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 aluminum tube with a side window. It's really frustrating to work with this one and 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.

However, there is an ultimate solution for all of these problems: the Barlowed collimator. 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).

Below are my short notes on the subject and my experience working with the DIY barlowed collimator.

Use notes
  • It's extremely simple mod - just insert your misaligned collimator with distorted laser spot in any barlow lens you have - that's all! Zero cost!
  • The only prerequisite - your mirror must have a good center spot marker on it (I've made one on my newly re-coated mirror using a fat black Sharpie marker).
  • If the diverged by the barlow lens laser beam highlights the mirror's center mark - it does not matter how precise your laser collimator is in fact - it will work perfectly well.
  • The procedure is the same as with the centering of the reflected red laser spot in the hole of the laser collimator screen, visible in the side window. But now you need to center the dark shadow from the mirror's center spot on the red background over that hole. 
  • The shadow reacts on slightest moves of primary collimation screws, so be careful and patient.

Good to know hints
  • The standard mirror's center mark might be too small or too big for a comfortable collimation. 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), 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).
  • The 1.25" collimator has quite a small screen to observe the shadow comfortably. The center spot image may end 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. Fix the tilt with screws. The impact on accuracy will be nearly unnoticeable. With fast mirrors moving the focuser's tube out to the maximum might also help a bit.
Overall, I'm very excited with the possibility to collimate comfortably at the mirror's end of the telescope with almost the Star test precision! Highly recommend this method to everyone.

1 comment:

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