This medium focal length Pentax is a dual-purpose eyepiece for both terrestrial and astronomical observing.
Terrestrially, it is an excellent moderate power eyepiece for detailed long- and medium-distance nature studies with the 80mm and 100mm Pentax spotting scopes under virtually any lighting conditions, dawn to dusk.
Astronomically, it provides superb medium power wide field lunar and planetary observing with long focal length refractors, reflectors, or catadioptric scopes. With short focal length refractors and fast focal ratio reflectors, it gives a “picture window on space” image. An f/4.5 Dobsonian reflector, for example, will yield a 3.1mm exit pupil, for bright images of emission nebulas and galaxies. Its sharpness across the field makes smaller globular clusters vivid and nearly three-dimensional. The 650mm focal length TMB-105 triplet apo refractor provides 46x, a 2.25mm exit pupil, and a 1.3-degree field (almost three times as wide as the Moon). These figures are all very usable for observing open clusters such as the Pleiades, large nebulas such as the Lagoon, and large globulars such as M-13. All will fit nicely into the field of view, with a framework of black velvet around them to set off their subtleties.
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 36x magnification and a 102’ 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 37x magnification and a 99’ field of view at 1000 yards.
With the 100mm aperture Pentax spotting scope this eyepiece provides 45x magnification and an 81’ 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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