A few months ago I purchased a Celestron C11 EdgeHD SCT scope together with a Moonlight Edge focuser. I knew that collimation of such a long focal length scope would be critically important. I convinced myself that purchasing the Hotech Advanced CT Laser Collimator for $455, a substantial sum for a calibration device, would be worthwhile. After much effort, I have used this device to perform a good collimation of my scope. This posting shares my learnings. Please keep in mind that these learnings are specific to my setup.
The device itself is simple and is well designed. The challenge is using this device properly, and unfortunately the Hotech instructions fall a bit short in that regard. Hotech has put quite a bit of work into the documentation - you will find various Hotech manuals (v8 and v9) and YouTube videos on-line. All have pieces of useful information, but none of them had a complete solution for me. My first collimation attempt, following the v8 instructions included with the collimator to the letter, resulting in a very poor star test that night. I knew that the problem wasn't with the device but with how I was using it. I wasn't going to let my $455 go to waste, so I did some more reading, research, and experimenting, and I was able to piece together a procedure which works. I don't think that most people have the time to do something similar, so I am posting the results here with no promise that it will work for you.
This is the most important thing: Most people (and much of the Hotech instruction) focus mainly on the alignment of the 3 screws of the secondary mirror. If this is the only alignment that you check, the collimation will not likely pass a simple star test. Two other alignments also impact your collimation - the tilt of your focuser, and the centering of your secondary mirror. All of these interactively work together to determine your collimation result. The beauty and simplicity of the star test is that it incorporates all 3 of these effects, although a star test can only help you adjust the secondary mirror to get the best result. In most cases, alignment of the secondary mirror alone through a star test will be sufficient to get good results. The beauty of the Hotech system is that, if done properly, it aligns not only the secondary mirror, but also the focuser tilt and the secondary centering. Alignment of all 3 of these results in the best collimation.
- Check your collimator
The Hotech collimator is not some magical, black box device. It is "just" 4 lasers in parallel, but they have to be lined up exactly. Check this by photocopying the face of the collimator and putting the copy up on a wall some distance from the collimator. Point the collimator square at the copy and check if the lasers line up with the copy holes. Mine lined up well.
- Optional - Create a jig
Hotech is right in that setup to achieve perfect alignment of the collimator is 90% of the work. This is a tedious and frustrating process using the recommended telescope/tripod approach, especially since for most people the telescope has to be horizontal. After messing with that twice, I decided to craft a jig for $10. This jig enables much quicker and consistent alignment, at my telescope's normal elevation. I attached a thin steel bar (36"x1/8") to the dovetail plate of my scope, and then attached the collimator holder to the steel bar. I took apart the original collimator holder and reassembled it as shown in the image. The holder adjusts for up/down tilt and left/right tilt. The bar can easily be moved left/right, and I use weights at the end of the bar to adjust the elevation as needed. The distance from the dovetail bolt to the holder bolt is 20-3/8" and never needs to be changed.
- Align the collimator
Turn on the collimator to mode 1. Adjust the bar and collimator holder until the outer crosshair tips from the primary mirror reflection are centered on the same ring of the collimator. This needs to be done accurately. Keeping the outer tips are centered, adjust the collimator so that the crosshair lines are centered on the printed cross on the collimator. This will take some time, so don't rush this process immediately before a night's imaging.
- Check the secondary mirror centering Check the inner tips of the crosshair to make sure that the secondary mirror shadow is centered. If not, use the Hotech guidance in the procedure to center the secondary mirror.
- Align the focuser Remove the secondary mirror and install the reflector mirror in the focuser. Turn on the collimator to mode 2. The task now is to center all 3 crosshairs - the primary mirror reflection seen before on the collimator, the new reflector mirror reflection now on the collimator as well, and the crosshair seen on the reflector mirror in the focuser itself. My Moonlight focuser had 2 methods of tilt adjustment - I used the cam adjustment.
- Perform collimation Install the secondary mirror and adjust the 3 secondary mirror adjustment screws until the three large reflected collimation spots are on the same collimator ring. Check to make sure that each of the much smaller red dots are hitting the collimation holes. In the focuser, the center of the 3 dots should be at the center of the reflector mirror. Numerous iterations will be required, especially since you have to keep checking to make sure that all of the above alignments from the previous steps still hold true.
Done properly, your collimation will be very close when starting a star test. A star test is still required to make any minor final tweaks (lasers are not perfect, and neither is the alignment process, while star light always travels perfectly!). Following my most recent Hotech collimation this week, I had to adjust two screws each by 1/32 a turn during the star test to achieve final "perfect" collimation.
If you are reading this because you are thinking about buying this Hotech device so that you can quickly and simply calibrate your SCT, my advice for you is to stick with the simple but very effective star test instead. I star test by using short exposure images of a defocused star and it works like a charm. You just need to take your time and pay attention to detail. The Hotech device is for those who are serious and have the time and money to take the next step. I am happy I bought it for peace of mind that my focuser tilt and secondary mirror centering are now both good, which is hard to verify accurately through other means. But I have to say that I expected such an expensive device would be a bit easier to use, especially after reading all of the Hotech ads and testimonials before purchasing it.