Considering the almost universal frustration encountered while collimating these scopes, perhaps you can share your insights about where the miscollimation is coming from? What have you figured out that we have not such that a good single dot laser does the trick?
There are finite degrees of freedom we’re dealing with here. One the z-axis, we have primary to secondary distance (focal length) as well as focuser to primary/secondary distances (focus). On the x and y axes we have focuser to primary, focuser to secondary, and primary to secondary alignment/centering versus a perfect 0,0.
Let’s assume for a moment the primary and secondary are properly spaced (verifiable by pixel scale analysis). This will give a focal length of 1375mm for an RC6 and 1600mm for an RC8.
if my focuser/primary mirror are throwing a correctly aligned laser dot reflection right back onto the laser dot viewed on a white surface at an appropriate distance. the primary and focuser are aligned in x and y.
if the primary/focuser pair hits the center dot of the secondary with a laser beam, then the focuser/primary are aligned with the center of the secondary in x and y.
finally if the secondary then throws the laser reflection right back onto the laser, then both the secondary center and the secondary tilt is aligned with the focuser/primary in x and y.
EXCEPT… It doesn’t work and many of us are ripping out all our hair, tossing our RCs in a closet, spending some months growing new hair, bringing our RCs back out of the closet for another go… lather, rinse, repeat.
If anyone has a method that works and are able to explain so that others may reproduce the results, that’s the holy grail. So far, we’ve got a lot of talk but no holy grail.
I di not want to hijack the thread but if it is of interest to you or somebody else, I will try to explain what I have learned.
This video https://youtu.be/a3UOGDUaq6o from my post #49 above gave me the idea. The trick is to collimate and fix the secondary first and use it as a fixed reference for the primary.
But that is the main problem with collimating (my) RC: the lack of a fixed reference to stablish the optical axis of the scope. By that I mean a line that is orthogonal to the center of the secondary and coaxial with the focuser. I did a lot of cheshire collimation and always ended up with perfect circles and bad pictures. So that was the first thing I had to solve.
In the video, a tilt plate, the focuser and a laser is used to align the focuser and secondary together first. But I do not have a tilt plate and I do not want one as collimation with it fixes your camera rotation at that angle. At any other angle the sensor will be tilted.
To align the focuser, I used a 1.25 HG laser and put it thru the center hole on the secondary holder. However, in the video you also see that the first step is to align the edge of the primary with the scope tube. As you know, on these RCs the GSO focuser is attached to the back plate. So I "reset" the primary all the way back. I actually took the scope apart and checked that this would put the plane made by the edge the primary perfectly parallel with the back of the M91 thread where the focuser is attached. I put the primary assembly on its back on top of the most flat and leveled surface in my house (the kitchen counter) and took measurement. Since the threads are made in a lathe (probably CNC) I know I can trust perpendicularity/parallelism. In doing this, when I align the bright outer ring of the primary with the tube, the laser should go thru the center hole of the secondary ... and guess what: it did. But even if it did not, I would have tilted the primary a little bit to get the laser thru the center hole of the secondary. That is the reference I need to get the actual reference (the secondary) spot on in place. Done with focuser for now.
For the secondary now.
The first thing I did not like about the RC designs is that the secondary which requires tiny little adjustments is mounted on springs that gives an infinite amount of DoF. It makes no sense to me to have all that freedom and then have to control it with coarse thread screws. I needed to make a flat surface to butt the secondary against. That surface will also put the secondary very close to centered on the mechanical axis of the scope which will also make it easy to align with the focuser now aligned with the hole that supports it. But also, that surface had to allow for the tiny little adjustments and only tiny no matter what while still firmly holding the secondary. A rubber surface of some sort would meet the requirements. It acts as springs keeping positive tension on the bolts and still a flat. Rubber washer about 1" OD x 1/8 thick from the hardware store to the rescue. My scope (bought it used) had not spring on the center screws. Great! that encouraged me to "fix it". So, with the washer in place, I retracted the secondary collimation screws and snug the secondary against the washer. I put the laser in the focuser and with my hand/fingers I directed the beam on the secondary back to the laser while peeking from the front. Then adjusted the collimation screws to keep the beam on on the laser. When the two bright spot fall on top of each other, the secondary is collimated and as the video says you do not touch it again.
And yes, I can hear the reader asking "how are you going to adjust the FL". Well, in my opinion based on what I have learned, the right way to adjust the FL on these RCs is actually the big, fine thread and the retaining ring you see there where the secondary mirror assembly screws on to. When you use that you can change the distance of the secondary very accurately (fine thread) without disturbing your collimation. That is the same mechanism on guide scopes such as the Orion Mini and many others where the front element moves up and down.
At this point, I was ready to collimate my primary which now requires very little adjustment. I did not use the HG laser concentric circles here. The accuracy of those methods that use visual assessment of the roundness/concentricity depend too much on visual acuity that I do not have. So I use an iterative combination of reflection of the laser laser beam for the primary and cheshire for the focuser. For the focuser I only care to about putting the center spot of the cheshire back on the center donut of the secondary. For the primary, laser beam fall back on its own.
I again hear the reader saying "what about the GSO focuser issue". Well, in my opinion based on what I have learned, my GSO focuser has no issues. The problem is how you sit the focuser on the scope flange. Mine has a conical flange end and that is a "built in" tilt plate. By design or otherwise? I do not know. But in order to put the cheshire dot back in the center on this step, I place the scope face down, loosen the M91 collar, put a flashlight on the 45 deg cutout and center carefully while retightening the collar. When tightened and centered, I go back to the laser for the primary. Then I repeat until I can not improve the situation anymore.
An the result is what you see on the picture I uploaded. I went outside and the first out of focus start field was very clear to me I could no improve the results. So the picture you see went from my table to capture. No other adjustment were done.
Another very important thing to fixe before collimation is nosepiece tilt and that is well known. I bought a HG Paralizer with a T tread (not the one for eyepieces with a srew) and that works great.
Sorry for this long post but I do not how to explain it any shorter. The devil is always in the details.
I hope this helps you or someone else with the "universal frustration" . lol
I personally would not like this hobby without its complexity.
Let me know if you have questions.