NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/astro/13553

Internet Message Format | 1992-12-31 | 3.2 KB

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From: [e-mail] (Duke P. Briscoe)
Newsgroups: sci.astro
Subject: Re: Binoculars, exit pupil size, etc...
Date: 31 Dec 1992 06:06:16 GMT
Organization: California Institute of Technology, Pasadena
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In regard to the discussion of pupil size, I would like to recommend
the pupil gauge kit available from Sky & Telescope. It consists of a
small plastic strip with various markings which enable one to measure
one's pupil size, and also a two page reprint of "A Pupil Primer for
Stargazers" from the May 1992 S&T.  The reprint is very informative
about the issues of exit pupil in regard to various observing
situations.  It is only $3.95, but there is a $2 shipping and handling
charge, but maybe if you order something in addition to the pupil
gauge, the handling charge won't be such a high percentage of your
total cost.  The phone number is [phone], or mail P.O. Box
9111, Belmont, MA  02178-9111.  I was surprised to find out that my
pupils are about 5mm in a partially sunlit room.  And right now, at
night in a moderately bright room lit by a typical incandecent
fixture, I measure my pupils as 6mm.  After dark adaptation, I have
measured my pupils as 7mm.  I had thought my pupils would be smaller.
I am 34, and according to the reprint's graph of pupil size in
darkness vs. age, my pupils are larger than those of about 75% of
people my age.

This pupil measurement has made me rethink my consideration of getting
10x50 binoculars to replace my Orion 10 x 70's.  I still might get
some 10x50's, hopefully with better coatings than the Orions, although
I have done no side-by-side comparisons which would lead me to believe
there is a problem with the Orions.  I am pretty happy with the
Orions, although they are somewhat large to hold for a long period of
observing.  The 10x is slightly shaky to hold, but not too bad for me.

Another issue with regard to exit pupil vs more magnification is that
the eye's ability to resolve detail falls off in dim light, so that
you may be able to see more detail in faint nebula by magnifying them
more highly (small exit pupil) rather than seeing a smaller but not
quite as dim image (large exit pupil).  I believe this is why high
magnifications are supposed to be useful for observing galaxies'
structure even though the image is fainter than a lower power.  Low
light vision is fairly complicated, with things like averted vision,
and sensitivity to movement (can be exploited by tapping the eyepiece
to make a faint object become visible).

Also, I would highly recommend people interested in binoculars to look
at Phil Harrington's book "Touring the Universe through Binoculars".
You might find it in a library; I was somewhat surprised and pleased
to find it in our public library. It has a pretty good appendix which
discusses waht to think about when buying binoculars.  It would also
be worthwhile to get the pupil gauge, so you would know your own pupil
size.

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