On Thursday, June 7, 2001, at 05:43 PM, William R Wood wrote:

> Can anyone explain how to calculate how many arc minutes are visible in
> the
> FOV of any particular EP and how you would determine how far from
> center, in
> arc minutes, a given star was in your EP. For example I have a 12mm
> Nagler
> that has an advertised 82 degree apparent FOV but I dont know what that
> means in terms of how many arc minutes a star on the extreme right side
> of
> the FOV is from the center of the FOV.

The actual field of view on the sky is the apparent FOV (as quoted by
the manufacturer, 82 degrees in the case of Naglers) divided by the
magnification. Magnification, of course, is the scope's focal length
divided by the eyepiece's focal length. So if you have an 8" LX200,
focal length 2000mm and a 12mm Nagler that gives 167x and an actual FOV
of 30 arc minutes. So it's 15 minutes from the center to the edge.

These numbers have to be taken somewhat approximately. The quoted specs
are sometimes off by a few percent. If you want more accurate numbers
then time how long it takes a star to drift thru the field with the
drive off. Choose a star with Declination close to for simplicity. The
actual FOV of the scope/eyepiece combination in arcminutes is 15 times
the number of minutes it takes to drift across the field.

--
Bill Arnett
http://nineplanets.org/ 37 27 38 N 122 16 11 W

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