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    Re: Correcting Night Vision
    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...
    
    
    

       
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