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RE: RE: How Do I Find The Correct Mirror Distances

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Posted by Robert May on October 23, 2000 22:18:02 UTC

Actually Kip, he's going about it the better way. The only question is whether he actually has enough room to install the primary.
The best way is to setup the tube so you can see a distant mountain (more than 2 miles away is prefered) and move the primary up and down so that you can focus on the mountainside. I'd put a 25mm or larger EP about 1/4"-3/8" from fully racked out on the foucser (put the focuser there and move the primary to focus!) and that's where you want the primary. You will also have to check that the alignment of the primary is sort of there before locking things down.
My favorite way of mounting a primary cell on a tube is to put some blocks on the tube wall at where the cell is supposed to be and to do the removable screws into those blocks through the mirror cell so that when you remove the cell, you can put it back in at the same distance up the tube without worry. Most of the poorer mountings for the primary mirror cell end up with the screws that hold the cell in place are going through the tube and the only thing holding the cell in place is the screws going through the tube wall. This is very poor because the only thing holding the cell in place is the friction of the screw on the wall of the tube against the cell. Sure method of having a scope that moves all over the place!
If you are doing all of the work by yourself, you may want to make up some thin shims so that you can put a good friction fit on the cell against the tube.
If you have the primary cell that has a tensioned type bolt arrangement, fill that in with a block or ring of wood or other material and proceed from there. The primary is so massive that that system will vibrate when the tube is touched.

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