I'm wondering if anyone has heard of astigmatism being caused in a secondary mirror in a newtonian when it is just stuck to a smaller diameter angled cylinder with the edge of the mirror free and not restricted at all?
Could the compound that holds the secondary on shrink during curing and distort the mirror to cause astigmatism?
anything is plausible, but the factual evidence is whether or not similar manufactured materials exhibit the same properties you are talking about Mike. Our resident scientist may hold the clue to this question, Bert any ideas?
do other users with the same scope find they had or have this problem or are you the only lucky one? is it only noticable at the edge or throuhgout the field?
Mirror and cell have been replaced (with a known astigmatism free mirror and cell) and astigmatism is still evident so I am guessing it is coming from the secondary?
Thing is, the secondary is simply glued to a cylinder cut at 45deg that is smaller in diameter than the secondary mirror leaving the mirror edges completely free..?
question - does the manufacturer acknowledge the problem, or are they saying that since the optics were perfect at our end, they must be ok at yours? Very hard to test things once they are fixed. You could always create a test bench with the mirror and the diagonal free of the tube and rotate the mirror cell. PIgs might fly too but its a suggestion?
Ah yes but silly me I could rotate the whole assembly and re fix in the next set of holes around huh Too late all packed up and ready to go back, just been thinking that's all
Very near either side of focus star images are linear and the linearity shifts through 90deg inside and outside focus. Further from focus the donnuts are oval shapped with the major axis of the oval shifting through 90deg inside and outside focus. The out of focus donnut of a star in the centre of the field shows the secondary shadow off centre too with the off centreness shifting through 90 deg inside and outside focus.
At focus the stars appear like little triangles with what appears to be a hole in the centre..?
I'm not sure now but I think the last session I tested it the linearity is first parallel to the secondary then shifts to 90deg to this?
The astigmatism must be coming form the secondary but how?
You might be on the right path Mike,
I guess the secondary on the ASA would be rather large and heavy and if the stalk is rather small it could lead to flexure around the outer zones. I would prefer a stalk just a fraction smaller than the secondary.
When it returns it would be interesting to give it a ronchi test to see if you can see any astigmatism, but lets hope that it returns with no signs of it....
cheers....
Mike, Your astigmatism could well be caused by the bonding of your secondary. I think `bonding' mirrors in any photographic scope is quite probelematic. You don't want the optics to shift around as the scope tracks across the sky but at the same time if the silastic bond is too thin it will stress the mirrors as it cures. In my experience diagonals need to have at least a 3mm thick silicone blobs or twisting can occur.
I think mechanical edge clips are really the only sound way to go for this purpose. The first Meade 16" Lightbridge I looked through up at Kulnura had obvious astigmatism at high power but where do you start the diagnosis when both mirrors are stuck down with silicon ?
Is it possible to rotate the primary mirror by actaully spinning the whole cell until the mounting holes line up , to see if the astigmatic orientation changes.
Too late all packed up and ready to go back, just been thinking that's all 
