NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Correcting Night Vision
From: Frank Reed CT
Date: 2005 Aug 11, 15:13 EDT
From: Frank Reed CT
Date: 2005 Aug 11, 15:13 EDT
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 a
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. The article
notes that this phenomenon was first described in the scientific literature back
in 1789 by none other than Nevil Maskelyne, the Royal Astronomer who can be
counted as the grandfather of the Nautical Almanac.
This same article goes on to discuss the benefits and possible downside of
the latest techniques in laser surgery with respect to amateur astronomers and
others who value their eyes as precision optical systems. 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. It's
interesting that the latest laser surgery technique captures and corrects for
these higher order problems so that star images should come out nearly circular
and sharp with effective resolution as good as 20/10 in some cases (20/20 is 1.0
arcminute resolution by definition, 10/10 is 0.5 arcminute resolution --which is
right at the theoretical limit for ordinary resolution).
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars