This is why you shouldn't rely solely on the laser collimator to check your Newtonian or Dobsonian's collimation. The other night, I spent an hour or two getting my scope precisely aligned using a Cheshire collimation tool. Just out of curiosity, I inserted the laser collimator in the focuser drawtube and this is what I see...
The laser dot should be pointed precisely in the center dot of the primary mirror. At first, I thought I had not collimated my scope very well, but when I put the Cheshire back in, everything was dead on. And the Cheshire does not lie. The reason the laser is so far off is because the laser itself is not collimated very well (or at all) from the factory. The laser is also prone to huge errors due to mechanical slop in the focuser and elsewhere. The Cheshire method is much more reliable, as this image proves.
NOTE: I have since added a tutorial on how to fix this issue!
Clear skies!
Monday, February 4, 2008
Why Not to Rely on the Laser Collimator
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7 comments:
Thanks for this tip.
A couple of questions: First, did you use the whole Cheshire collimation tool set (including the auto collimator) or just the Cheshire tool? Did you also use the sight tube (I don't understand everything in this yet so if the question seems stupid it probably is)?
Also, Zhumell has a deluxe collimator. Would it be any better than the one included with the scope, do you think?
I don't currently have an autocollimator, but the Cheshire I have is a sight-tube/Cheshire combo tool. It functions both as a sight tube and as a Cheshire, so I can use the same tool to do everthing from centering the secondary to aligning the primary with one tool.
As far as the Zhumell deluxe laser, I'm sure it might be a little nicer, but the inherent problem with a laser collimator is that it needs to be collimated, just as the scope needs to be collimated. Putting the laser in a Barlow will diffuse the beam, which can be quite useful. By placing a paper target on the Barlow, you can center the shadow of the center dot on that target. Then you know that you are perfectly collimated, as this is not dependent on the laser's collimation.
Hope this helps. Once I get a hold of a Barlow, I'm going to do a write-up on Barlowed laser collimation.
February 12, 2008 2:29 PM
DO you have a recommendation as to where to buy the collimation tool set from? I'm looking at getting an order turned in so I can really see!
I got mine at the Cloudy Nights Classifieds section. This item at telescopes.com is similar to my Cheshire eyepiece.
February 28, 2008 10:34 PM
Your headline would have been more accurate if it had said: Why Not to Simply Rely on a Laser Collimator.
Perhaps a good follow-up post would be how to collimate your laser collimator. Assuming it has a round body, this is fairly straightforward.
Clamp a piece of angle iron or aluminum in place and lay the collimator in it (pointing at a wall). Now roll the collimator (rotating it in place 360 degrees). If perfectly collimated, the laser should stay at the same point on the wall. If it travels in a circle, it is out of collimation.
Good laser collimators can themselves be collimated. I have an Orion Lasermate Deluxe. I'm told the collimation screws are under a self-adhesive label.
You bring up a good point, cheekygeek, as most laser collimators do have a means to collimate them. I've been meaning to make such a jig, but school lately has been hectic!
On my Zhumell laser, the collimation screws are set into the body, inside some little holes. If I remember correctly, they are allen screws.
Anyways, another way to collimate a laser is to actually put it in the telescope, and adjust your secondary WAY out of whack, so that the laser beam misses the secondary entirely on the return trip and exits the objective. Then your telescope has become the jig, and you can get a decent collimation of the laser in this manner. If I get time this weekend, I'll do a write-up of the process for future use.
Thanks,
Sean
April 10, 2008 9:18 PM
I had the same mechanical slop problem with my 10" LightBridge using the LaserMate collimator until I found a company solves the slop problem with their new SCA technology. I went ahead and bought the SCA laser collimator last week, and the laser works like a charm.
It uses its own self centering adapter to lock the laser without the thumbscrew and automatically centers the laser in the focuser. It is about time to have someone to come up with this brillient idea. You should check out their website, hotechusa.com. The laser is so accurate with repeatable installation, I don't even need to barlow the laser to compensate the mechanical slop.
Clear skies!!
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