Quote:
Originally Posted by multiweb
Hi Ray, check your secondary as well. Astigmatism is almost always caused by the secondary offset or the surface not being flat so it may be stressed by the rubber gel that's bonding it to the plastic container. I cut mine lose, then glued some cork under the retaining clip and packed the sides with paper shims. Then I put only one dab of gel grip on the spider side of the plastic. So it's just 'hanging there' by that and the front clip.
The primary is too heavy to be siliconed I reckon. I'm not sure it would deform the figure as to create aberrations. It's still a thick chunk of glass. Although rubber gel as an incredible pull when it cures. I managed to deform the primary on my C11 by putting too much gel at some stage and that was much thicker. Since then I don't take chances with glass and tend to fit it tight but not bond it or restrict it in any way. The 3 unpainted bracket are what's holding the mirror weight. It makes contact with the glass sides. You can just rotate the mirror but it will not move sideways and it will be very hard to remove it up unless you're dead square. Like pushing a bearing on a shaft or inside a bush.
A good trick if you still have astigmatism is to rotate the primary 120 degrees, then check again. You'll find a position where it's gone. Mark it, then always remount your mirror in that position. Also mark the cell so you always match its orientation to the tube.
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thanks for the advice Marc. The first thing I checked was the secondary - without any edge support on the main mirror, there was no astigmatism, so that ruled that out. Seems that even that much glass is pretty bendy when talking in microns
. I was still surprised though that cured silicon could distort the mirror so much, although I recall a post by Mark Suchting on a distortion problem he had with silicone attachment of larger mirrors. Regards ray