Below is the five step process that I used to collimate my 10” Astrotech RC. Obviously, the solid tube design may be a bit harder to work with, but the principles remain the same. This thread is to fill in the gaps that I did not understand until many nights of testing. No tools are needed, but, a Cheshire eyepiece or some foil with a pinhole may be helpful. This is not MY method as it uses many notes from the great folks here on CN as well as references the DeepSkyInstruments guide to collimating RCs. This is simply a combination of steps I found to work well. It is worth noting that it has been a while since I have done this and I welcome those who have more to add/object.
Collimating an RC is “easy” as long as you are methodical and do not turn screws aimlessly. I currently work in an experimental optics lab as an undergraduate student. When building optical setups, many times it is the case that a bad initial alignment will be detrimental to your end goal no matter the time spent trying to "clean" it up. If the first step in the alignment is not performed properly, the remaining 50 steps will be affected as well forcing you to start from the beginning (perhaps I have too much experience in this regard........). Eliminate as many factors as possible to give yourself the best chance of achievement your goal. Start with a solid base and the rest should follow.
*this assumes collimation needs to be redone entirely*
1. NUMBER 1 is KEY in saving time when collimating the system:
Ensure all three adjustment screws on primary and secondary are at equal turns.
-Assume the geometry and build dimensions of the scope are “acceptable” in placing your mirrors where they need to be regardless of looking through the optics. Meaning, if your collimation screws are protruding from the secondary assembly/primary plate at equal lengths with respect to each other, the mirrors should be “close” to being aligned. This basically gets rid of the screws in a sense and forces the mirror alignment to conform to the optical tube assembly. For GSO, this is a bold assumption but is usable.
2. Adjust the secondary mirror: Perform this outside or in a highly lit environment.
If looking through the focuser at the secondary mirror, the image of your eye (literal eye) should be in the center of the mirror.
-It is a mirror. If the reflected image does not reflect back to you, obviously, something is wrong. This is where the foil with pinhole or Cheshire eyepiece becomes handy. Your eye can be a bit hard to make out but a bright surface will be easy to see in the reflected image.
3. Adjust the primary mirror:
Using a star field, adjust the primary mirror for ON axis de-focused star shape.
-This comes directly from the DSI method. Don't worry about which screw to turn. Just turn one of the three adjustment screws by a small amount and observe the affect. If it does not produce the outcome you expect or want, undo what you just did. Move on to the next screw and try again. Repeat until the defocused star seems concentric (or as close as you can get it). Again, this is ON axis or at the very center of the image.
4.
Adjust Secondary for OFF axis star shape.
-Also from the DSI method. Turn screws in a similar fashion as you did with the primary. Adjust-check-undo if needed. I found being near focus was more beneficial at this step as it more closely represents what you will be seeing in an actual image.
5.
Repeat step 3 and then 4 (and back again) until satisfied.
This is obviously not the end-all be-all problem solver. Practice is your best friend. The steps above are simply a baseline and procedure to ensure the adjustments you make are "meaningful". The DeepSkyInstruments method is a great source and has many images that correspond to the steps in this guide. As stated above, please add or correct any of this if desired. While this method may work for me, there is always more to learn and can prove more beneficial. I'll likely come back and correct any spelling errors/wording but for now I am pressed for time.
Hope it helps,
-Josh
