From: chaloux@mandolin.mitre.org (Dave Chaloux)
Subject: Re: binocular question
Reply-To: chaloux@mandolin.mitre.org (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Mon, 28 Dec 1992 21:33:57 GMT

Dana A. Bunner writes:

>I'm not familar with these 16x50's however one other drawback, stability
>is indeed a concern ... even 10X's exhibit a fair amount of jitter,
>is the small exit pupil diameter.  A 16X50 will have an exit image size
>of 3.125mm.  Most people in their 20's and 30's have a maximum eye pupil
>size of 7mm.  Since surface area is a ratio of squares, the size of the
>image will only illuminate about 20% of your pupil, wasting 80% of your
>night sight capability.  16x50's are designed as daytime glasses, when your
>eye contract to about 3 mm.  

>Usually a minimum exit image is 5mm for night use.  You'll need a 5mm 
>image to achieve even 50% image coverage on your pupils.  However if you
>are over 35-40 years old, that maximum pupil size usually begins dropping
>to the 5.5-6.0 mm range.  The standard night glass has a 7mm image.  Thus
>the popular sizes are 7x50, 8x56, 10x70, and 11x80.  You'll find the 
>images in these sizes to appear much brighter.  Note that Celestron does
>not advertise the primary use of their 12x50 Pros as night glasses.  The 
>smallest 16X models I've seen advertised for night use are the Fujicon 
>16x70's and many consider them too dim for night use.

>This is not to say they can't be quite useful at night.  I believe one
>sci.astro poster routinely uses 20x60's.  However there is a marked
>decrease in image brightness once one begins to drop below about 4.5 mm
>exit images.

I can't pass this one up. It is true that the surface brightness on these
binoculars will be quite a bit less than the maximum. However, my own
conviction is that maximizing exit pupil is overrated even on extended
objects. For example I use my 20" at 195x frequently. In fact the other
night when I was testing my NGC MAX setting circles I basically used 195x
the entire night. This is an exit pupil of just 2.6mm. This is even less than
the 16x50s. When looking for faint small galaxies, I will sometimes run it
up to 283x or even 360x. Maximizing the exit pupil is important on low
surface brightness large objects. However, there are more objects of fairly
high surface brightness but small angular diameter out there to try to see.

The greater magnification will also help out on double stars.

I personally would have gone with the 10x50s but for a different set of reasons.
1) Jitter already discussed.
2) The reason I use binoculars is for large extended objects like the Plieades,
the Double cluster, the beehive, bright comets, etc. I want the bigger field of
view.
3) Good collimation is harder to achieve with the greater magnification.
4) Finding objects will be easier with the bigger field of view.

This having been said, high quality 16x50s will actually show more Deep Sky
objects than 10x50s which will show more than 7x50s. Further, they will show
dimmer stars in clusters.

As Dana points out, the quality of the coatings, etc. is critical. Dana is also
fair enough to point out that binoculars with small exit pupils can be useful.
I just wanted to take it further and say that exit pupils can turn into a
sacred cow that should be slaughtered from time to time. People might see far
more instead of less as far as Deep Sky goes if they used more power. Even on
"low surface brightness" objects more power sometimes helps. An example of this
is M33 which in large scopes which can show a huge amount of detail at higher
powers.
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