Rx for the astigmatic skywatcher.
People who wear glasses like to take them off when using a telescope. This way they can get their eye closer to the eyepiece, for a wider field of view and better shielding from stray light. If the glasses are for simple near- or farsightedness the technique works well. After minor refocusing the telescope can do two jobs at once: play its normal role as a light-gathering magnifier and correct the person's vision for infinity.
Unfortunately, about half of all people who wear glasses need them to correct astigmatism too. When I remove my own glasses there is no way to get a perfectly clear low-power view through a telescope by refocusing. Star images assume various elliptical and distorted shapes; they never become sharp pinpoints of light. (The very term astigmatic, derived from Greek, means "not a point.") The defect is caused by a cornea or eye lens that has an ovoid instead of a truly spherical shape.
Astigmatics have several options. We can wear glasses at the telescope and put up with tunnel vision, thereby missing out on wide-field views of the universe. Or we may switch to contact lenses if our astigmatism is mild enough to be corrected this way. Finally, we can apply the needed correction at the eyepiece itself.
That's the solution offered by Custom Ophthalmics of Fredericksburg, Virginia, which recently announced its Starpoint line of correcting lenses for astigmatic observers (S&T: July 1995, page 43). The firm's owner, Myron Wasiuta, is a practicing optometrist and avid amateur astronomer whose electronic images have appeared recently in CCD Astronomy.
The ordering procedure is simple. Mail or fax your current eyeglass prescription to Custom Ophthalmics with a note stating the brand and focal length of the eyepiece the corrector will be used with. Be sure to specify which is your "observing eye."
The corrector is inserted very close to the eyepiece's eye lens, where it won't interfere with getting your eye where it belongs (see the facing diagram). To be sure of a proper fit, Custom Ophthalmics asks you to send a cardboard template that matches the eyepiece top or fits snugly within the eyecup. The firm edge-grinds the corrector down to size, mounts it on a polycarbonate carrying disk, and scribes the disk with fiduciary marks so it can be rotated to the angle appropriate for the customer's astigmatism.
BUT WHO NEEDS IT?
A look at your current prescription should tell whether this product can help you achieve better telescopic views. The correction for near- or farsightedness is called "sphere," and that for astigmatism "cylinder," both expressed in diopters. The sphere value is irrelevant since it can be focused out with the eyepiece. But if the cylinder is 0.75 diopter or more (with either a + or - sign in front) a Starpoint corrector will probably improve sharpness, especially with a wide-field, low-power eyepiece.
My own observing eye has a cylinder error of -2.50 diopters, and I've found glasses (or a corrector lens) mandatory for all critical observing at low and medium powers - in fact, anytime the telescope's exit pupil is 1 millimeter or larger. Above 30x per inch of aperture, however, I can see planetary markings and double stars just as clearly without my glasses as with them. The tiny exit pupil at such powers "stops down" the eye and improves clarity, in much the same way that a photographer closes down a camera's iris to compensate for poor lens quality.
Before deciding on the purchase of a corrector, it is a good idea to rule out the telescope itself as a source of bad images. Poorly figured, pinched, or misaligned optics can produce effects that mimic observer astigmatism. To determine the source of the problem, rotate your head while looking through the eyepiece without glasses. An oval or irregular star image will rotate with your eye if your vision is at fault; otherwise you can blame the telescope itself. (A Starpoint corrector could also cure instrumental astigmatism, but deciding on the proper prescription would require special optical tests.)
To evaluate this product I selected two optical systems likely to give fits to anyone with glasses. One is the 20-mm Nagler Type 2 eyepiece from Tele Vue, whose very short eye relief allows it to present an 82 [degrees] apparent field of view. The other is a pair of Unitron 11x80 binoculars. In either case I cannot see the full field with glasses on, even after folding the rubber eyecups down.
I faxed in my order on February 17th. On March 22nd Wasiuta called to say that he had the finished, edge-ground lenses and was about to send them out for antireflection coatings. The package of correctors arrived April 19th, and Sky & Telescope sent them out to be tested by optometrist Lawrence Phillips of For Eyes Optical Co., Cambridge, Massachusetts. Using the neutralization method on a lensometer, Phillips found that the spherical correction was nil (as expected) and that the astigmatic component matched my prescription exactly. He added that the lenses appeared "very well made. I've seldom seen such crisp optics and fine surface finish."
Mounting the Starpoint lens on the Nagler took me about one minute. By tugging gently on the rubber eyecup, I was able to work the Starpoint's 42-mm carrying disk into position under the eyecup's internal lip. The fit was perfect, leaving the outer surface of the Starpoint lens just 5 mm above that of the Nagler. I noted, however, that popping the corrector in for yourself and then out again for your guests at a star party would not be practical.
Pressing my face against the rubber I got a view no glasses-wearer can ever behold - easily seeing by peripheral vision the entire rim of this extraordinary ocular's field stop while looking straight ahead. My eyelashes brushed the Starpoint at times, but this was not annoying. Besides, the corrector is much easier to clean, when necessary, than any eyepiece.
Next I inserted this 2.4-pound ocular in a 5-inch rich-field refractor to try it on the stars. The first glance was something of a shock; the star images were horrible, with perhaps twice as much astigmatism as my usual view sans glasses! Then I remembered to rotate the eyepiece until the red dot and line on the corrector's mount were vertical. The stars crisped up into perfect pinpoints. It wasn't necessary to view the marks in the dark - rotating the eyepiece to the proper orientation came as naturally as focusing itself.
Before adding the pair of correctors to the 11x80s, I previewed several familiar objects with glasses off while bracing the binoculars against a picnic table. I easily separated Mizar and Alcor and also saw many light and dark markings on the gibbous Moon. Only Copernicus was definitely recognizable as a crater, and Plato was just a dark spot. But after installing the correctors, the walled plain Clavius and two of its internal craters came into view. I could even resolve Mizar's 14-arcsecond companion!
The instructions provided for installing the correctors on binoculars are straightforward and easy. But again it was clear that the installation is semi-permanent - you would not want to remove the correctors to share a view with a friend. The correction was maintained for near and far objects because the Unitron binoculars are of the popular center-focus type. Units with individually focusing eyepieces would be a hassle for terrestrial use, however, because the astigmatic angle would need resetting with each change of focus. There would be less problem if such binoculars were dedicated purely to astronomy, since infinity focus is basically a set-and-forget operation that requires only minor tweaking.
The Starpoint lenses did correct my astigmatism, but I'm also 2.75 diopters nearsighted. I found that the eyepieces had to be moved all the way in against the binocular body, with no ability to "pass through" infinity. If the focus travel on your telescope or binoculars is limited, or if you are extremely near-sighted, you should probably ask Custom Ophthalmics to correct for that error in addition to astigmatism.
Eyeglasses for astigmatism have been manufactured ever since 1824, when the English astronomer George Biddell Airy became the first person to order a cylindrical lens for his own left eye. But the frustrations felt nightly by astigmatic observers are still very much with us. The potential market for Wasiuta's device is vast - perhaps one quarter of all people who use a telescope or binoculars.
RELATED ARTICLE: Exploring the Options
Does a mildly astigmatic observer need a corrector for each and every eyepiece? Certainly not. Unless your astigmatism is worse than about 3 diopters only medium and low powers will require correction, and then only if the eyepiece itself lacks the eye relief you need to wear glasses. My Vixen 9-mm LV eyepiece, for example, already has adequate relief; so do others in the LV line.
Making your own correctors is not difficult either. In this magazine for November 1976, page 376, I explained in detail how anyone with a little abrasive, pitch, and rouge can grind and polish tiny cylindrical lenses. (I made seven of them in a single evening and have used them ever since.) While clearly the least expensive way to go, this solution does require the know-how and materials left over from at least one mirror-making project.
Yet another option is to visit a local eyeglass center and buy unmounted lenses polished with the correction you need. Such "blanks" cost upward of $40 apiece, and they still have to be cut down from their huge meniscus shape on a drill press with abrasive and a cookie-cutter bit. Considering the extra fuss this work would entail, the Starpoint lenses are reasonably priced.
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|Title Annotation:||includes related article; Custom Ophthalmics' Starpoint Lens|
|Author:||Sinnott, Roger W.|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 1995|
|Previous Article:||Understanding celestial coordinates.|
|Next Article:||The Observer's Guide to Astronomy, vol. 1.|