Tele Vue 4mm Radian 1.25" Eyepiece.

By limiting the field to 60 deg. (a Radian = 57.3 deg.) and establishing a uniform 20mm eye-relief, Tele Vue reworked the Nagler concept using the latest glass types to achieve optimum full field sharpness, freedom from distortion, freedom from ghosting (for critical planetary viewing) along with a unique adjustable eyeguard assembly which Tele Vue calls "Instadjust" Instadjust was developed from the need for proper eye position for non-eyeglass wearers. It is essential, since with Radian's long eye-relief, non-eyeglass wearers would easily tend to get too close to the eyepiece, resulting in vignetting the field, just as you would moving out from the exit pupil. The click-stop, Instadjust eyeguard permits fast, accurate positioning of the eye to the exit-pupil with maximum suppression of environmental stray light. Instadjust is topped off by a soft (for comfort and eyeglass protection) fold-down rubber eyeguard and protective cap. Usual Tele Vue features such as full multicoating, blackened lens edges, parfocalization, and grip-ring complement the Radian line.