[pasmembers] Re: need suggestion on purchasing binoculars
- From: LPhxAZ <lphxaz@xxxxxxxxx>
- To: <pasmembers@xxxxxxxxxxxxx>
- Date: Wed, 14 Nov 2012 14:11:18 -0700
that is a good point about the exit pupil, Bob.
"When the rings are well-positioned (tilted to our view), the resulting image
looks more like an egg."
that was Galileo's experience too. it was only when Christian Huygens viewed
Saturn 40 years later with a larger telescope that the rings were seen as a
separate object.
----- Original Message -----
From: Bob Christ
To: pasmembers@xxxxxxxxxxxxx
Sent: Wednesday, November 14, 2012 2:02 PM
Subject: [pasmembers] Re: need suggestion on purchasing binoculars
Great pointers Leah. I would also add that Mr. Grove be advised that exit
pupil size should he considered when purchasing binoculars. A lot of folks
think 7x50 binoculars are great for astronomy purposes because the lower power
(7x) is easier to hold than those that deliver greater magnification and the
50mm aperture is sufficiently large. The holding-still premise is correct, but
.....
The exit pupil, the size of the focused image exiting the eyepiece through a
binocular or telescope, is measured in millimeters. The simple formula to
determine exit pupil size is aperture ÷ magnification. In the 7x50 example (7x
magnification ... 50mm aperture), the exit pupil is greater than 7mm. A
child's eye could potentially dilate to an upper range of 7mm when dark
adapted, but not an adult's; typically falling in the 4-5mm range; or even
less. Therefore, the 7x50's provide focused light that the human eye cannot
use - a portion of the delivered image is simply lost and the anticipation of
the view can be a disappointment.
In my 15x70 binocs, mounted on an exceptionally sturdy tripod, Saturn is a
small, bright object and the rings are simply short and thin faint lines
extending out each side when viewed edge-on. When the rings are
well-positioned (tilted to our view), the resulting image looks more like an
egg.
We usually display through a pair binoculars at star parties, and they are a
great night sky viewing tool when aimed at appropriate objects.
Bob
On Nov 14, 2012, at 1:08 PM, LPhxAZ wrote:
I don't think you can see Saturn's rings with binoculars, unless they are
as large as a telescope. (70-100 mm)
http://stargazerslounge.com/topic/40112-saturn-with-binoculars-recommendation/
http://www.wwnorton.com/college/astronomy/astro21/sandt/binoculars.html
it always "gets" me when people brag about faint objects they have seen
with "just binoculars", and then it turns out that their "just binoculars" have
a 3" or 4" lens on each side.
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