so.

This method was the one favored by observers for, apparently, hundreds of years. Madler, doing his epochal lunar observations in Beer's observatory at the beginning of the 19th century, used full lantern lighting while making the observations. Later observers followed the practice through the 19th century and well into the 20th. Such 20th century observers as Goodacre and Wilkins noted that the use of a red light was not only not necessary, but undesirable, and that observing in a lit environment was not prejudicial to the view. In more recent times, Harold Hill in his book A Portfolio of Lunar Drawings describes a strip lighting arrangement he has made for his sketch board. Famous telescope maker Roland Christian reports also using white lighting while making lunar and planetary observations.

My principal caution about this method is not to allow any light to directly fall on the eyepiece - or else you are asking for annoying reflections off the eye lens. Lamps with shades and lights placed behind the observer work quite well.

This solution does assume that there are no other observers nearby who are wanting to look at celestial objects other than the moon. If there are, then douse the lights - don't be impolite!

The second method is, in my opinion, far less preferable, and that is to introduce a neutral density filter into the telescope to quell some of the light. In general, I do not favor introducing unnecessary optics into the system, although admittedly the effects of a filter should be minimal if placed close to the focal plane. But more importantly, the moon's brightness will be quelled considerably, and will become dark enough that the eyes must have some dark adaptation to see small features. Unfortunately, dark-adapted photopic eyesight isn't really all that good, and by doing this you are asking your eyes to perform in conditions it wasn't designed for.

Nevertheless, a good deal can be seen through the use of filtration, and it is a technique favored by many observers. Adopt whichever you prefer.

On the Use of Averted Vision:

There is no use for averted vision while observing the moon. Both physiology and experience are against it. The rods, employed in averted vision, are not as good detectors of either fine resolution or low contrast as the cones, which are used in direct vision. Observers who have had success with averted vision are actually enjoying the benefits of having a few cones even in the areas of the retina where rods are thickest.

On the Use of Lunar Charts

When using a lunar chart with a Newtonian telescope, all that is needed to properly orient the chart is to rotate it until it matches the view through the eyepiece. The common assertion that a Newtonian telescope inverts the image is a myth - it will only be inverted with a particular orientation of the eyepiece relative to the ground. Much more accurate would be to say that the image in a Newtonian is arbitrarily rotated.

For users of refractors, telescopes in the Cassegrain tradition, and other instruments that require the use of a star diagonal or an odd number of reflections, the situation is a bit more grim. The image in these telescopes is mirror reversed, most commonly experienced at the eyepiece as a left-right flip of the image. Sky & Telescope markets a simple moon map that is flipped for users of such telescopes, but it isn't terribly detailed. Mentally flipping the charts of a conventional map works for some but is difficult for others.