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laser beam on secondary

Started by BABOafrica, 03/08/2011 06:58AM
Posted 03/08/2011 06:58AM Opening Post
I'm trying to tweak the collimation on a home-made 10" dob. I have a laser pointer which I can fix exactly in the center of the focuser (using a small home-made brace). I assume the laser beam should, for peak performance, hit the center of the secondary and get reflected to the center of the primary.

I do NOT have a store-bought collimator of any kind.

Any tips?


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In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 03/08/2011 07:58AM #1
Here are a couple.

If you have a 1.25 inch focuser, obtain a plastic film canister from a roll of 35 mm film. (I suppose they still exist.) Drill a small (1/8 inch or so) hole directly in the middle of it. Put it in the focuser, look into it through that little hole, and see that all the mirrors edges are concentric.

Rather than use a brace, mount your laser pointer into a film-canister type of arrangement so that it can be removed and replaced in the focuser by simply moving the film canister. (In other words, the canister is the holder.). Now, take that canister and roll it along a flat table while watching the laser beam on a distant (five feet or more) wall. Does the beam describe an absolutely straight line, or does it go up and down? If it does not go stright, adjust the laser in the canister until it does. Unless you do this, you do not know if your laser is giving you a useful reading. When it does go straight, put it in the focuser and proceed.

For all practical purposes, you can use the center of the secondary. The purists would have you offset where the laser beam hits, but at the level of precision of your tools, that is delusive exactitude. Have the beam come down the focuser, off the center of the secondary, to the center of the primary, back directly to the center of the secondary, and back up the tube to the same place it started. Then you are as collimated as the laser can get you. Note that the laser collimation (and this is true even of those made by Howie Glatter or David Ho) is not necessarily good collimation, because the beam can be fooled if the secondary is out of position. But if you used the film can to get concentric edge of mirrors as described earlier, and then the laser pointer, it should be pretty good.

By the way, unless you can verify that the laser collimator is really collimated itself (it draws a straight line when rotated), you are better off with a simple film can with a hole.

Posted 03/08/2011 08:57AM #2
Do you have a Barlow lense? If so, you can easily make the easiest and best collimation device I have found so far.