NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Correcting Night Vision
From: Bill B
Date: 2005 Aug 19, 16:28 -0500
From: Bill B
Date: 2005 Aug 19, 16:28 -0500
Frank Fascinating indeed. Thanks for the heads up. It answers many questions I have been pondering. When I obtained my Astra in January the scope was driving me nuts. I would leave my dimly lighted living room, focus the scope, but the focus kept drifting off over time. Considered it was the due to temperature change, so popped out, did a quick focus, and left the scope outdoors for an hour and took another quick look. Still in focus. Did the reverse and still in focus. Also noted that the the longer I looked after coming out the harder time I had as stars/planets would become increasing distorted. An IE check done right after I popped out was almost dead on, and after 15 minutes of looking (tripod mounted) I was lucky to get within plus/minus 4 minutes because of distortion. I attributed that to "eye strain." It would appear from the article that soon after the eye starts to dark adapt (pupil opening expands) distortion becomes progressively worse for all but perfectly shaped lenses. Which suggests the question, are most of us shooting stars better to *not* dark adapt our eyes for first magnitude stars? Just pop out, shoot quickly, and be done with it? Does the list find that using glasses with a sextant helps to alleviate star spikes, tails, and other distortions? I have not done enough controlled experiments to draw any conclusions. Glasses make sighting more clumsy for me, and if there are any nearby light sources the reflection of the lenses outweighs and possible benefits. A related question. Most people have a dominant hand, foot, ear and eye. Competitive target shooters I have spoken with claim they use their dominant eye exclusively for sighting. Are their any opinions from the list concerning using the dominant eye for sextant observations? I am right-eye dominant, and my left eye suffers to a greater extent from myopia and distortion than my right eye. Yet in tests of over 150 observations (without glasses) I get consistently better results with my left eye. Any thoughts? Bill > There's a fascinating article in the latest issue of Sky & Telescope > magazine on night vision. It starts out with a detailed account of "night > myopia" --that tendency for vision to become unfocused as our eyes adapt fully > to darkness. The article suggests that observers (of all ages) who want to see > the night sky at its best should consider getting eyeglasses specially made > for night use. A prescription 1.0 diopter stronger than normal is suggested as > rough guideline though there are details in the article on doing your own > testing. Note that this is not necessary when looking through a telescope (on > a sextant or otherwise) since refocusing can correct for night myopia.... Unless there is astigmatism. > > ...You'll have to read > this yourself to get the full story, but there are some nice descriptions of > the higher order aberrations in the eye that can lead to distortions in > images of stars. This seems relevant to celestial navigators since many > people describe star images as appearing "spikey" and "flared" even at the > best focus...