Pentax 5mm XW Extra Wide Angle Eyepiece

This short focal length Pentax eyepiece has a very wide 70° field of view and very long 20mm eye relief, a uniquely useful combination not matched by other eyepieces. It uses eight lenses in five groups to provide outstanding medium to medium high magnification astronomical observing with short focal length refractors and fast focal ratio/short focal length Dobsonian reflectors. Its long eye relief is particularly helpful if you must wear eyeglasses while observing, although all observers will benefit from its wide field and sharp images. It provides about the maximum usable magnification with a typical 8" or 10" f/10 Schmidt-Cassegrain telescope. It has good contrast for seeing subtle lunar and planetary details, and for seeking out small planetary nebulas.

It will generally be useful with long focal length refractors, reflectors, or catadioptric scopes only under very good or better seeing conditions. Average seeing will limit its usefulness. Using it with a 2000mm 8" f/10 Schmidt-Cassegrain, for example, will result in 400x (50x per inch of aperture) and a 0.5mm exit pupil (about 1/50th of an inch across). Both of these are very close to the generally-accepted maximum values of magnification per inch of aperture and exit pupil size for usable observing under good to very good seeing conditions for a reasonably experienced observer.