Pentax 40mm XW Extra Wide Angle 2" Eyepiece

This long focal length 2" Pentax eyepiece is outstanding for low power/wide field astronomical observing with long focal length refractors and catadioptric telescopes . . .

This long focal length 2" Pentax eyepiece is outstanding for low power/wide field astronomical observing with long focal length refractors and catadioptric telescopes. However, it is not that well suited to fast focal ratio reflectors. For example, it provides a huge 8.9mm exit pupil with an f/4.5 Dobsonian reflector. As the eyes of most observers are unable to dilate to this size, this eyepiece will probably waste a good portion of the light-gathering capacity of a fast focal ratio scope. It is better suited to longer focal ratio refractors, Schmidt-Cassegrains, and Maksutov-Cassegrains than it is to reflectors with focal ratios shorter than f/6. Using it with a 2" star diagonal in a 2000mm 8" f/10 Schmidt-Cassegrain, for example, will result in 50x and a wide 1.4° field of view. That is almost three times the diameter of the full Moon and a large enough field to fit in all of the Pleiades, with room to spare. It will also work well for rich field observing with moderately fast focal ratio refractors. For example, with the 600mm focal length TeleVue TV-85 it provides 15x, a 5.7mm exit pupil, and a huge 4.7° field. With the 650mm focal length TMB-105 triplet apo, it provides 16.3x, a 6.45mm exit pupil, and a 4.3° field. On both scopes, the fields are eight to nine times as wide as the Moon. These figures are all very usable for capturing galaxy clusters, viewing emission and reflection nebulas and open star clusters in their entirety, scanning the star clouds of Sagittarius and large and faint nebula remnants such as the Veil, etc. You can use the calculator below to see how wide a field it will give you with your particular scope.

The long 20mm eye relief is ideal for those who must wear eyeglasses while observing due to astigmatism. The substantial 24.7 oz. weight of the eyepiece may require rebalancing of some Dobsonians if you switch between this 2" eyepiece and a lighter 1.25" eyepiece.