Collimation Using a Webcam
Submitted: Friday, 15th June 2007 by Scott Tannehill
The popularity of short focal ratio Newtonians – and in particular “fast” Dobsonians – has brought the importance of accurate collimation into focus (snicker) for owners of such telescopes. The light gathering ability of these telescopes can only be maximally exploited when these instruments are properly collimated. And truss design scopes require collimation each time the telescope is assembled.
There are many techniques and philosophies of collimation. Don Pensack recently published his review on this forum; this is an excellent resource. A regularly updated publication by Vic Menard is a more in-depth review on the subject, and there are many online sites illustrating the principles and practice of collimation, and several articles in Sky and Telescope. The features and merits of things like fully off-set collimation versus partially off-set collimation and other such nuances are beyond me, much less beyond the scope of this short discussion.
A valuable development was the publication by Nils Olof Carlin of the use of the barlowed laser collimator technique. Laser collimators are commercially available and almost all vendors offer the Barlow attachment described by Nils. While not necessary for accurate collimation, these barlowed laser collimators simplify (some say oversimplify) collimation for Newtonian users who can afford or fabricate such a device. By using a triad of tools (barlowed laser collimator, sight-tube, and autocollimator) a Newtonian can be most precisely collimated. With practice collimation of even the average truss-style Dobsonian can be done in minutes by an experienced person. Unless that experienced person happens to be me. But that’s off topic. I won’t discuss the use of these collimating tools – that you can find on many websites – but I will say that an understanding of how to use a barlowed laser collimator is necessary for you to understand what follows.
Nils and other collimation experts (I hesitate to call them amateurs after reading some of the posts) can easily be “heard” by joining the very active and educational Yahoo group Collimate_Your_Telescope. On that group these experts chime in and help answer questions from amateurs that range from the most basic to the most complex. And, best of all, it’s free.
Despite years of experience I still find myself occasionally playing collimation ping-pong with my telescope: I adjust a bolt, dash to the Chesire eyepiece or the back of the focuser to look at the laser collimator (more often I use the laser collimator), snort in disgust when I realize that I turned the bolt the wrong way, go back and turn it the other way, and repeat the process usually too many times. Some nights it still seems to take forever, even though it’s probably more like 20 minutes. But on those bad nights I sometimes consider converting exclusively to refractors.
I stumbled upon this fun and perhaps useful technique for collimation. My wife and I had set up a webcam video-chat with friends in the U.S. a few nights back. My laptop was set up on a counter next to my GSO Dobsonian, and my laser collimator was laying out nearby in preparation for a battery change. I looked at all three objects sitting there and experienced a small-magnitude epiphany. My wife declared it was no epiphany but rather some natural consequence of the beef curry meat pie I’d eaten for lunch. She warned me to clean up after I tinkered and then went to bed.
This technique requires a laptop, a webcam and associated webcam operating software, and a barlowed-laser collimator. And, as stated above, you should know about collimation and barlowed laser collimators in particular.
I scanned the IIS Articles list, did a quick Google search, and a quick search of the Collimate_your_telescope Yahoo group. I could not find this technique mentioned. However, it was a quick search, and I’d bet that someone, somewhere, has done this webcam thing and published the technique somewhere…I just couldn’t find it on a quick search.
Really, this is just a “digital visual” method of letting you see what you are doing at the focuser while you turn the collimation bolts at the back of the telescope.
I set the webcam to point at the back of my laser collimator when it was set in the focuser. Your technique will vary based on your scope and webcam. In my case, I just taped some foam to the spring-loaded built-in clamp at the base of my webcam, and attached it to a spider vane. See Figure 1. This way, I could quickly and easily attach and remove it and get it to point at the back of the collimator with no trouble each time.
I attached the webcam to my laptop, and set the laptop near the front of the telescope (my cheap webcam cable is short!) but pointed the screen back to the rear. (For nighttime use I have a red acrylic shield over my screen to preserve night-vision, and I set the laptop on a folding table I use for my eyepiece box and charts.) I adjusted the webcam settings using the software and found the best image quality. You should focus your webcam to get the sharpest image. See Figure 2. In this picture you see the laptop in the background. I used a flash for this photo and consequently the screen image of the back of the collimator is washed out. Real time it’s very easy to see the image of the webcam in daylight or at night.
The laser exiting the central hole of the white back-plate of the Barlow attachment (the white disc in the back of the laser collimator in Figure 3 below) caused glare I could not resolve by adjusting the settings of my webcam. As a consequence on the screen I could not make out the precise center of the collimator’s back-plate. This is necessary for accurate collimation. I solved this problem by drawing four straight black lines intersecting on a piece of white paper, cut a small hole out of this at the intersection center, and taped this over the hole on the back-plate. See Figure 3. With this, it was easy to see on the screen when the center-mark shadow was centered over the laser collimator hole (or rather, centered within my fabricated reticle overlay which itself was centered over the collimator hole).
Figure 4 shows the webcam image beside a direct photo of the back of the focuser with the barlowed laser collimator in the focuser drawtube. With the webcam software I can save video or still photos. The black and white-like web-cam images were in a totally dark room using the webcam and the red laser light only. It simulates dark conditions.
I can watch the back of my laser collimator on the laptop screen while I twiddle the collimation bolts, and dial in the primary collimation exactly. Figures 5 illustrates the donut shadow of the primary center-mark moving toward, and arriving at, the center of my hand-drawn reticle as I turn the collimation bolts.
I still use an autocollimator after primary collimation (and often repeat this cycle again to perfect collimation). But secondary collimation is rarely the rate limiting step. It’s primary collimation that takes the most time and causes the most frustration for wanna-be expert collimators.
Happy collimating and clear skies.