RE: Binoculars

Matson, Robert (ROBERT.D.MATSON@cpmx.saic.com)
Mon, 21 Dec 1998 13:32:59 -0800

On the subject of binoculars, I agree with Lee:  9 x 63 is a bit much for
extended
satellite observing sessions.  8 x 56's are perfect.

> Celestron has a nice 8x56 pair at around $250 (mail order).

The very binoculars I own!  They have been fabulous for satellite tracking.
They're
not too heavy, they have a wide apparent field of view, 7mm exit pupils,
good
eye-relief, fully-coated optics, a nice flat field, and only minor
vignetting (loss
of light-gathering power/contrast at the periphery).  You might want to look
through a few pair to compare star images at best focus -- I noticed some
have better alignment than others, and astigmatism varied a bit from pair to
pair.

Light-gathering power is good -- I can routinely reach magnitude +8 even
in my somewhat light-polluted Long Beach skies (where the unaided limit is
about +5).  I definitely prefer 8 x 56 to 7 x 50, primarily for the improved
ability to acquire and track very dim satellites.

Terry Lisansky wrote:

"Usually 7 power is the most you can use and still hold your
binoculars steady."

As I mentioned, 8x still works for me, but I'm sure some
find they have difficulty keeping an 8x image steady.  So
try 7 vs. 8.  You may find 8x is fine.  Of course, you can
always buy the fancy image-stabilized binoculars to get
around this problem, but then you're no longer talking
$300!

Terry continues:

"Also this is related to exit pupil.  This is the diameter
of the light beam as it exits the eyepiece.  Divide the
objective diameter by the magnification and you'll get it.
6 millimeters is about the size of your dark adapted eye
pupil."

I would add that maximum pupil size decreases with age.
Children can have pupils as large as 8mm.  I'm 36 and
mine measure 7mm, so my 8 x 56's are a good match.  (On
a side note, I wonder if belladonna (atropine) increases
adult dilated pupil size back to childhood levels?  All
things being equal, 8mm pupils vs. 7mm buys you 0.3
visual magnitudes.  ;-)  Of course, I'm not advocating
the use of topical alkaloids to improve scotopic vision!
Just curious.)

Best wishes and clear skies,

Rob