Quote:
Originally Posted by 5ash
It is often mentioned that final collimation on a star is important in fast newtonians. Some posts on the web suggest looking at the symmetry of the defocussed star image as a guide , however this works fine with SCT 's and maksutovs etc where the secondary is central , but where the secondary is offset as in a fast imaging Newtonian the defocussed image will show asymmetry due to the offset secondary, " not being central"! Looking at the stars "airy disc" requires very still conditions ( I don't think I've ever seen one) , so what are the options besides hoping your mechanical collimation using a Cheshire is sufficient?
Regards philip
PS I've tried laser collimation after using a Cheshire and found no need to touch anything .
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I think that the slightly out of focus star image is primarily a diffraction pattern and the offset secondary geometry does not show up as asymmetry. For example, the star image near focus has a central dot plus light and dark zones that have no direct geometrical equivalent - it is primarily a diffraction pattern.
I find that if I align to get a symmetrical star diffraction pattern near focus and then wind the focuser further out (or in), the pattern will revert to the "offset secondary" - by that stage it is too far out of focus to use the star test.
If you are aiming for high resolution, star testing is a very sensitive way to get there - coma starts the instant you move off the central axis and can harm resolution even when the "seagulls" are not obvious.