You need to know four numbers to use this program:
The last is typically measured from the center of the secondary to the place
in the focuser where the average eyepiece's field stop sits when focused.
To be cautious you may wish to use the eyepiece that comes to focus
furthest out from the diagonal.
You can use any units you wish; the program works with inches, millimeters,
Just be consistent!
Diagonal Minor Axis
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