GSO RC mirror cell tilt?

[E_ri_k]

Hi all. I had a thread going quite a while ago, where I was getting conflicting results between my TAK collimating scope, and star testing, with the RC10, in regards to the secondary spot.

In the end I gave up and trusted the star test collimation, and I think the images I got were pretty reasonable, although there were focus issues towards the edge of the field, which got me thinking.

I am re-investigating the initial problem at the moment, and trying put a bit more effort into finding out what is happening.

Whilst the star test looks correct, the TAK scope doesn't agree in regards to the secondary spot, and vice versa.

I have discovered, whilst rotating the focuser 360 Deg, and looking through the TAK scope, the FOV I can see from the reflection, moves in a circular, no concentric pattern, like a wheel on an axle which is not perfectly centred. If I use my laser collimator, and do the same thing, the laser spot never centres on the secondary, and follows a non concentric path around the secondary spot. I have measured the secondary spot, and I'm sure it is correctly placed.

I find the same results weather using the Moonlite focuser, or the stock focuser. I'm sure that the male threads for the focuser are part of the mirror cell. If so, would it be reasonable to assume that the mirror cell may be tilted, or not centred somehow?

In my mind I am picturing that I have collimated the secondary (by star) to suit any tilt present in the primary cell. That is why the TAK scope would show an error. If I have, would a star test would show miss-collimation towards the edge of the field? I haven't been able to test for this yet.

Does this theory sound plausible, or are there any other suggestions out there?

If so, how would I go about adjusting, or checking for this. Could I use the Moonlite's collimation screws to compensate for this?

Erik

[Paul Haese]

When you say a non concentric pattern around the center spot, do you mean an oval or something else?

If it travels around the center spot then it is most likely not tilt of the primary. If it is like an oval with one part being very near the center spot and the other part of the elipse way off then this could be tilt.

When you use the Tak scope though and you get concentric rings this indicates that the primary and secondary are orthogonal to each other. One of those rings is the primary.

You have to center the primary as part of using the Tak Scope. You might like to check Ken Crawfords site for the video on how to sort this out.

[E_ri_k]

To me it looked like a circular motion, but I'd have to verify that. I will check that video out, thanks.

I understand that all the rings need to be concentric when viewed through the tak scope. I have had trouble getting enough light into the diffuser and in my working environment in general to clearly see the rings.

I'll have another crack tomorrow.
Erik

[E_ri_k]

I watched Ken's video, I think I have seen it before actually. All that is very straight forward.

I have some photos of what I am seeing through the tak scope. I have the "Tak Dot" centred, could maybe do with a whisker to the left? I can't see the light annulus through it unless I grossly miss-collimate the primary, so my reference is the secondary mirror baffle.

This is the procedure I followed originally, expecting a star test to look somewhat good, however, it showed a large error. This is where the conflict arose between the two methods, where really, it shouldn't?

[Paul Haese]

Even the image on the left looks out of collimation. The outer ring with the large black ring is not concentric. That ring is where most of the problem is I think.

[Paul Haese]

Just a thought, is the actual secondary assembly in the center? Just measure from one side to the other on all four vanes.

[E_ri_k]

I measured the secondary the other day, but just did again. I am getting 76mm on each one.

Is the green arrow the non-concentric part you are referring to? I thought that was just light leaking into the camera, as I has with difficulty holing it up to the tak scope. Maybe I have miss-identified the reflections I am seeing? But the green dot, is 100% the adaptor for the tak to screw into the moonlit, because of the smudge marks on it.

I have been looking for a concentric pattern between the Pink, and Blue parts. So the primary baffle and the secondary baffle(as I cant see the light annulus" through the tak scope.

Rotating the focuser will through the concentricity off in regards to each part.

[E_ri_k]

I had another look with with the camera to try and identify that outer ring. It is there on the cameras LCD, but I cant see it through the scope....until, I pulled the focuser of the tak scope out so far it nearly came off. I can just make out some kind of blurry light ring, and it it matches the photo, and is not concentric. I thought I should be focusing on the central part of the image. The part contained with in the white portion of the image?

[ReaPerMan]

I have had simular problems with my RC10 and as I don't have a Tak scope ive have been using a Catseye and a Hotectch laser. I find that if i get the sight picture near perfect with the cats eye (like your picture 1, I get a terrible return from the laser. If I get a perfect return from the laser it almost looks like your picture number 2. In the end I used David courtner's hall of mirrors technique and star testing. This has got me the closest to full collimation and I gave up looking at the rings. CCD inspector tells me i'm on and the rings don't agree.

all the best

Paul