Pentax 3.5mm XW Extra Wide Angle Eyepiece

This very short focal length Pentax eyepiece has a wide 70° field of view combined with a long 20mm eye relief for eyeglass use. It's a combination of virtues you won't find in any other eyepiece in its focal length range. It uses eight lenses in five groups to provide outstanding high magnification observing with short focal length refractors and fast focal ratio/short focal length Dobsonian reflectors. The wide and flat field of view allows longer undisturbed viewing with a Dobsonian. This allows several people to observe before an object has to be recentered in the field, a help for family viewing or star parties. 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. Using it with an 820mm 4" f/8 Takahashi FS-102 fluorite refractor, for example, will result in 234x (58x per inch of aperture) and a 0.44mm exit pupil (about 1/50th of an inch across). Both of these figures 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.