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Re: Placement Of Minor Axis Offset

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Posted by Thad Robosson/">Thad Robosson on December 7, 1998 15:03:29 UTC

: I know that the proper placement of the diagonal must be offset from the optical axis, but all the material I have read does not give steps for measuring. Is the measuring done from the sides of the tube? What if the tube is not perfectly round, as in a cardboard tube? Thanks for any Info.

Hey There,

I picked up a copy of Star testing Astronomical Telescopes at the local telescope shop, and it's a wonderful book. It explains fairly thoroughly the process, and even gives figures on the amount of offset. The method given is 1000 times better than just setting everything until it's all concentric. Untill I read and followed this method, I had no idea what an airy disk even looked like. My 'scope is a Meade 8" f/6, and it turned out fine once I set it up right. At least I can tell when the atmosphere sucks, or if the scope isn't cooled down enough.

As far as cardboard is concerned, mine is, and I had no problems. I use a set of machinist's calipers to make the measurement, but one could use a bent coathanger. I suppose trial and error is in order if your tube is not perfectly round, but unless you're looking for PERFECT collimation, you can get it close enough for enjoyable visual observing.

Hope I've helped,

Thad

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