The title of the thread......Minimum fl.....
As the focal length increases so to does the size of the seeing disk - larger seeing disks can accommodate larger pixel sizes and still achieve good sampling.
It think it comes back to larger apertures = fainter targets, longer focal lengths = smaller FOV (unless we increase the camera frame size)
For 20 years the f10 SCT almost ruled supreme....add the x0.63 reduce and you have a long focal length solution.
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