4mm Radian Eyepiece.

"What sets Radians apart from other long eye-relief, short-focal-length eyepieces is superb image quality. Even discerning planetary observers who customarily choose Plössl or orthoscopic designs are switching to Radian." — Sky & Telescope, December 1999

Instadjust was developed for maximizing contrast while aiding in proper eye positioning for non-eyeglass wearers. It is essential, since with the Radian's long eye-relief, non-eyeglass wearers could easily get too close to the eyepiece, resulting in vignetting where edge field rays miss your pupil. The push-pull, click-stop adjustment permits fast positioning and maximum suppression of environmental stray light.

Another unique development is the Pupil Guide. It helps non-eyeglass wearers, especially novices, zero-in and stay centered on the exit pupil. The Pupil Guide is included with all Radian and Nagler Type 4s and fits under the soft, fold-down rubber eyeguard.