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True field of view (degrees) = imaging sensor size x 57.3 / focal length of telescope or lens.
If you had a 35mm camera with 36mm x 24mm sensor (film) you would have a true field of 36x57.3/200=10.3deg (long axis) and 24x57.3/200=6.9deg (short axis) for a 200mm f.l. lens. With a 2000mm focal length telescope the figures would be 1.03deg x 0.69deg.
If you use a DSLR with reduced sensor size (scale=1.6) then your sensor is not 36x24mm but 22.5x15mm. Therefore the true field will be 22.5x57.3/200=6.4deg and 15x57.3/200=4.3deg for a 200mm lens and 10x smaller for a 2000mm lens. Note this is the same as multiplying your focal length by 1.6x and using 36x24 (full size sensor) in the calculations.
The 200mm lens wide open (at 18mm mark) would be f3.5 but not at the 200mm mark typically f5.6 or greater. On a 2000mm f10 SCT the f ratio will still be f10 as the focal length and aperture and therefore focal ratio of the system has not changed, only the sensor size is in effect cropped.
Since the invention of compact digital and DSLR cameras without full 35mm film size sensors, camera manufacturers/dealers have introduced a method of comparing sensor size and effective focal lengths to that of 35mm format and whilst this is good for 35mm format it is rather meaningless when you are comparing to non 35mm format.
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