Mirror Distortion with Tube Rings?

[Astrosurf]

I'm sure my coma has got worse over the years. I've just bought a coma corrector, but I wondered if tightening up the tube rings too tight could distort the mirror? It's the sort of habit one can get into without even realising. Just a thought.

Alexxx

[Uplooker]

I would not have thought it possible. The rings are generally positioned nowhere near the primary cell and there is significant clearance between the cell / mirror and tube. Enjoy your coma corrector, which one did you go for?

Ian

[SnakeyJ]

Alex - actually distorting the mirror sounds very unlikely, but I have found with my 200P that over tightening can distort the ota which puts the scope out of collimation.    I suspect this is due to poorly cast/finished tube rings that are not quite circular.    To reduce this on my black diamond explorer 200P, I re-felted the tube rings with DC-FIX flocking which was slightly thicker and softer than the standard Synta material and avoid over tightening - collimation is then performed with the OTA mounted and balanced for imaging c/w filter wheel and camera.    I'm also equally cautious when adjusting the spider to avoid any distortion.

[mikeobrien]

I managed to distort my primary mirror when I had just got it by fastening the mirror clamps too tight this gave me triangular shaped stars. I backed them off until they just gripped it and it was fine. The mirror had been sliding about in its cell!

Sent from my Nexus 7 using Tapatalk

[Astrosurf]

Thanks guys. I may have over-tightened my mirror, but I did do it very gently. I can't understand why, when I rotate the tube, that I can hear the mirror move slightly with a little thump. It's only on one particular place though. I can't see it moving and the screws and Allen key sockets aren't loose. Any ideas?

I've bought this CC. I hope it works!

http://www.astroshop.com.au/products/docs/mpcc-options.pdf

I must get confident enough to collimate before each session.

[SnakeyJ]

I managed to distort my primary mirror when I had just got it by fastening the mirror clamps too tight this gave me triangular shaped stars. I backed them off until they just gripped it and it was fine. The mirror had been sliding about in its cell!

Sent from my Nexus 7 using Tapatalk

Good point from Mike - over tightening the mirror clips them selves will distort the mirror and cause astigmatism.  Though I tightened mine just enough to remove any noticeable flop.   Using a sheet of paper between the clip and mirror, such that it can still be easily removed was suggested here and seems to do the trick.

[lensman57]

Hi,

I have a lot of problem with coma and my Quattro 8s too. I have also replaced the dovetail bar with the longest that SW does and have fastened another on the top of the rings to increase stabilty. I also have suffered the pinched optics effect when overtightening the mirror clips. The baader MPCC is supposed to be very good but mine seems to be very sensitive to spacing but then again it is an F4 scope.

Reagrds,

A.G

[Astrosurf]

Mirror clips? Maybe it's something I don't see. All I see at the edge are small pads where the mirror rests on the tube base.