$AB - an excellent observation report - really enjoy reading them!
Quote:
Originally Posted by §AB
The 12" dob has 4, count 'em 4 fans on the mirror cell but I don't have an AC power supply to run em with. So cooling naturally takes a hella long time. Before observing, I had to place paper shims between the mirror edge and a couple of the silicon blobs supporting it as I found that collimation was slipping as I moved the scope in altitude. Luckily my plan worked and collimation barely moves during the course of the night. As for the secondary, I *think* its collimated - I WANT CATSEYES!!! The scope I think has a little bit of astigmatism, not sure if that is a byproduct of thermals around the mirror, seeing, or maybe the secondary is pinched (it appears to be held in a sleeve similar to those of the GSO dobs).
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Astigmatism is unlikely to be caused by tube currents or dodgy seeing. In theory it could be the result of uneven cooling, but since you did not use the fans that is not very likely either (that's the good news about sloooow cooling - much less chance of temperature gradients and deformation in the mirror itself
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Assuming the optics are basically fine, you may want to look into the paper shims you used - they would likely cause the mirror to be supported unevenly and thus cause an ever so slight saddle-like deformation that will show up as astigmatism. If it is serious enough, try to see whether the direction of the astigmatism correlates with where the bits of paper are - if you can live with it I'd leave it for now...