Misadventures in collimation
I confess to being ‘that guy’ who bought a RC scope, attempted to tweak the optics with little or no knowledge and have been trying to rectify it ever since. I’m not proud of this fact, but it is my pride that keeps me persisting with trying to find a solution. Its not like I hadn’t been warned - the forums online catalogue tale after sorry tale of those suffering with the blight of collimation woes. There are various tips, tricks and methodologies, some of which involve stripped your scope back down to its molecules, resmelting the glass and reassembling ( I exaggerate). I don’t have the stomach for wholescale surgery, I just want to tweak those little allen keys to give an acceptable performance - is that too much to ask? I started this thread before finding a solution - so let this either act as a warning or inspiration - history will decide.
1. What’s the problem?
My images had stars that were teardrop shaped not round. This could have been down to bad guiding - something I am only now beginning to master, but having bought a Cheshire eyepiece, it appeared the optics were out as the central dot reflection in my secondary mirror was off - so little so I started to tweak.
Ask yourself ‘how perfect does it need to be?’ before entering this rabbit hole. More time collimating means less time observing and many wasted clear nights, so check you’re up for this adventure.
Howie Glatter laser
2. Don’t touch the primary
The consensus online appears to be don’t touch the primary mirror - it is unlikely to be too far wrong. I would agree, but only after having fiddled with mine. In my defence I only did this after adjusting the secondary and not getting the desired results. Using an expensive laser collimator and following an online video tutorial I adjusted the rings to be concentric and the dot plumb on the secondary mirror - what I assumed would be bang-on. A star test revealed however that this was a long way off - probably worse than before. Both my gut and my wallet felt that particular blow.
3. Perfect pattern
I found a description online for the ‘reset method’ to get yourself back to factory settings before starting afresh, so that’s what I did. Concentrating on the secondary mirror, I started a new round of tweaks. What we’re aiming for is example of concentric rings, the large central dark spot being the shadow of the secondary mirror. You need a wall to point the scope at, which probably means setting up somewhere inside, letting the optics adjust, finding space and crafting a pop-up collimation lab. It can be a weekend-sapper, and you only know if it’s been worth it with a star test. I find this unsatisfactory and want more certainty to be able to perfect these optics in daylight. Surely doing this by day means you can see what you’re doing, it warmer and you’re not wasting clear nights.
4. Artificial star
I read about using something called an artificial star for testing - essentially a little box that producing a mock star for collimation tests. However their usefulness appears to be mixed, most units are expensive (I’ve already shelled out a few quid on the laser) and you need a decent distance (focal length x 20) from the scope to the ‘star’ - like, more than my living room. Here’s a description of artificial star method with a special but cheap pinhole torch. You still need a decent distance from star to scope to perform properly, but I like the element of control.
5. The Ball Bearing Method
Googling the artificial star method I came across the ball bearing method, which involves creating a DIY artificial star by pointing a bright torch at a ball bearing mounted/stuck on a black background. Apparently this method is even used by professional collimators and comes in less than £5, including new batteries for your torch. I immediately bought some ball bearings off eBay and later, a something called a Saw Horse I found in my local DIY store. Onto this I screwed a Losmandy dovetail plate so I can mount my scope horizontally, and securely. I now need to fix my ball bearing to something and test the focus at a suitable distance.
6. Three Tools
As a reminder - I have accumulated three collimation devices since getting my RC; a Cheshire Eyepiece, a laser eyepiece and a projecting holographic laser (Howie Glatter). Cost wise (same order), these range from £20 to £200 and one could assume the more expensive is better, more accurate and easier to use. Despite following various tutorials on how to use the Howie Glatter, when taken to a star test, the tweaks I had made by day against a wall surface had never cut the mustard when put into practice.
7. Daytime adjustments and progress
My route back to collimation (good enough for me) was this:
Reset my primary mirror back to factory settings by loosing all the black locking screws and tightening all the silver screws. I used the Cheshire to centre the dot of the secondary, adjusting the secondary mirror. Using the laser eyepiece, having loosened the secondary screws equally, I manually adjusted the beam back into the centre of the laser and re-tightened. I did final tuning on the laser EP on the primary mirror. I alternated between the Cheshire EP to tweak the secondary and laser EP to tweak the primary. Note I didn’t use the expensive Howie Glatter at all.
8. Ball bearing test on artificial star cluster
I set up a test using a single ball bearing set into some black foam (from one of the hard shell cases I use to transport my equipment). Around this I inserted a handful of dressmaking pins , liberated from my wife’s sewing box. Having created this artificial cluster, I positioned them outside facing south and found focus from a location inside the house (to maximise the physical separation). I had the telescope on the saw horse and the ball bearing/cluster in a box using my tripod for vertical adjustment. The distance between scope and ball bearing was 13m using an extension tube to achieve focus. I’d recommend around 15m.
I performed a few tiny tweaks to the mirrors but the sun was beginning to to be obscured by cloud, so my light source was fading. So I waited until dark, and now with a torch focused on the cluster, checked it again. I was getting even de-focussed star shapes with the central obstruction in the middle. I first check it out of focus then inside perfect focus - everything was looking pretty good. Having spend a full day doing this I decided it was good enough and was ready for a real star test.
Single artificial star test on the ball bearing - ok not quite perfect (could be little more towards 2 o’clock?) but from where I was this is a vast improvement! What was great was that the image was so still - no atmosphere to make the star jump around and distort. This stability made the process controllable and less like trial and error.
5min un-calibrated guided image. I can see defraction spikes around the core of the image and pretty round stars.
9. Proper Star test
On Sunday night I performed a star test and fired off some test shots with my CMOS cooled one shot camera. I’m pretty happy. I’m not going to say everything is perfect, but from where it was - with stars that looked like bananas - I’ve learned my lesson. I wont’ be going gung-ho at those mirrors again for a while, and as for the Howie Glatter - it’s going on eBay. It was the £1 ball bearing and a torch that fixed this problem, not the £200 laser.
10. Conclusion
I would have chosen to never started this process if I’m honest, and I have probably achieved nothing more to putting the scope back to where it was before I tinkered with it, but hindsight is a wonderful thing. What I would definitely say is that artificial star tests by day using the ball bearing method (or my own variation, the Star Cluster Method) was 100x easier than performing a first star test on a cold dark night with jumpy stars. It also didn’t waste a clear night. It is lo-tech, cheap and effective - unlike most things about this hobby. Good luck with your collimation.
A few months on, I have been able to take images like this - which I am delighted about.