While doing the mirror refit as per
Geoff's Howto being discussed on the
GS Dob optics thread, I've noted that the mirror is only supported on three cork pads at its edges. From the little I know about mirror cells I suspected that this may not be enough to stop an 8" mirror from flexing too much. I don't know what the mirror cells are like on the bigger Dobs, so this may only be relevant for us little dobbers.
After a bit of digging around, I found the relevant information in a review article on a bunch of 8" Dobs:
Alan M. MacRobert,
Sky & Telescope, Jan 2000, volume 99, issue 1, page 60.
One of the Dobs considered was the Orion XT8, which has the same construction as the 8" GSO Dob. I am sure that the S&T people will forgive me for posting a few short relevant passages from the article:
Quote:
I disassembled the Orion and Bushnell mirror cells. Both proved to be holding the mirrors only at the edges. A mirror should be supported more toward its center if it is not to deform under its own weight, especially if it is as thin as these, which had a width-to-thickness ratio of 9:1 (S&T: April 1996, page 75).
The Orion mirror cell offered the chance for an easy fix. I put three stacks of felt pads, made of cheap weatherstripping held together with double-sided tape, on the thick metal plate behind the mirror. The mirror now rested on pads 70 percent of the way from its center to its edge. I was careful to replace the mirror clips very lightly on the mirror's face so they would apply no pressure. Sure enough, once the telescope was reassembled and recollimated, the star test showed that the astigmatism was much reduced!
As a check, I took the pads out and restored the mirror cell to its original condition; the astigmatism came back.
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So there's another mod to add to the list. Stuffing felt under the mirror does not sound all that appealing to me though. How much felt is enough how much is too much? The problem really is that any more than 3 fixed support points and you no longer have all of them in a plane... Has anyone got a better suggestion on how to improve mirror support?