This short focal length Pentax is a high power eyepiece usable for both terrestrial and astronomical observing.
Terrestrially, it is superb for detailed long-distance nature studies with the 80mm and 100mm Pentax spotting scopes under average lighting conditions.
Astronomically, it’s an excellent high power lunar and planetary observing eyepiece with long focal length refractors, reflectors, or catadioptric scopes under average seeing conditions. Its 10mm focal length is an ideal match for an f/10 Schmidt-Cassegrain, providing 200x and a 1mm exit pupil that closely matches the resolution of the scope to that of your eye. Some of the larger globular clusters, such as M13, will fill its 0.35-degree field with such a scope. It is also exceptional for medium power viewing with short focal length refractors and fast focal ratio Dobsonian reflectors. It has seven lenses in a six group optical design for a very wide 70° field. It is regularly useful for astronomical observing with long focal length scopes, but below average seeing conditions may limit its performance somewhat. If so-so seeing is the norm where you observe, its 10mm focal length will often represent the maximum practical with a long focal length scope.
The long 20mm eye relief is ideal for those who must wear eyeglasses while observing due to astigmatism. Even those who do not wear glasses will find the long eye relief a comfortable change from the cramped eye relief that is typical of the usual short focal length eyepiece.
For terrestrial observing . . .
With an 80mm straight-through Pentax spotting scope this eyepiece provides 50x magnification and a 74’ field of view at 1000 yards, which are the values given in the specifications to the left.
With the 80mm angled Pentax spotting scope this eyepiece provides 52x magnification and a 71’ field of view at 1000 yards.
With the 100mm aperture Pentax spotting scope this eyepiece provides 63x magnification and a 58’ field of view.
For astronomical observing . . .
The field of view of 70° degrees given in the specifications to the left is the apparent field of view. The actual field of view will be equal to that apparent field divided by the magnification of the eyepiece/telescope combination.
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