Four element designs give a flat, beautifully corrected field of view. Optical elements are exactingly ground and hard-coated with magnesium fluoride. Precision metal parts are black-anodized for antireflection and machined to standard 1 1/4" diameter. Parfocal mechanical designs - no need to refocus when changing eyepieces. Comfortable exit pupil distance for observing ease. Threaded at the base to accept all 1 1/4" Vernonscope mounted glass filters.