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My take on GSO Ritchey-Chrétien collimation

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#101 Stonius

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Posted 26 July 2021 - 10:03 AM

Here's what I do; maybe it'll help some people;

 

  1.     Put the Howie Glatter in the eyepiece holder (single dot mode, we're not up to the hologram rings thing just yet). Use the focuser plate to  centre the red dot in the secondary donut. Yes, this may not be the optical centre of the secondary, if it's the geometric centre that's enough to align the focuser more or less with the centre axis of the OTA. manipulating the tilt of the secondary doesn't move the donut so we can say that the focuser - donut (and to a reasonably accurate degree the OTA) are aligned, That's all we can say at this point - who knows about the primary and secondary.
  2.     Now for this bit, I prefer an orion cheshire laser. Blasphemy, I know! But look - under the sticker there are three collimating screws and I collimate mine to match the Howie Glatter. Chuck it in the eyepiece holder and adjust until the laser bounces right back into the hole. (fig 1 - you can see I've replaced the grub screws with more accessible tech screws). At this point, you have the focuser aligned with the secondary, both of which are reasonably closely aligned with the tune itself.
  3.     Now for the last part. I read a lot about projecting the Howie Glatter hologram rings (if you have that attachment) onto a nearby wall. The thing is, if you're in the bush, white walls are in short supply. So I just cut a circle of diffusion gel such that it fits over the spider knobs (fig 2). Diffusion gel is plastic and transparent and non-flammable and you can get it from professional film lighting shops like John Barry. I used Lee Filters 216 'White Diffusion'. You should be able to buy it in smaller squares rather than a whole roll. Or, you know, improvise. It really just needs to be translucent like frosted glass - it needs to diffuse the light that hits it. Maybe you could get one cut from some kind of rigid plastic and it'll probably work even better than mine. Anyway, now the projected rings are showing you the projection of the light path through the scope. Use the Primary adjustment screws to centre the rings and you're done. It's best to only use two screws to get it in the right spot so you don't change the focal length of the entire system because that would be a bad and not very good thing.
  4.     Now here's the thing; remember how I said the donut may not be the optical centre of the secondary? If a star test shows this to be the case, you have to revisit the focuser alignment. The secondary doesn't move much - it more 'rotates around a central axis' so if your focuser needs to be looking at a different part of the secondary, you need to readjust focuser tilt. Adjust it in the direction of misalignment, then repeat steps 2 & 3. If it's worse, you adjusted the wrong way (make a note of the correct direction for nest time). Artefacts and reflections in between the rings can help you spot that something isn't right, so keep an eye out for these (fig 3). By far the most critical adjustment is the focuser adjustment. Tiny errors make much larger errors in the Secondary and Primary Mirrors that are difficult to adjust and result in a lot of tail-chasing.

Anyway I hope that helps someone.

 

Cheers

Markus

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Edited by Stonius, 26 July 2021 - 10:04 AM.

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#102 TinySpeck

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Posted 26 July 2021 - 10:52 AM

@Stonius -- thanks for chipping in; there are some clever ideas in there.  Your photos show why I don't like lasers these days, though.  The reflection from the secondary is a big misshapen blob so centering it is really a crap shoot.  And look at those hologram rings -- they're thin in spots and thick in others, ultimately due to the semiconductor laser bar shape being stopped down to a dot only approximately.  It makes it hard to judge centering.

 

Also, have you checked that your hologram is collimated to the laser?  When I turned mine in the focuser I could see the hologram wobble around its axis, even though the plain laser dot didn't.  (This is also more difficult to judge with varying band thickness.)


Edited by TinySpeck, 26 July 2021 - 10:53 AM.

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#103 Stonius

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Posted 26 July 2021 - 07:59 PM

@Stonius -- thanks for chipping in; there are some clever ideas in there.  Your photos show why I don't like lasers these days, though.  The reflection from the secondary is a big misshapen blob so centering it is really a crap shoot.  And look at those hologram rings -- they're thin in spots and thick in others, ultimately due to the semiconductor laser bar shape being stopped down to a dot only approximately.  It makes it hard to judge centering.

 

Also, have you checked that your hologram is collimated to the laser?  When I turned mine in the focuser I could see the hologram wobble around its axis, even though the plain laser dot didn't.  (This is also more difficult to judge with varying band thickness.)

Hey Gerrit, thanks.

 

Yes, I agree the only perfect test is a star test. I figure this one allows you to get close in a 'bench test' environment before heading out which has the advantage that only small adjustments will be necessary in the field. As you know, large adjustments make stars move out of the field of view which can be frustrating with recentering all the time.

 

There were a lot of steps I omitted for the sake of simplicity. Yes, you should check the focal length of your scope first with a platesolve. Checking the alignment of the lasers by rotating is a good idea. Doing the same with the focuser rotation and extension is also worthwhile. It's even worth having a look with a Cheshire to  confirm you're on the right track. Lots of double checking can be done to give confidence in the results.
 

And yes, there is the 'fuzziness' of lasers, but I think its within an acceptably small tolerance. Once under the stars, I guess you can use the on axis/off axis astigmatism DSI method. But in a bench test environment, I can't think of anything that delivers more accurate results.

 

Cheers,

Markus



#104 TinySpeck

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Posted 29 July 2021 - 07:21 PM

I'm now happy with my new no-star-test, no-laser collimation technique, which I've posted here.  Take a look if you're interested in the next step.



#105 nebulachadnezzer

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Posted 29 July 2021 - 09:17 PM

I'm now happy with my new no-star-test, no-laser collimation technique, which I've posted here.  Take a look if you're interested in the next step.

Looks like a reasonable process, and I definitely like the elimination of the lasers.

Is it worth noting that the diagram showing the Cheshire reflection isn't exactly what you'll see through the Cheshire? At least if I recall correctly you won't see the *outer* edge of the primary as depicted without removing the baffle, which is a procedure I don't recommend anyway. Your notes and call-outs are correct, but that one aspect of the illustration may confuse someone.

There's always the risk that the Cheshire is off-center. I can't decide if the rubber gasket compression style self-centering adapters are a good or bad thing. I have the Hotech one that came with a laser but I've put a Cheshire into it before. I tend to prefer a close-tolerance milled aluminum 2" to 1.25" adapter that came with one of my Moonlite focusers, but still nothing is perfect.

I'd like to see a Cheshire with a threaded front so I could screw it into the focuser (with appropriate adapters). That's one somewhat nice thing about the Russian-made adapters you can buy for the Takahashi scope, which is sort of a magnified concept of the same thing (but unfortunately also prone to internal tilt in my experience).

I haven't seen the Hall of Mirrors effect before. I will definitely look at it next time I'm staring down the business end of one of my RCs. (Weather permitting I'll be playing with the SCT again this weekend as Saturn approaches opposition.)

EDIT: Couldn't wait. I just looked own the front of my RC10 and it looks good. However, upon re-reading that section of your process I needed some clarification. You wrote, "Tighten a primary silver screw (CW) to pull the reflected 'tube' toward it." Could you explain which tube is being pulled where, and what screw would correspond to that? I *think* you are saying that if you see asymmetry in the Hall of Mirrors that an adjustment is required to make it symmetrical but I'd appreciate it if you could elaborate on that step.

Thanks for putting this together!


Edited by nebulachadnezzer, 29 July 2021 - 09:26 PM.

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#106 TinySpeck

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Posted 29 July 2021 - 10:58 PM

Is it worth noting that the diagram showing the Cheshire reflection isn't exactly what you'll see through the Cheshire? At least if I recall correctly you won't see the *outer* edge of the primary as depicted without removing the baffle, which is a procedure I don't recommend anyway. Your notes and call-outs are correct, but that one aspect of the illustration may confuse someone.

There's always the risk that the Cheshire is off-center. I can't decide if the rubber gasket compression style self-centering adapters are a good or bad thing. I have the Hotech one that came with a laser but I've put a Cheshire into it before. I tend to prefer a close-tolerance milled aluminum 2" to 1.25" adapter that came with one of my Moonlite focusers, but still nothing is perfect.

I'd like to see a Cheshire with a threaded front so I could screw it into the focuser (with appropriate adapters). That's one somewhat nice thing about the Russian-made adapters you can buy for the Takahashi scope, which is sort of a magnified concept of the same thing (but unfortunately also prone to internal tilt in my experience).

I haven't seen the Hall of Mirrors effect before. I will definitely look at it next time I'm staring down the business end of one of my RCs. (Weather permitting I'll be playing with the SCT again this weekend as Saturn approaches opposition.)

EDIT: Couldn't wait. I just looked own the front of my RC10 and it looks good. However, upon re-reading that section of your process I needed some clarification. You wrote, "Tighten a primary silver screw (CW) to pull the reflected 'tube' toward it." Could you explain which tube is being pulled where, and what screw would correspond to that? I *think* you are saying that if you see asymmetry in the Hall of Mirrors that an adjustment is required to make it symmetrical but I'd appreciate it if you could elaborate on that step.

Thanks for putting this together!

Hello @neb!  Would you mind posting this on the new thread?  It all pertains to that.  It's getting late here now, but I'll reply there tomorrow.



#107 nebulachadnezzer

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Posted 30 July 2021 - 09:52 AM

Hello @neb!  Would you mind posting this on the new thread?  It all pertains to that.  It's getting late here now, but I'll reply there tomorrow.

Done, with a few edits.




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