Thursday, January 19, 2012

DIY Barlowed collimator

I've got a stock laser collimator with my Dob. Its construction is based on some primitive China made keychain laser pointer enclosed in the aluminum tube with a side window. It's really frustrating to work with, so I've been looking for alternatives for a while.

After searching online I've realized that this construction is quite common in cheap accessories from many vendors, the Orion for example.

Common problems
  1. The laser spot might be not perfectly round, mine is more like a 1:3 ellipse, what makes it tough to understand where is the true center of the reflected spot;
  2. The collimator's body is not precise enough, so it can "play" in the focuser making the beam dancing quite significantly around the center spot mark of the primary mirror.
  3. The laser beam might be misaligned easily from simple handling (the laser pointer can shift inside), and sometimes right after the intended use (the on/off button of the pointer inside is triggered by a side screw of the collimator, what may interfere with the alignment screws setting).

In other words, the collimation is severely inconsistent and may be wrong, especially if done in the field, where it's hard to verify the collimator's collimation itself.

Long story short, I have finally found the ultimate solution for all of these problems: the Barlowed Laser Collimator (BLC). The physics behind this device, as well as the step by step instruction how to work with the Barlowed collimator is very well described in this article (the image above is taken from that website).

BLC Quick Overview
  • It's extremely simple mod - just insert your (possibly badly misaligned) collimator (with its possibly severely distorted laser beam spot) into any Barlow Lens (or a TV PowerMate like device) you might have 1.25" or 2", $10 or $500 one, doesn't matter... And that's all! Zero cost if you have a Barlow already (which is a must have anyway)!
  • The only prerequisite - your mirror must have a good center spot marker on it (I've made one on my newly re-coated mirror with an ordinary fat black Sharpie marker and a paper center mask as I didn't like sticker rings dimensions (loo large diameter and too wide contour).
  • If the diverged by the Barlow lens laser beam highlights the mirror's center mark (as visible from the UTA side), then it does not matter how precise your laser collimator is. It would not only work perfectly well for the task but the accuracy of the collimation on a typical Newtonian would surpass most of the other visual methods used in the field in the dark bar the classic Star Test.
  • The procedure is the same as with the centering of the reflected red laser spot in the hole of the laser collimator screen (in the side window of the collimator body). But now you need to center the dark ring shadow from the mirror's center marker on the red lit background making it concentric with that hole. Way more accurate not only because of the Barlow return beam magnification, but also as you can see and judge the accuracy of the whole picture clearly, not just assuming that the narrow spot is centered somewhere in the hole.
  • The shadow reacts on slightest moves of primary collimation (and even stopper screws when they are bending the mirror cell). Just be more careful and patient compared to the direct laser collimation alignment.

Good to know hints
  • The standard mirror's center mark might be too small or too big for a comfortable collimation with the BLC. Mine was just 4 mm initially and it was almost disappearing in the return hole of my collimator. After the recoating, I've made it 6mm which is about perfect fit on the screen. For a given mirror the perfect size can be calculated (sorry, no formula from me now, maybe later).
  • The diverged laser beam is much weaker than the focused one. So if your laser collimator batteries are almost dead (or if you trying to collimate in a cold temperature environment, or in the dusk), the image brightness might be insufficient for a good contrast of the center spot shadow to deal with even in the dark (since then I had my collimator modded with the USB power port). With practice using your DIY BLC it's less and less crucial.
  • The 1.25" collimator has quite a small screen. The center spot shadow image may end up outside of the screen's edge if the telescope was significantly misaligned initially. Fortunately, the laser collimator is still there, just take it out from the Barlow and use as intended for rough initial collimation (I'm thinking of a bigger DIY 2" collimator rig with a different laser beam emitter already).
  • The diverged laser beam spot might be too small (or/and a little offset) to cover the entire mirror's center mark. In that case, you can try to move the collimator a little out of the Barlow and wiggle both around looking for a better coverage of the center spot before securely clamping each in place. The impact on accuracy from that will be nearly unnoticeable. With fast mirrors, moving the focuser's tube out to the maximum might also help to expand the highlighting spot a bit. Or you can try refocusing the inner laser collimator some day to improve the beam to your aperture (some lasers inside allow that easily with a flat screwdriver).
Overall, I'm very excited with the possibility to collimate comfortably at the mirror's end of the telescope with almost the Star Test accuracy! I would highly recommend this method to everyone.


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