Your telescope's focuser is used to compensate for your near- or far-sightedness;DIOPTRX compensates for your astigmatism. These units attach and lock onto the tops of over 20 long eye-relief Tele Vue eyepieces to achieve the sharpest full-field viewing possible. DIOPTRX models are available in 1/4 diopter steps, from 1/4 up to 3.5 diopters, and are rotatable for tuning to the best orientation. A series of engraved letters on the barrel helps to monitor orientation. Simply choose the DIOPTRX model that matches your eyeglass prescription for astigmatism. All lenses are multi-coated glass in anodized aluminum housings with rubber eyeguardsThe visual experience in astronomy is maximized by stripping away limitations: the limitations of the observing location (dark skies and steady seeing) the limitations of the telescope (aperture, field, resolution and contrast) the limitations of the eyepiece (field, resolution and contrast) the limitations of our eyesight (e.g., astigmatism)Tele Vue has historically been dedicated to overcoming the first three of these limitations: with wide angle, flat field telescopes easily portable for traveling to great observing locations matching ultra-wide angle eyepieces with full field image quality to generate what we call "spacewalk" viewing and a variety of long eye relief eyepieces to allow eyeglass wearers to enjoy the maximum visual experienceTo overcome the fourth limitation (a