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Tweaking your Newtonian's collimation with a standard laser collimator


paul
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I guess this is obvious stuff to the old-hands but I thought to share my learnings in case they are helpful to someone else.

  1. Don't trust the laser collimator until you have checked its own collimation. Even expensive ones might not be perfect. The easiest way I've found to do this is to put the Newtonian scope on a table with the focuser pointing up. Place the collimator into the focuser and secure it loosely, rotate the collimator in the focuser and look at the primary mirror (carefully!). As you rotate the collimator, the laser point on the primary should not move at all. If it draws circles on the mirror then you need to adjust the collimation of the collimator. Typically, there are three adjustment grub screws around the collimator for this (often hidden by black gunk).
  2. If you can adjust the laser brightness, use the dimmest setting you can see. This often makes the smallest spot and can improve accuracy particularly for the cheaper collimators.
  3. The collimator fires its laser through a big hole - 3+ mm on some designs I've seen. When you adjust the primary mirror knobs you are trying to make the laser point go back down the hole. But how do you know when the spot is dead centre? The way I do it is to adjust the knob so the laser point is on the edge of the hole. Then, without letting go of the knob I rotate it so the point reappears on the opposite edge of the hole. I judge how far I have rotated the knob. I halve that rotation back to put the laser in the centre. I repeat that step with each knob in turn several times. With practice this is very quick and you get a feel for it.  

 

 

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31 minutes ago, MarkAR said:

Cant agree more about last collimators. I checked mine and it was miles out, 2 feet from the wall and it was making a 4" circle. Managed to improve it a lot but it's still off and won't line up true.

The smallest circle you can make will indicate how far off the central line the laser pointer is. It should not be more than a millimeter or two if it is a reasonable quality.  The laser beam will at least be parallel to the collimator body. Unless you have a very fast scope that is probably alright IMO. If it is worse than that the laser pointer has probably slipped out of its anchoring point. I had that with a collimator that used a presentation laser pointer inside. The cheaper ebay collimators seem to use a laser module that is moulded in to place so shouldn't be offset by very much.

 

 

Edited by paul
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