One of the leading considerations in eyepiece selection at the turn of the century is eye relief. Eye relief is simply the distance from the eye lens of an eyepiece to the location of the exit pupil. If you have your eye too far from the eye lens, then you can't see the whole field of view of the eyepiece. With some eyepiece designs, if you put your eye too close to the eye lens, the entire image blacks out, or parts of it do.
Short eye relief describes a condition in which you have to have your eye pretty close to the eyepiece in order to see the entire field. The amount of eye relief is dependent upon the design of the ocular, but normally, short focal length eyepieces have the shortest eye relief, so usually the condition is encountered in high-power viewing of the moon and the planets. Short eye relief presents several potential problems for observers:
There are two basic methods for dealing with eye relief. The first method is to purchase eyepieces having generous eye relief to begin with. The second method is to apply some observing skills and techniques to better use whatever eyepieces you have. We'll discuss both of these here.
The first-order solution to the eye relief problems is to pick eyepieces that have generous eye relief, which is why, as stated above, it is now a leading consideration when choosing eyepieces. For eyeglasses wearers, 15mm of eye relief is about the minimum needed to see the whole field of view. Designs such as the Vixen Lanthanum and the Tele Vue Radian have generous eye relief even in the shortest focal lengths. The drawbacks are that these eyepieces are considerably more expensive than some other options, and eyepieces optimized specifically for long eye relief are not necessarily well corrected in ways important to lunar and planetary observers. When this is the case, eye relief is achieved at the sacrifice of image quality - not, in general, something I would recommend. On the other hand, Plossls and orthoscopics can be had for fifty dollars per ocular, and have been recognized as excellent eyepieces for over a century - but they are not optimized for good eye relief. When it comes to selection, then, I'd say get the long eye relief oculars if you can afford it and are convinced you want or need them. Otherwise, it is best to learn how to take full advantage of a less exotic ocular.
If you have chosen to go with an eyepiece that is not optimized for long eye relief, you can do yourself the favor of at least getting an inexpensive design that doesn't have outrageously short eye relief. Currently, Plossl oculars are quite popular, and deservedly so - they are good performers and are cheap and easy to manufacture, so they represent a good value for the money. However, in short focal lengths, Plossls have astonishingly short eye relief. An 8mm Plossl in a friend's eyepiece case is routinely referred to as the "8mm contact lens," referring to the fact that you have to press your face firmly against the eyepiece barrel to see the entire field of view (it offers about 5.4mm of eye relief). On the other hand, an Abbe orthoscopic of the same focal length will have about 9.5mm of eye relief - which is plenty to keep you comfortable if you aren't wearing glasses. To this extent, then, my recommendation for buying a short focal length eyepiece, if eye relief is a concern, would be to avoid Plossls and go with something a bit more forgiving. Here is a table of eye relief "rules of thumb" for common eyepiece designs:
Eyepiece Type | Eye Relief |
| Abbe orthoscopic | focal length x 1.1 or 1.3 |
| Brandon | focal length x 0.7 |
| Kellner | focal length x 0.5 |
| Plossl | focal length x 0.7 |
Table 1: This table provides "rules of thumb" for calculating the eye relief of traditionally favored high-power oculars of common designs. The rules of thumb are imprecise; for example, at longer focal lengths, the Abbe orthoscopic has about 1.4x its focal length in eye relief. Other eyepieces are similarly not handled by a simple factor, but this will do to get you in the ballpark for a given eyepiece design. If specifications are published by the eyepiece maker, check them, rather than relying on this table.
Another workaround you can handle when selecting the eyepiece is well known, and that is choosing a longer focal length eyepiece, and purchasing a barlow or telenegative unit to boost its magnification. For example, a 25mm eyepiece used with a 5x Powermate gives you the equivalent of a high-power 5mm ocular. The tradeoff here is that the barlow lens introduces more optics. This is not inherently bad, but adding more optical surfaces gives the opportunity to introduce more errors (i.e., the barlow could introduce spherical aberration or added astigmatism if it is improperly designed and made), more narrow-angle scatter from the coatings, more absorption which might affect the perceived color of the image, and other potential problems. These problems should not be overstated, but are seen frequently enough that many hard-core observers prefer to go with a "pure" short focal length eyepiece, rather than introduce extra gadgets
Besides buying things, there are several techniques that can be applied to make viewing through a short eye relief eyepiece more comfortable and productive.
First, eyeglass wearers should determine whether or not their eyeglasses really need to be worn at high powers. Only observers who have astigmatism in their observing eye should ever wear their eyeglasses at the eyepiece. Nearsighted and farsighted observers can completely correct their vision by operating the telescope's focuser, and eyeglasses will have the opportunity to be bumped and scratched by the eyepiece unnecessarily, and will provide substantial reflective losses to boot.
Observers with astigmatism in their observing eye may need to wear their glasses at low powers. At low powers, the exit pupil is large, and takes advantage of a large proportion of the eye's pupil, and this gives astigmatism the opportunity to rampage freely. On one side of the position of best focus, an astigmatic observer will see stars as a short line; on the other side of focus, the same short line will be seen, but it will be oriented 90 degrees from the first line. In between the focus positions where the line appears, the star can appear like a bloated circle or a boxy shape with corners. If you rotate your head relative to the eyepiece, and the lines rotate with it, then the astigmatism is in your eye, rather than in the telescope or eyepiece.
However, at high powers, only a small portion of the eye's lens is used to form the image, due to the fact that at higher and higher powers, the exit pupil of the eyepiece is smaller and smaller. According to eye doctors, of those patients who have astigmatism, 95% are completely astigmatism-free when using a 1.3mm exit pupil. For this reason, even astigmatic observers usually do not need to wear their glasses when using high powers. Obviously, those that do not absolutely need their glasses at high powers should not wear them, and this will solve any eyeglass-induced problems with eye relief. To find out if you are one of the unfortunate 5% who do need eyeglasses even at higher powers, simply do the test described above with a high-power eyepiece. But at some exit pupil, your astigmatism will disappear. In practical terms, local defects in the eye lens and cornea overwhelm even very strong astigmatism at exit pupils of 0.2mm or smaller, so that should provide a lower limit to eyeglasses use.
If you are one of the few who need glasses for astigmatism at any practical magnification, you can get your optician to make a corrective cylinder that can be fitted to a filter ring and screwed into the bottom of the eyepiece. This is well worth doing for such extreme cases, and will probably be necessary for even low-power oculars under these circumstances.
The remaining problems of short eye relief are all ones of technique and sometimes practice. Finding the field of view in a short focal length eyepiece can be difficult unless the observer is comfortable. A strained posture at the eyepiece will not allow the observer to keep the eye centered and steady over the exit pupil, and will result in constant problems with short eye relief eyepieces. This is a side effect of an what is really poor technique to begin with. Uncomfortable, strained observers do not - can not - see as much as when they are relaxed, and I do not see the point to making amateur astronomy painful in any case. The simple solution is to address the problem. Get an observing chair so that observations can be made while seated. If you are using a big telescope, install half-steps in your observing ladder. Keeping the head still while looking through any eyepiece is a fundamental observing skill, and observers who can't do this have other problems without adding short eye relief to the list.
The problem of occasionally bumping the eyepiece can also be aided by better positioning and comfort at the telescope, but not always. For example, that 8mm Plossl is impossible to use at its full field without pressing against the eyepiece, and that shakes all but the most solidly-mounted telescopes. One of the most straightforward solutions here is simply to admit that you don't need the whole field of view. To offer a typical case as an example, lets pretend we have a 4mm Abbe orthoscopic, with a 40 degree apparent field, used with a 10" f/6 telescope. This will offer us about 375x, which is high power by any standard, and will have a field of view of about 400 arcseconds. This is enough room to put eight or nine Jupiters across the field, and sixteen of Mars at its absolute largest. Obviously, for even the largest planet, the vast majority of the entire true field is not needed, so the observer can afford to back the eye off the eye lens a little bit. You can afford to do this because, even when the entire field cannot be seen, none of the rays for the paraxial region will be vignetted until you are a long way back from the eyepiece (typically, six inches or more, while moving back a half inch is sufficient to solve the 'bump' problem). This is the simplest solution to the problem but, again, you have to keep your head still.
Finally, short eye relief eyepieces tend to accumulate skin oil, mascara, and other crud that hangs out on the observer's eyelashes. Some improvements can be had by simply not wearing mascara while observing, and washing the eyelid before observing, although some people may consider this advice tedious. The design of the eyepiece can help - in barrels where the eye lens is at the top of a cone, rather than beneath a lip, less gunk will accumulate on the lens, apparently because in the absence of a lip there is not enough mechanical action to jar really large amounts of stuff free. Backing the eye off when you don't need the whole field helps here too. But fundamentally, every so often an eyepiece simply needs to be cleaned, and when it gets really horribly dirty, it may be necessary to disassemble it and do a thorough job.
Hopefully, you've picked up some useful tips when selecting or using your eyepieces here - so go out there and have some fun!
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