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Calculating distance, FOV, etc through the eye piece

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Is there a calculation for these figures? I see people taking about 'arc minutes' and 'minutes' and expressing FOV in degrees (of course). How is all of this determined?


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I'm not quite sure about what you would like to know. A degree divides into 60 minutes of arc (arcminutes) and an arcminute into 60 seconds of arc (arcseconds.)

The field of view afforded by a particular focal length of eyepiece installed in a particular focal length of telescope is a little more complicated. There is the eyepiece's apparent field of view, which is very large, and the true field it will deliver in a given focal length of telescope. Here is a link to a calculator which should help you find your true field in any specific setup;

Eyepiece Calculator


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to calculate actual field of view through the lens you simply divide the field of view of a lens (say 50degrees for a generic Plossl) by the magnification the lens will give.

E.g. focal length is 1000mm for my telescope, the 32mm TV Plossl will give 31.25x magnification. (focal length divide lens size)

The actual field of view will be 50degrees divide 31.25mag = 1.6 degree of view


field of view




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Thanks Alex , thats a good question....and Thanks for the links guys :o..this is something I have been wondering about aswell ,now both bookmarked in my astro folder (it's a big folder now!)and I can now include stuff like this in my observations etc.

regards craig

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Alex, this might help you out, it certainly helped get to grips with it.

Great article! Very cool. Thanks!

Now if I could just get a clear night. :o It's been cloudy for a full week now.

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