[alexch (Alex)]

Hi All,

I recently got a Skywatcher 12" F/5 dobsoinian telescope. I guess it's a classic example of "aperture fever" after observing with an 8" Skywatcher for 6 months...

I collimated the 12" telescope with a laser collimator then a cheshire (laser collimation of the primary was very very close). DSO looked great, but Saturn was less sharp than in 8".

So I did a star test yesterday and found diffraction rings elliptical, direction changed in and out of focus. I tried 2 eyepieces and a barlow (200x, 400x and 600x magnification), rotated them, tried another eye - all the same. An ellips became a circle only after 2.5mm out of focus travel, which to me indicates a severe case of astigmatism.

Then I tried to identify which component is responsible. The ellipse orientation remained approximately the same near zenith and at low angle (hard to judge the low angle because of the turbulence).

Then I removed the primary mirror from the cell (it was loose enough, nothing pressing on it), rotated it 120 degrees and put together with the cell positioned exactly the same as before. Ellipses still had the same orientation. I was relieved that the main cause was not the primary mirror itself.
The secondary mirror in Skywatcher (Synta) dob is glued to a metal cylinder with a piece of foam, so it should not be stressed.
So I decided to rotate the primary mirror with the cell together and noticed that ellipses changed orientation.

Am I correct assuming that the primary mirror cell is responsible for the astigmatism?

I disassembled it again today and there was nothing obviously wrong. The mirror rests on the 9 circular cork pads positioned on 3 triangles and is not touching the meatal at the bottom.

Did any of you have this issue and how the Synta mirror cell may be improved? Or should I just get Skywatcher fix it under warranty?

Your advice is very much appreciated.

Many thanks,
Alex