Hi Mark,
You are right to continue to ask your question. I haven't seen anyone answer it. I didn't read all the posts, but all the "references" cited that I read seemed to be strongly worded opinions. No one explained why/how a laser and Cheshire measured different things. Even a perfectly collimated laser and perfectly square focuser (which isn't necessary) with no slop can allow the two to show different things.
One way to understand this is to perform an experiment with your telescope and a laser collimator. Place the laser collimator in the focuser and rotate the secondary so that the spot is half way to the edge of the mirror. Then "collimate" the scope using just the secondary collimation screws. You can get the spot to hit the center of the primary by tilting the secondary off to the side. And, you can get the return beam to fall back on itself. Then take a look using the Cheshire. It will show those deviations because it offers more information about the relative positions of the edges, reflection of the spider, etc. Make sense?
Consequently, I always rotate the secondary mirror so that the spot falls as close to the center of the primary mirror as possible, then tweak the secondary adjustment screws. When I do this, collimation stays very good and I only have to check with a Cheshire every so often.
Albert
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From: Mark Brada ^lt;mpbrada_at_yahoo.com>
Date: Thu Mar 13 2008 - 19:25:22 MST
At any rate, I still don't understand this. You use your focuser with a cheshire, no? So aren't exactly the same errors being introduced? Mark
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Received on Fri Mar 14 00:49:38 2008