Advice needed final collimation on a star?

[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 .

[glend]

I am very satisfied using the Chesire. I regularly do collimatjon checks on my refractor using defocusing either side but generally don't bother on the newt if the Chesire is centred and my camera test shots show good stars (with my coma corrector and have used the focusing mask o produce clear diffraction spikes.

[5ash]

Hi Glenn ,
Thanks for your advice.
Regards philip

[Shiraz]

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.