127 Mak Collimation

[JamesF]

Has anyone actually ever done this themselves?

I've recently had cause to attempt it on mine as I had to take it apart. An initial star test showed the diffraction rings were offset when slightly out of focus, but by eye I really couldn't see anything wrong with the collimation (using a makeshift collimation cap).

Next time the sky was clear I took it out again and using Procyon did a star test to get the offset diffraction rings. I adjusted the mirror until the rings looked perfectly concentric and that, I thought, was that.

I have noticed however that when viewing Mars, if I adjust the scope until its out of focus and I get a bright image of the primary mirror with a dark circle in the middle, the dark circle is definitely slightly offset.

I'm wondering if perhaps I have been fooled by the atmospheric conditions and the rings I had on the star test after all my adjustments were still off centre, but not by enough to be visible given the slight shifting of the rings due to atmospheric distortion. I'm thinking about making up an artificial star and using that for collimating instead.

Thoughts, anyone?

James

[SiriusB]

Duno if this helps any James?

Makes interesting reading anyway.

http://stargazerslounge.com/discussions-scopes-whole-setups/136866-maksutov-focus-failure-help.html

[JamesF]

Thanks for that. Interesting reading.

Because I'm an inveterate tinkerer this afternoon I completely disassembled the Mak, discovering in the process that the baffle tube didn't appear straight. I could get the primary collimated nicely based on viewing the mirror from the front, but looking through my improvised colli-cap it was clear that whilst the mirrors were as close to right as I could tell, I could see more off the baffle tube on one side than the other. I've refitted it and it seems much better now. Next step is to do a star test.

I also discovered that the OTA is actually just a tube that is threaded internally at both ends. The corrector/secondary assembly screws in at one end, and the mounting plate for the backplate and baffle tube assembly screws in at the other. The mounting plate doesn't screw in all the way however. It's just screwed in until it's about flush with the end of the tube and has a fair bit of play at that point. It is totally reliant on the pressure between the seal, the backplate and the end of the OTA to hold it in place. It seems a fairly naff piece of design to me, but it does have the benefit that by loosening the three allen screws holding the backplate to the mounting plate the entire assemly can be rotated to leave the focuser knob in whatever position you'd like it. Obviously collimation will be required afterwards.

For anyone who is interested, the primary mirror is mounted on an externally-threaded tube that slides up and down the baffle tube as the focus is changed. There's a flange on the tube that the front of the mirror presses against and it's held in place from the back by the focuser bracket (which is threaded to fit the tube). An o-ring is between them. A locknut screws down against the focuser bracket to stop everything coming loose.

The focuser bracket is "teardrop" shaped, with the "fat" end behind the mirror. The focuser knob turns a threaded rod that pushes or polls the thin end to move the mirror assembly up and down the tube.

Anyhow, more news when I've done another star test.

James

[JamesF]

Well, having had the scope apart several times, cleaning lots of bits, removing the slack from the mirror retention mechanism, realigning the baffle, and then starting collimation from "so far out that I could see around corners" using both a home-bodged collimation cap and just working visually by gauging the symmetry of the mirror reflections when viewed from the front, last night I finally got to do a star test. Procyon was handily placed and suitably bright to be usable through the drifting cloud.

Here's what I now see:

If I've understood what I'm doing properly, that's pretty close to what I'm after. I can't claim it's perfect, but I reckon it's not far off and if I want to do better I'll need an artificial star so I can work without atmospheric effects interfering with the process. It looks to me like there might still be a tiny bit of adjustment required vertically.

There's also still a very small amount of mirror-shift when the direction of focuser travel changes, but I think that's inherent in the design of the scope. The focuser spindle doesn't move the mirror directly, but pushes on one end of a 50mm long lever that screws up to the back of the mirror. Given an asymmetric design offset as much as that I think it's going to be very difficult to avoid some degree of shift. From the point of view of imaging it only really starts to become a nuisance when I get out to about f/35, which is probably getting towards the limit for the little Mak anyhow.

James