Magnificent images. 5080mm is stoking it! Your seeing is undoubtedly good or at least your arcsec/pixel combo suits the conditions very well.
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Originally Posted by AstroJunk
How do I calculate/measure the image circle?
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Image circle at the focal plane is defined by two factors. Firstly, the i.d of your focuser. If you’re using a typical 2” (50mm) focuser then obviously you’ll never get an image circle larger than this. Most astrographs have a 3” (76mm) or 4” (101mm) focuser to cater for larger format chips. Secondly, how much of the light is usable or put another way - the flatness of your light path. Commonly, coma in parabolic mirrors distort the edge of the FOV, thus the full light path can’t be utilised without some form of flattener in place. I did have a formula for calculating field flatness in an optical design, but it was based on a refractor’s objectives. As you’d expect, this also depends on chip size. It is typical, that even if you have a large 4" focuser, the image circle will not be 101mm. An example of this is the Takahashi FSQ, it uses a 4" focuser, but the image circle is 88mm.