Choosing eyepieces: It makes little sense to buy lots and lots of eyepieces. You’ll spend more time trying to pick the right eyepiece to use than you’ll actually spend observing. It makes more sense to have just three or four well-chosen eyepieces that cover the full range of observing possibilities.
Most telescope manufacturers supply one medium power eyepiece with their scopes. Its power is low enough to show you a generous field for deep space observing (but not so low that light pollution masks faint nebulas and galaxies), while still high enough for reasonably-sized planet and star cluster images.
Keeping in mind the power per inch of aperture and exit pupil guidelines above, your first optional eyepiece might be one with about twice the power of the eyepiece supplied with your scope. This will give you larger planetary images and will more easily resolve globular clusters and binary stars. If your skies are fairly dark, your second choice might be a low power eyepiece with about half the power of your scope’s standard eyepiece. In other words, if you start with a 25mm medium power eyepiece, you might first add a 9mm to 13mm eyepiece for high power observing and then a 40mm or 42mm for low power.
If you develop an interest in very high power lunar and planetary observing, you might later add a very short focal length eyepiece for those rare nights when seeing conditions allow the highest powers. If you develop in interest in hunting for the faintest of deep sky objects, and you have access to dark skies, you might add a 2" star diagonal and a long focal length eyepiece (such as a 2" 56mm Plössl) for maximum image brightness and an extra-wide field.
There are many other choices you can make, of course – wide field medium power eyepieces for observing the larger nebulas; filling in the gap between medium and high powers for observing the smaller planetary nebulas; etc. – but the three or four basic eyepiece focal lengths outlined above will give you a good foundation for every type of observing you might want to do.
Once you have chosen the appropriate focal lengths for your eyepieces, give some thought to what eyepiece type(s) to get. The best type of eyepiece to buy is sometimes influenced by how you’re going to use your telescope.
For example, if you have a Dobsonian reflector, you may find it easier to locate objects in the wide fields of Panoptic, Super Wide, Nagler, or similar eyepieces, rather than in the narrower fields of Plössls or modified achromatics. Also, it takes longer for an object to drift across the field of view of a wide field eyepiece. This allows several less-experienced family members to view an object before it drifts out of the eyepiece and you have to move the scope for them. A wide field also means you can observe the planets longer before you have to move a Dobsonian. Wide field eyepieces are also excellent for star party use with Dobsonians.
If you have a scope with a motor drive, however, the need for a wide field eyepiece to extend your observing time at high powers before you have to adjust the scope position may diminish, as the drive will keep objects centered automatically. Also, very few objects require both high magnification and a wide field of view simultaneously, so the high power of a short focal length wide field eyepiece on a long focal length catadioptric scope is often a waste of the eyepiece’s wide field. The higher contrast of narrower field Brandon, Plössl, and Ultima eyepieces for planetary and planetary nebula observing may outweigh the need for lower-contrast wide field eyepieces to help locate those objects.
The type of telescope you have can also influence the type of eyepieces to buy. For example, if you have a reflector, its inherent coma can be emphasized by the wrong choice of eyepiece. The faster a reflector’s focal ratio, the more pronounced its coma, an optical defect that makes stars appear progressively more fan-shaped as you get closer to the edge of the field.
The diameter of the effectively coma-free field of a reflector is roughly equal (in millimeters) to one-half the square of its focal ratio. An f/5 scope has a coma-free field of approximately 12.5mm (5 squared = 25, divided by 2), an f/4 scope has an 8mm field (4 squared = 16, divided by 2), etc. A 40mm eyepiece usually looks at an area about 28mm in diameter in the center of the telescope image, while the diagonal of a 35mm film negative spans 44mm and that of a DSLR with an APS-C sensor is typically about 28mm. Both are much wider than the coma-free field of a fast focal ratio scope. At low powers,then, up to 50% of a fast reflector’s image can show coma’s distracting fan-shaped stars. Optional coma-correcting lenses can eliminate this problem.
Inexpensive modified achromatic eyepieces typically suffer from curvature of field (an inability to bring the center and edges of the field into focus at the same time). Not only are some of the stars fan-shaped in a modified achromatic eyepiece due to coma, but they’re out of focus as well! Reflectors with fast focal ratios (below f/6) are therefore at their best with flat field eyepieces such as Plössls and those designed for fast focal ratio use (Naglers, Panoptics, Super Wide Angles, etc.) rather than modified achromatics.
Coma is not usually a problem at high magnifications. A high power eyepiece looks only at the central 5mm to 12mm area of the image and does not see the comatic outer portions.
An f/6 reflector (36mm coma-free field) is much more tolerant of eyepiece field curvature, as are refractors, which rarely show any coma. Schmidt- and Maksutov-Cassegrain catadioptrics are generally not prone to coma, although Schmidts have field curvature that is emphasized by inexpensive eyepieces at low powers. These scopes can all be used successfully with a wide variety of eyepiece types, including modified achromatics. However, higher quality eyepieces will always get more out of them than a simple three-lens modified achromatic. With these telescope types, the kind of eyepiece you buy should be determined by your budget and observing requirements rather than by any telescope limitations.
Naturally, your budget will have a say in choosing your eyepieces. Ultimately, however, you’ll get more enjoyment out of one good eyepiece that pulls all the performance possible out of your telescope than you’ll get out of two lesser quality eyepieces that can limit its performance. Buy quality. It pays dividends.