You can calculate the true field of view when Astro imaging using the same formula as for eyepiece TFOV. i.e.
TFOV = Sensor size in mm x 57.3 / Telescope or lens focal length
E.g. For APS-C sensor of 24 x 16mm and 420mm lens
TFOV = 3.3 X 2.1 degrees
For full size 36 x 24mm sensor and 420mm lens
TFOV = 4.9 x 3.3 degrees
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