The thread referes to fisheye lenses. The 50mm and the 90mm isn't a wide lens by any means.
The FoV from the redcat (250mm) is 5 times the fov from the 50mm. So the calculate Error is the same for both lenses.

EKos/KStars calculation:
50mm on the asi120mm = 330' x 247,5'
250mm on the asi120 = 66' x 49,5'

if i use www.12dstring.me.uk/fovcalc.php to calculate the values it get:

50mm on the asi120mm = 5.50x4,12'
250mm on the asi120mm = 1.10'x0,83'

These are the values i would expect.

When i calculate the Pixel Size back from the 330' on the 50mm i get 17 [?] for a pixelsize - Thats a huge sensor

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