SPECIAL REPORT NIGHT MYOPIA SPECTACLES ... - …

1 30 September 2005 Sky & Telescopemount pinos, california, circa half decadeinto a newfound hobby, I found myself part of the monthlynew-Moon gathering upon the 8,300-foot-high mecca ofLos Angeles area astronomy. Entranced with the magic ofstar-hopping, I had demoted the naked-eye sky to a scatter-ing of celestial steppingstones mere highway signs tellingme 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 findingdistant galaxies and ghostly nebulae with my recently ac-quired reflector. The stars had never really looked thatsharp to my naked (that is, telescope-free) eyes anyway,even when my glasses were on.

2 And unlike my telescope,my glasses didn t come with focusing one NIGHT , on impulse, I borrowed a fellow stargaz-er s much stronger eyeglasses I was modestly nearsight-ed, while my companion was blind as a bat without opticalaid and slipped them on. I felt as if I d stuck my headinto an upside-down fishbowl. But despite the distortion Isaw more stars, and they seemed a bit brighter and sharperthan proverbial light bulb went on over my head: maybemy regular glasses didn t quite cut it for stargazing. Butmore than a decade would pass before I would gain anyreal insight into my modest discovery or fully exploit it byordering eyeglasses that fully compensate for my , though, such eyeglasses have become my most im-portant piece of optical equipment.

3 Using them when myeyes are fully dark-adapted, I can detect stars about a halfmagnitude fainter than the dimmest ones that my regularglasses bring into view. My stargazing SPECTACLES havesharpened the richly textured edges of the Milky Way sdark lanes and pools; they have enabled me to spy theplanet Uranus where Sky & Telescope s charts place itGo the final mile to correct your eyes worstastronomical shortcomings, and you may see a whole new sky at SkiesSPECTACLESforALAN DYERBy Joshua RothSPECIAL REPORTNIGHT MYOPIAUNDEREXPLORED VISTAL ight pollution is one reason that some star-gazers are relatively unfamiliar with the Milky Way s richly textured starclouds and dust lanes, but another reason may be that they ve never beenable to bring the naked-eye sky into crisp the stars.

4 And they have brought a number of deep-sky objects into naked-eye view for the first I routinely resolve Perseus s Double Cluster intotwo distinct knots of starlight, and sometimes I can see theCoathanger s hook in Vulpecula s faux cluster, Collinder399. Then there are the challenging double stars I now cansplit in Taurus s Hyades. Not bad for an investment thatran about $300 less than some premium eyepieces andwill last for several years!One Phenomenon, Many TheoriesWhy wouldn t the eyeglasses I d been getting since fifthgrade (with an exam and an updated prescription every fewyears) work particularly well for stargazing? Simple, I naivelyfigured: the wall chart with the increasingly little letterswasn t at infinity, so each part of the chart sent slightly diverging rays through my eyes to the corresponding partof my retinas.

5 In contrast, the light rays from each star onthe dome of the sky are essentially parallel because theyare vastly more it turns out this wasn t really the reason. Rather, aphenomenon called NIGHT myopiawas the culprit. The termfirst came to my attention thanks to Barry Santini, a NewYork based optician who has optimized eyeglasses for adozen-plus amateur astronomers. NIGHT MYOPIA , Santiniexplains, is the tendency of people to become more near-sighted in darkness than they are in daylight. It was thiseffect and not the wall chart s relative proximity thatmade the stars come into better focus when I used eye-glasses that were stronger than my nominal daytime the optometrists, opticians, and vision scientists I interviewed for this story had heard of NIGHT MYOPIA .

6 Butthey don t entirely agree on what causes it!Like Santini, Colorado optometrist and amateur astrono-mer Keith Bowen attributes NIGHT MYOPIA mostly to instru-ment MYOPIA ,a hardwired psychological and physiologicalreflex that causes us to focus on nearby things in the absence of strong visual stimuli. Because most people losethe ability to focus nearby as they age, NIGHT MYOPIA shoulddiminish with advancing age as well, Bowen adds and indeed, one 55-year-old amateur astronomer, Joel M. Mos-kowitz of New York, has watched his own NIGHT MYOPIA diminish significantly over the past decade. A day-to-nightoffset of 1/2or 3/4diopter*is typical, says Bowen, who pre-scribes NIGHT -driving glasses for some University optometry professor Arthur Bradleyblames the eye s chromatic aberrationas well as the unwantedaccommodation that Bowen describes.

7 The human eyebrings light of different colors to focus at different dis-tances from the cornea, Bradley explains. Moreover, therods on the periphery of the retina (which are principallyresponsible for NIGHT vision) are sensitive to light that isslightly bluer than the light most easily seen by the conesat the retina s center (which are optimized for daylight). Al-though the wavelength difference between the peak sensi-tivities of retinal rod and cone cells is only a modest 50nanometers (roughly one-tenth the wavelength of yellowlight), it can produce a half diopter s worth of NIGHT myopiawhen combined with the eye s substantial chromatic aber-ration, says Donald Miller, also at Indiana third camp includes two scientists who study the eyeas an optical system: Jim Schwiegerling, a University of Ari-zona professor of ophthalmology and optical sciences, andDavid R.

8 Williams, director of the University of Rochester sCenter for Visual Science. Schwiegerling and Williams bothSky & Telescope September 200531 Smart Shopping for Stargazing GlassesWhile the primary advantage of stargazing eyeglasses is theirability to compensate for NIGHT MYOPIA , other factors can beoptimized as well. Note that these should be considered evenif you re not modifying your nominal eyeglass York based optician Barry Santinirecommends normal plastic (index of refraction ), medium-index plastic ( ), or crown ( ) or hi-crown ( ) glass though he particularly favors spherical hi-crown glass lens-es, which are particularly scratch-resistant. Refrain from theuse of polycarbonate lenses, he warns, which, althoughlightweight and very impact-resistant, are soft (scratch easily)and exhibit the worst color aberration of any ophthalmic lensmaterial available.

9 For astronomical use can benefitfrom antireflection coatings, whether or not NIGHT MYOPIA isan issue. But don t use any yellowish NIGHT -driving tints on stargazing glasses, warns optometrist Keith Bowen: theyreduce light transmission. And bear in mind that antireflec-tion coatings do require frequent cleaning, since oily de-posits from one s skin render them inef-fective and can permanently damagethem if not removed for a long time.(Some experienced stargazers do with-out coatings for just this reason.) Bowenrecommends first rinsing coated eyeglasses withwater (to remove dust) and then using mild dish soapor a specialty spray cleaner and a soft cloth to AND FRAME of Santini sclients have especially enjoyed the sphericaleyeglass lens shape that comes from ContourOptics (see the upper photo at right).

10 These wraparound lenses give a nearly all-sky view,without the peripheral cutoff from conventional eyeglasses. As far as mechanical factors go, I especially like the double-sprung hinges on my eyeglasses(lower photo); these allow me to repeatedly remove the glasses without worrying about wearing out S&T: CRAIG MICHAEL UTTER (2)*Highlighted termsare defined in the glossary on page the lion s share of the credit (or blame) for NIGHT my-opia to the eye s substantial spherical aberration,which be-comes noticeable only when pupils are dilated. Light strik-ing the periphery of a dilated pupil comes to a focusfarther ahead of the retina than does light striking thepupil s center, they explain.