Many peoples’ eyes exhibit enough astigmatism to require correction with eyeglasses. Astigmatism is caused by a change in the shape of the eye’s cornea that causes it to magnify images by different amounts in different directions. The difference in magnification (the astigmatism) is expressed in diopters. If your eye has a dioptric power of 2.00D in the vertical plane, but a power of 2.50D in the horizontal plane, the difference between these two powers (2.50D – 2.00D) is 0.50D. Your eye would then be said to have an astigmatism of one-half diopter (0.50D).
When using a telescope, near-sightedness and far-sightedness can be compensated for by focusing, but not astigmatism. Astigmatism shows itself as elongated, oval, or distorted star images. As cornea curvature changes are most visible at the periphery of the cornea, astigmatism is more of a problem when using low power eyepieces that have a large exit pupil. When using higher power eyepieces (with their smaller exit pupils centered on the less-astigmatic portion of your cornea) astigmatism may not be noticeable and eyeglasses may not be necessary. Through experimentation, it has been determined that 2 diopters of eyesight astigmatism is detectable down to a 1mm exit pupil (a 10mm eyepiece on an f/10 scope, for example), and 1 diopter is detectable at a 2mm exit pupil (a 20mm eyepiece on an f/10 scope).
Eyeglasses can be uncomfortable to use at the eyepiece in many telescope orientations and invariably cause a loss of field because they push your eye back from the correct eye relief position. In addition, they are prone to ghost images, reflecting an image of your own eye back to you, and a substantial light loss from their uncoated lenses. Bi-focal, tri-focal, or vari-focal (progressive) designs are also very disconcerting to use when trying to view a large field at infinity. To overcome the image-blurring effects of astigmatism without using eyeglasses, TeleVue has developed a corrector lens for your eyepiece that compensates for eyesight astigmatism and replaces your eyeglasses at the telescope. The Dioptrx Astigmatism Correcting Accessory Lens extracts the maximum visual performance from your telescope by matching your eyesight to the eyepiece.
TeleVue Dioptrx lenses attach to the top of all long eye relief TeleVue eyepieces in place of the eyepiece’s existing rubber eyeguard. They are available in ¼ diopter correction steps, from ¼ diopter up to 2½ diopters. The multicoated lenses rotate to let you match the orientation of the Dioptrx correction precisely to your vision for the best images. The Dioptrx system gives you the potential for optimizing the optical correction for your astronomical viewing in both the amount and orientation of your individual astigmatism. The ability to change the amount of correction at low cost as your eyesight changes is another advantage. With an inexpensive Dioptrx multicoated high quality lens accurately aligned to your eyepiece, you can maximize your ability to see the faintest stars over wide fields and the subtlest of small planetary details. You’ll have the satisfaction of knowing that you are getting the very best visual experience possible with your prized telescope and eyepieces.
To choose the correct Dioptrx accessory lens for your eyesight, simply choose the model that matches your eyeglass prescription for astigmatism (it will be the "CYL" number in your prescription). If you don’t know your eyeglass prescription, contact your optometrist. Ask him or her for the amount of astigmatism in your dominant eye (the one you primarily use for observing). The orientation of the astigmatism is not important, as the Dioptrx is rotatable for tuning to the best orientation. A series of engraved letters on the barrel helps to monitor the orientation. All lenses are multicoated precision-ground optical glass in anodized aluminum housings with rubber eyeguards.
The Dioptrx lenses attach to and lock onto the top of the following long eye relief TeleVue eyepieces only: 26mm and 31mm Nagler type 5; 22mm, 27mm, 35mm, and 41mm Panoptic; 22mm, 40mm, and 55mm Plössl; all Radians (18mm thru 3mm); and all Nagler type 4 (22mm, 17mm, and 12mm). Adapters to allow their use with 19mm and 24mm TeleVue Panoptic eyepieces and the 20mm Type 5 Nagler, as well as all TeleVue Type 6 Nagler eyepieces.