29-09-2008, 04:50 PM
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Registered User
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Join Date: Mar 2008
Location: Wollongong
Posts: 3,822
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There are a number of methods, but I don't have all the details at hand.
The most common is to divide the apparent field of view by the magnification (assuming you know the afov). Magnification = focal length of scope/focal length of eyepiece.
Better is to measure the field stop diameter and apply a formula I've forgotten - someone else will know it. 
Otherwise you can time the passage of a star on the celestial equator from one edge of you fov to the other through the centre of the fov. The star travels 360 degrees in (very close to) 24 hrs = 15 degree per hour = 15 arc minutes per minute. The hard part is making sure the chord crosses the centre of the fov. It's not too hard is you have an aligned eq mount but it would be a lot of trial and error on an alt-az.
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