Quote:
Originally Posted by Merlin66
Just a comment.....
Secondary mirrors are not exactly elliptical mirrors as required by the theory and formulae.
They are usually cut cylindrically from blanks.
The difference causes some additional light loss in the system.
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Sorry but there is no difference. A cylinder cut at 45° (or any angle other than 90°) gives an ellipse just like a cone does. That's why when looking through the focuser the secondary appears circular.
Dave