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We should take again about the issue of field distortion, or in familiar terms "fisheye" effect. A simple conversion factor from pixels to angle is not sufficient, since this assumes a constant scale across the image with no distortion across the field of view. There is always some amount of field distortion though you may not notice it in ordinary images. You need to photograph a set of points (stars will do nicely, if your camera can "see" them) at known angular locations. Then you can build a map connecting pixel position to angular distance from the center of the image. Once you have that map in hand, and assuming that it does not change from one session to the next (may well not be true!) a digital camera has great potential as an angle-measuring instrument.

Frank Reed
ReedNavigation.com
Conanicut Island USA