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Originally Posted by iceman
Someone write a how-to for centre-spotting, please!  the question comes up often and would be great to have a local article to point them to. |
Well, I center spot a little different than most, but I'm happy with the results.
This is how I do it with my Hardin (GSO). To start, place the tube on a table covered with a soft towel. Remove the mirror from your scope and from the mirror cell. This is easier than you'd think, just remove the 6 screws around the end of the scope and the whole rear mirror assembly comes off. The GSO's are rather a tight fit and, at least the first time, you may need to tap the lower ring off with a piece of wood and a mallet. I've found Synta's to be a little looser.
Once the rear assembly is off, remove the 3 mirror clips (details in the mirror clips forum - lol). Lift the mirror off and place it. face up, on a clean sheet of paper. Trace the mirror and cut out your pattern. Fold the circle in half, then in half again. Snip about 1mm off the point of the folded circle. Unfold it and you have a very small, perfectly centered hole. Lay it on your mirror and place a dot on the center with a fine point dry erase marker!
Remove the paper and draw 4 lines away from the center dot forming an 'X'. Be sure to leave about a 1mm gap bewteen the dot and your lines so the dot stands alone. Make the 'X' lines about 4mm long. If you make a mistake, just wipe it off with a damp Q-tip. That's the beauty of marking your mirror with a dry erase marker instead of a paper reinforcing ring. Fear not! The center of your mirror is behind the secondary and doesn't contribute to the image. If you prefer the paper reinforcing ring install it during this step instead of making the 'X'.
When re-installing the mirror, be careful not to over-tighten the clips. The slightest pressure applied by the mirror clips can change the shape of the mirror!! Some people say to leave enough room for a credit card between the mirror and the clip. I don't go that loose, but I'm careful that the clips do no more than barely touch the mirror. I've never seen any mirror distortion in star tests.
Before reinstalling the rear assembly, align your secondary!!! Use a chesire/sight tube. It's very easy to do with the mirror removed. No confusing reflected images. Rack the focuser in so you can see all of the opening at the rear of the tube. Just center the the rear end of the tube in the secondary. Don't forget to check the radial alignment of the secondary, too.
Now reinstall your primary and collimate normally, starting by double checking your secondary alignment with the mirror in place. I collimate with only the chesire/sight tube, but lasers still work with the dry erase marker mark on the center of the mirror. Rotate your chesire so that the cross-hairs are turned 45 degrees from your new 'X' lines. You'll get a very neat pattern to align to align by.
I remark the mirror after each cleaning, which for me is about twice a year.
Cheers mates.
