[The Mekon (John Briggs)]

The last few nights I have noticed significant astigmatism with my 18" scope. When aligning I use a 9mm Nagler (228x) and place the star as best in the middle of the field. When focusing at this magnification I have noticed the astigmatism. It is less obvious at my regular observing magnification of 118x (17.3 Delos)
Now I believe my mirror is good one, (made by Mark S.) so could the cause be uneven cooling? I am now putting the scope out only an hour before dark, and observing soon after so the temp difference between ambient and the mirror may be more than 3 degrees.

Can astigmatism be caused by a pinched mirror? I say this as the only recent change I have made is to firm up the sling tension.

What about the secondary? Can this cause aberrations? To me this is the worst part of the scope, it is a 3.5" one and I do not like the mounting holder. Am thinking of going to a 3.1" as I am strictly visual.

Any thoughts on this rainy night are appreciated.

John