SPECTACLES for Spectacular Skies

SPECIAL REPORT NIGHT MYOPIA SPECTACLES for Spectacular Skies By Joshua Roth Go the ﬁnal mile to correct your eyes’ worst astronomical shortcomings, and you may see a whole new sky at night. mount pinos, california, circa 1990. A half decade into a newfound hobby, I found myself part of the monthly new-Moon gathering upon the 8,300-foot-high mecca of Los Angeles–area astronomy. Entranced with the magic of star-hopping, I had demoted the naked-eye sky to a scatter- ing of celestial steppingstones — mere highway signs telling me where to point my Coulter 8-inch (20-centimeter) Dob- sonian and begin doing real astronomy. It wasn’t just that I had fallen under the spell of finding distant galaxies and ghostly nebulae with my recently ac- quired reflector. The stars had never really looked that sharp to my “naked” (that is, telescope-free) eyes anyway, even when my glasses were on. And unlike my telescope, my glasses didn’t come with focusing knobs. But one night, on impulse, I borrowed a fellow stargazer’s much stronger eyeglasses — I was modestly nearsight- ed, while my companion was blind as a bat without optical aid — and slipped them on. I felt as if I’d stuck my head into an upside-down fishbowl. But despite the distortion I saw more stars, and they seemed a bit brighter and sharper than usual. The proverbial light bulb went on over my head: maybe my regular glasses didn’t quite cut it for stargazing. But more than a decade would pass before I would gain any real insight into my modest discovery or fully exploit it by ordering eyeglasses that fully compensate for my nocturnal nearsightedness. Now, though, such eyeglasses have become my most im- portant piece of optical equipment. Using them when my eyes are fully dark-adapted, I can detect stars about a half magnitude fainter than the dimmest ones that my regular glasses bring into view. My stargazing spectacles have sharpened the richly textured edges of the Milky Way’s dark lanes and pools; they have enabled me to spy the planet Uranus where Sky & Telescope’s charts place it UNDEREXPLORED VISTA Light pollution is one reason that some stargazers are relatively unfamiliar with the Milky Way’s richly textured star clouds and dust lanes, but another reason may be that they’ve never been ALAN DYER able to bring the “naked-eye” sky into crisp focus. 30 September 2005 Sky & Telescope among the stars; and they have brought a number of deep- Like Santini, Colorado optometrist and amateur astrono- sky objects into naked-eye view for the first time. mer Keith Bowen attributes night myopia mostly to instru- Now I routinely resolve Perseus’s Double Cluster into ment myopia, a hardwired psychological and physiological two distinct knots of starlight, and sometimes I can see the reflex that causes us to focus on nearby things in the Coathanger’s hook in Vulpecula’s faux cluster, Collinder absence of strong visual stimuli. Because most people lose 399. Then there are the challenging double stars I now can the ability to focus nearby as they age, night myopia should split in Taurus’s Hyades. Not bad for an investment that diminish with advancing age as well, Bowen adds — and ran about $300 — less than some premium eyepieces — and indeed, one 55-year-old amateur astronomer, Joel M. Mos- will last for several years! kowitz of New York, has watched his own night myopia diminish significantly over the past decade. A day-to-night 1 3 One Phenomenon, Many Theories offset of /2 or /4 diopter* is typical, says Bowen, who pre- Why wouldn’t the eyeglasses I’d been getting since fifth scribes night-driving glasses for some patients. grade (with an exam and an updated prescription every few Indiana University optometry professor Arthur Bradley years) work particularly well for stargazing? Simple, I naively blames the eye’s chromatic aberration as well as the “unwanted figured: the wall chart with the increasingly little letters accommodation” that Bowen describes. The human eye wasn’t at infinity, so each part of the chart sent slightly brings light of different colors to focus at different dis- diverging rays through my eyes to the corresponding part tances from the cornea, Bradley explains. Moreover, the of my retinas. In contrast, the light rays from each star on rods on the periphery of the retina (which are principally the dome of the sky are essentially parallel because they responsible for night vision) are sensitive to light that is are vastly more distant. slightly bluer than the light most easily seen by the cones But it turns out this wasn’t really the reason. Rather, a at the retina’s center (which are optimized for daylight). Al- phenomenon called night myopia was the culprit. The term though the wavelength difference between the peak sensi- first came to my attention thanks to Barry Santini, a New tivities of retinal rod and cone cells is only a modest 50 Yo rk–based optician who has optimized eyeglasses for a nanometers (roughly one-tenth the wavelength of yellow dozen-plus amateur astronomers. Night myopia, Santini light), it can produce a half diopter’s worth of night myopia explains, is the tendency of people to become more near- when combined with the eye’s substantial chromatic aber- sighted in darkness than they are in daylight. It was this ration, says Donald Miller, also at Indiana University. effect — and not the wall chart’s relative proximity — that A third camp includes two scientists who study the eye made the stars come into better focus when I used eye- as an optical system: Jim Schwiegerling, a University of Ari- glasses that were stronger than my nominal daytime pre- zona professor of ophthalmology and optical sciences, and scription. David R. Williams, director of the University of Rochester’s All the optometrists, opticians, and vision scientists I Center for Visual Science. Schwiegerling and Williams both interviewed for this story had heard of night myopia. But they don’t entirely agree on what causes it! *Highlighted terms are defined in the glossary on page 35. Smart Shopping for Stargazing Glasses While the primary advantage of stargazing eyeglasses is their posits from one’s skin render them inef- ability to compensate for night myopia, other factors can be fective and can permanently damage optimized as well. Note that these should be considered even them if not removed for a long time. if you’re not modifying your nominal eyeglass prescription. (Some experienced stargazers do with- LENS MATERIALS. New York–based optician Barry Santini out coatings for just this reason.) Bowen recommends normal plastic (index of refraction 1.50), medium- recommends first rinsing coated eyeglasses with index plastic (1.55), or crown (1.53) or hi-crown (1.60) glass water (to remove dust) and then using mild dish soap — though he particularly favors spherical hi-crown glass lens- or a specialty spray cleaner and a soft cloth to remove UTTER (2) MICHAEL : CRAIG es, which are particularly scratch-resistant. “Refrain from the smudges. S&T use of polycarbonate lenses,” he warns, “which, although LENS AND FRAME DESIGNS. Some of Santini’s lightweight and very impact-resistant, are soft (scratch easily) clients have especially enjoyed the spherical and exhibit the worst color aberration of any ophthalmic lens eyeglass lens shape that comes from Contour material available.” Optics (see the upper photo at right). These COATINGS. Eyeglasses for astronomical use can benefit “wraparound” lenses give a nearly all-sky view, from antireflection coatings, whether or not night myopia is without the peripheral cutoff from conventional an issue. But don’t use any yellowish “night-driving” tints eyeglasses. As far as mechanical factors go, I on stargazing glasses, warns optometrist Keith Bowen: they especially like the double-sprung hinges on my eyeglasses reduce light transmission. And bear in mind that antireflec- (lower photo); these allow me to repeatedly tion coatings do require frequent cleaning, since oily de- remove the glasses without worrying about wearing out Sky & Telescope September 2005 31 SPECIAL REPORT NIGHT MYOPIA give the lion’s share of the credit (or blame) for night myopia to the eye’s substantial spherical aberration, which be- Daytime comes noticeable only when pupils are dilated. Light strik- Retina sees ing the periphery of a dilated pupil comes to a focus compact star image farther ahead of the retina than does light striking the pupil’s center, they explain. And while “you’re not actually fixing the spherical aberration of the eye” by augmenting your myopia correction, Schwiegerling stresses, you are pushing the “minimum-blur circle” — the most concentrat- ed bundle of starlight that your dilated eye can provide — onto the retina, enhancing night vision. Schwiegerling estimates that an amateur astronomer whose dark-adapted pupils span 6 millimeters typically will experience a half diopter of night myopia, while the rare stargazer whose pupil can open up as wide as 8 mm may Nighttime 1/ Retina sees experience a 1 2-diopter effect — though both numbers blurry star image vary widely within the human population. Since one’s pupil size, whether light- or dark-adapted, gradually dimin- ishes with age, this also could explain anecdotal evidence for night myopia’s being less severe in older astronomers. Image plane Measuring Night Myopia Scientific uncertainty about its cause notwithstanding, night myopia is “a documented effect,” says Bowen — in fact, it has been documented at least since 1789, when NOCTURNAL NEARSIGHTEDNESS Nevil Maskelyne described it to the Royal Society of Lon- Night myopia refers to the tendency — shown by many but by don.

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