How to determine astigmatism source?

I've come across an older first generation XT8 8-inch Orion Dob, and notice that it has a fair amount of astigmatism. Any recommendations on determining which mirror--primary or secondary--is the culprit? I've checked the primary mirror mounting brackets, ensuring that they are loose enough to just barely hold the mirror in place. My suspicion is the secondary, but that's a SWAG at this point. I've ruled out eyepieces, since the astigmatism is noticible with every eyepiece I've tried, the latter ok in other scopes. For what it's worth, I don't have extra primary/secondary mirrors to swap in place of the originals to sort this out by process of elimination.

Ron
Equipment Forum co-moderator

Firstly, I assume you are allowing maybe
1 hour for the scope to equalize temperature
with the outdoors. Also, it is best to test
looking nearly straight up.

Next... test the primary...
Carefully note the direction of the star pattern
elongation inside of best focus. Then rotate the
primary mirror by 90 degrees in its cell
(un-install mirror / rotate / re-install / re-collimate
as needed). The check the direction of the elongation
again. If it did not rotate at all, then its
pretty clearly the secondary mirror.

Next step would be to remove the secondary and
see if anything is causing undue stress on it.
Sometimes there is pillow batting behind the
secondary to press it in place, and removing
some of it will reduce the pressure on the mirror.
After that, maybe try replacing the secondary.

If both the secondary and primary are astigmatic,
you can usually find a rotation of the primary that
will minimize the total astigmatism.

If the astigmatism is small and in the proper
direction, you can sometimes make it go away
by heating the rear of the secondary mirror
with a small resistor. This will cause the
secondary to go slightly concave, hence
adding (or subtracting) a little astig.