camera calibration depth coverage

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Are you sure it has a focal length of 1500mm ? That's a very large telescope

Ideally you would calibrate your camera at the distance it is going to be capturing a scene - the lens characteristics may change when you refocus. This isn't always possible - if you are looking at something a long way off - in which case use the closest target to the real position you can.

There are online depth-of-focus calculators that will tell you the range of distances over which you are in focus without changing the lens focus