Astigmatism in Skywatcher 12". Advice needed (Resolved)
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?
What you describe doesn't make much sense, in that the astig. axis doesn't change when the mirror is rotated but does when the whole cell mirror combo is rotated.
I'd recommend just rotating 90 degree and carefully not the axis of stig outside and inside focus by putting your hand into the lightpath so you can identify the axis in the defocused star, and mark that on the outside of the tube. It could well be the secondary as I've seen they are glued on pretty crudely without any real space for some give .
Sounds like it's the mirror cell causing the primary to go out of figure. The primary maybe loose, but the clips that keep it in the cell must be pressing against it in such a fashion as to cause the astigmatism. They don't have to press really hard to make the mirror twist out of shape. If it were the primary mirror causing the astigmatism, when you rotated it, the astigmatism would change direction in the new orientation of the mirror. Since it didn't, then the cause must be something else. What gave it away was when you moved both the cell and the mirror....the orientation of the astigmatism changed then, so it must be the cell causing it. I'd suggest you modify the cell so the clips don't press down on the mirror at all. Maybe even replace the cell.
I tend to agree with renormalised that the cell must be pressing on the mirror somewhere.
The retaining clips have little adjustable plastic posts that stick out perpendicular to the mirror wall, I left about 1mm play between the mirror and the post.
What is more likely culprit - the support at the back of the mirror if it is slightly uneven (those corks on triangles) or the rubber retaining clips?
I can slide a business card between the primary mirror and the rubber retaining clip when the cell is lying flat on the table, didn't try to slide it in at an angle.
I will star test today again, marking the direction of astigmatism and rotate to confirm. It's just a pain in the back to lug the 12" solid tube 10 times a night.
Alex, See if the flotation triangles are rocking evenly in all directions with the mirror off. It may be that it is not sitting properly on the flotation triangles and that some points are exerting more force than others.
I checked the triangles yesterday and made sure they rock evenly.
I tested yesterday again with a wide ruler instead of a hand and it appears that my earlier test was inconclusive and the rotation of primary mirror itself (cell in the same position) changed the direction of astigmatism.
There is a little crack at the back of the mirror going 2-3mm deep and about 2cm long. I guess I will call the shop and ask them to take care of it under warranty, the astigmatism is just too strong.
Thanks for all replies. I'll update the thread once the issue is resolved.
