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Sunday, December 28, 2008

Collimate Your Laser Collimator


As an amateur astronomer, you'll periodically hear people talk about how inaccurate your typical laser collimator, as I've done in this post. The truth is, the typical laser collimator that ships with your telescope is likely off by quite a bit, and any collimation you do with it will only end in a poorly collimated telescope. This is fixable, however, and can be quite simple to fix. Below, I'll show you the easy way to do this without any fancy equipment.

  1. Insert the laser collimator into the focuser, as you typically would to collimate the telescope. Next, adjust the secondary mirror so that the laser completely misses the secondary mirror on the return trip and exits the objective. The telescope is now, in effect, a jig to hold the laser in a manner that it can be rotated without changing its position.



    When you do this, be very careful not to look into the laser beam. Always be mindful of where the laser beam is so you can avoid it.

  2. Put a piece of paper at the end of a hallway and position the telescope so that the laser beam is on the paper. Mark the position on the paper, then rotate the laser 90 degrees and make another mark. Repeat this two more times and you will have a pattern like the one shown below. Now you have a square that shows you how far off your laser is. If the laser doesn't appear to move, then you're one of the lucky ones to get a pre-collimated laser.


  3. Now use the adjustment screws to move the laser dot into the center of this square. On my Zhumell laser collimator, the screws are actually allen screws, so you'll need the appropriate allen key. The adjustment screws can typically be found on the main body of the collimator inside three holes. On some laser collimators, the adjustment screws may be hidden under the label.


  4. Now just repeat this process until you've got the smallest square that the slop in your focuser allows. You'll never be able to get it perfect, but you can see in the following picture that you can get this square much smaller than when you started.



Now you can actually use your laser collimator to collimate your telescope! Keep in mind, however, that laser collimators can only be used to collimate the secondary mirror. The return beam is not guaranteed to be 100% accurate so still can't rely on the laser to collimate the primary mirror. For this, you still need to use a cheshire/sight-tube combo, but the laser can now make your collimation process much quicker!

Clear skies!

18 comments:

Len Howlett said...

As usual, Sean, a succinct description that succeeds in making a seemingly difficult act easily visualized and thus better facilitated by the reader. As I inferred in an earlier communication to you, one of the big problems with going through the steps in everyday life is the explanations, including 'how to' and assembly instructions particularly, seem to confuse the issue more than to have their intended effect. Thanks for making this little mentioned potential problem soluble and for helping to make my experience with my telescope less of an obstacle and more an enjoyment.
Len

RevAaron said...

Wow! As a truss-tube dob owner, I use my laser collimator extensively. These instructions should have been in the box. I'm putting this on my to-do list. Thank you!

Mathias said...

Hi! I just recieved my not so collimated HoTech collimator! How did you ensure that the tube with the collimator in it was fixed in the same position while you did the collimation on the laser? Thanks. /Mathias

Sean Welton said...

I actually put the laser in my 1.25-to-2" adapter, then turned the adapter. For the purposes of this tutorial, any slop in this joint is going to affect your collimation, but one can only collimate to the least accurate tool in his kit. In this case, we are making the focuser the least accurate part, since most modern focusers are quite sturdy and more accurate than the stock laser collimator.

Hope this helps,
Sean

April 5, 2009 at 8:13 PM

Mathias said...

I turned the collimator in my focuser adapter as well and managed to get the dots down to 1-2mm from each other. The dot is still doing a small circle in the primary when turning the collimator and consequently the dot exits the hole in the collimator in certain positions. Have you experienced this?

Also, Dave at HoTech told me to try and rotate the collimator in the 2" Hotech-adapter for the collimator instead since it's more accurate. Maybe that's a way to collimate the laser at least...

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