Re: ATM Lowest allowable magnification

["B. Pepin" <atmjedit@conceptsnet.com>]

Well there is really *no such thing* as the lowest allowable magnification,
unless you are concerned about the size of the exit pupil being enlarged
beyond the point where all the light from the optic enters your eye pupil.
As Al Nagler has pointed out, you really can't find any astronomical
telescope design that will give you full aperture illumination when you
reduce the magnification below about 12X, anyway -- so if you want really
low power, you simply sacrifice some effective aperture to the gods of
wide-field.

There is no optical law says you can't do this. I find that 8x or so (around
what a moderate-power binocular gives) is the lower useful limit for
magnification for a 3 or 4 inch telescope of most types. Below this the sky
becomes a little too gray for my taste, and the lack of contrast keeps my
eye from distinguishing objects well.

BP


----- Original Message -----
From: John Lynch <john_p_lynch@hotmail.com>
To: <atm@shore.net>
Sent: Tuesday, March 13, 2001 2:32 AM
Subject: ATM Lowest allowable magnification


>
> To the ATM listserv:
>
> I'm sure this topic has been discussed before, but I'd like to
> ask just in case it hasn't.  I'd like to know what is the lowest
> allowable magnification, i.e., longest focal length eyepiece,
> that can be used before you run into problems.  I'm sure it has
> something to do with the exit pupil and vignetting.  I'm interested
> in the solution, in general, and specifically in the following
> two cases:
>
> 1.  6" f/6 with standard 1 1/4" I.D. rack and pinion focuser
>     and 1.52" secondary mirror
>
>     I'm planning on using 8mm to 25mm eyepieces.
>     How about 32mm?
>
> 2.  8" f/6 with 2" I.D. Crayford focuser and 1.83" secondary mirror
>
>     I think 17mm to 22mm eyepieces would be O.K.
>     How about 32mm and 48mm?
>
> Thanks.
>
> John Lynch
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