Dioptrx: Prescription for Perfection

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By Dennis di Cicco Conceptually, as It’s usually called “cylinder,” and well as optically and mechanically, is given as a plus-or-minus (±) With the right Dioptrx lens the Tele Vue Dioptrx ($98) is a value in diopters. For the purpose attached to an eyepiece, simple device. But a chance obser- of selecting the right Dioptrx, the observers suff ering from vation made me really appreciate ± is irrelevant (as is the angle); astigmatism can enjoy pin- what this astigmatism corrector you need only know the absolute point stars without needing can do. value in diopters. Observers with 2 to wear prescription eye- Our view of the heavens depends diopters of astigmatism often see glasses. Although custom on the optical performance of our its image-degrading eff ects with CICCO DENNIS DI made to fi t more than two S&T: S&T: eyes as well as that of our tele- exit pupils as small as 1 mm. My dozen Tele Vue eyepieces, scopes. A twist of the scope’s focus minor 0.5-diopter astigmatism isn’t Dioptrx’s 43-mm-diameter knob compensates for near- or far- particularly obvious until the exit (1.7-inch) mounting ring sightedness, but astigmatism — the pupil reaches 4 mm or larger. can be jury-rigged to fi t aberration aff ecting nearly one in Your mileage may vary. other eyepieces. three adults, rendering stars as lit- Dioptrx lets you see pinpoint tle dashes rather than sharp points stars in the eyepiece with- — is more problematic. Observers out wearing your eyeglasses. with astigmatism typically have to Simple enough. I tried it, and it wear their eyeglasses while look- works nicely. End of story — or so I of smaller companies were built on ing through a telescope. thought. Last spring I fi tted a Diop- similar foundations. But Tele Vue’s Dioptrx transfers correction trx to a 35-mm Panoptic eyepiece milestones, either by chance or by for the eye’s astigmatism from and turned the TV-NP127is toward design, have always brought wide, eyeglasses to a lens atop the tele- the Double Cluster in Perseus. breathtaking views of the starry scope’s eyepiece. Serious observ- There’s a reason why every astron- sky to more and more observers. ers will fi nd that the pros of this omy author through the ages has My experience with the Double approach far outweigh any cons. lauded this pairing of open star Cluster made me think of Glenn A big advantage is that Dioptrx is clusters as one of the fi nest targets E. Shaw’s article on rich-fi eld a high-quality, multicoated glass for amateur telescopes. To see the telescopes (S&T: March 1980, lens made to fi t more than two Double Cluster at a magnifi cation page 192). Like others before dozen Tele Vue eyepieces (there’s of 19× swimming in the Panoptic’s him, Shaw pondered what type of a list at www.televue.com). One of 3.3° fi eld with perfect, tack-sharp telescope would show the most the few negatives is that a Dioptrx- stars was proof I didn’t really need. stars in a single view. But he also equipped eyepiece isn’t convenient But as I was scanning the fi eld rationalized that lots of easily seen for a group of people taking turns for the orange tint of red-giant stars would trump a view of stars with a telescope, though the cor- stars, something singular hap- teetering at the limit of visibility. rector can be removed in seconds pened. In the tranquil moment of He concluded that a 5-inch f/5 (which also makes it easy to trans- that chilly evening, the telescope refractor with a 33-mm eyepiece fer among eyepieces). just seemed to melt away. It was was the perfect instrument. “The Not every observer with astig- as if there were nothing between magnifi cation is 19×, and the exit matism needs correction at the me and the stars, not even my pupil 6.6 mm across,” he wrote. telescope, but determining who eyeglasses. Were it not for a lone “This little telescope is potent — it does and who doesn’t isn’t an spring peeper calling out for a provides truly breathtaking views exact science. Those with sig- mate, I could imagine myself fl oat- of some two hundred billion suns in nifi cant astigmatism generally ing in space looking at an infi nite our galaxy.” need correction. It’s also more of starry sky. It was a moment of ob- By dumb luck, my view of the an issue with low-power optical serving Nirvana. Double Cluster was with Shaw’s systems having exit pupils that ap- It was also a moment that made ideal prescription. But I also had proach the 7-millimeter maximum me understand what Tele Vue the luxury of uncompromising op- of the typical eye. (You calculate founder Al Nagler means by optics tics (something that Shaw didn’t the size of the exit pupil — the light that “do no harm.” S&T columnist enjoy a quarter century ago with bundle exiting an eyepiece — by David Levy profi led Nagler in the his achromatic refractor and Erfl e dividing the eyepiece’s focal length June 1999 issue (page 99). The eyepiece). I’d say it doesn’t get by the telescope’s focal ratio, or story of boy telescope maker grow- any better than that, but knowing f/number.) ing up to establish a successful Nagler and Tele Vue, I wouldn’t bet Your eyeglass prescription tells company isn’t unique. Celestron, we’ve heard the last word on this what, if any, astigmatism you have. Meade, Astro-Physics, and a host subject.