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AT12RCT Primary Mirror Clips: Why?

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#1 Monkeybird747

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Posted 19 January 2020 - 11:07 AM

This truss RC has 6 mirror clips, in addition to a central locking collar. The locking collar does not make contact with the mirror (a 3mm or so gap) and does not employ any type of o-ring. I’ve not seen this in an RC design before, and even the tube versions of the GSO RCs just use o-rings and the locking collar without clips.

The clips influence the star diffraction pattern. Is there any reason I shouldn’t just ditch the clips and put a rubbler o-ring on the locking collar myself and thread it back on? I mean the collar will prevent the mirror from falling out, so what is the point of having clips? The question there would be how tight should the locking collar and o-ring be? Should it even make contact with the mirror, or be spaced with a sheet of paper or similar?

 

Pictures of the cell and the locking collar with an o-ring I had lying around as a test (I’ve sense found a better one). There is also a picture of a mock-up of a custom nichrome heater I was thinking of building to place behind the primary on the cell (I’m not ready to glue something I built to the actual mirror). The purple paper is the shape the heater will take on, and the wire will be sandwiched between two pieces of high temp silicon sheet glued together with RTV.

 

Thoughts appreciated.

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#2 charlesgeiger

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Posted 20 January 2020 - 02:18 AM

Seems  like a lot of clips for retaining a 12" mirror.  You do have a nice 18 point flotation system.  You might want to reduce the number of clips from 6 to 3 evenly placed.  You may also want to blacken the ends of those three clips that appear shiny to me.  So with this in mind, you might want to take the cell out and do as you say and reduce the clips (darkened) to three and see how stable the mirror seems in the cell when you have it tipped up at 90+degrees.  If you have any second thoughts, you can always add back the three removed (but blacken the ends also).

Charllie



#3 Monkeybird747

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Posted 20 January 2020 - 04:17 AM

Thanks again Charlie. I ended up putting the clips back on for now. As a compromise with myself I shimmed the clips so they don’t protrude into the light path quite as much.

The o-ring project isn’t panning out. I collimated with TAK collimation scope with baffle removed. When I threaded in the baffle the primary was off. The baffle threads on to the locking collar, and the locking collar need to be fully seated so that it squares itself up on the cell. With the o-ring installed on locking collar it wasn’t threading fully into the cell behind the mirror. As a result the weight of the baffle tube was being partially supported by the mirror and o-ring.

I put things back to stock and collimated again with baffle tube removed. Between the Glatter and TAK collimation scope everything is spot on. But when I threaded the baffle on the primary was off again. But that doesn’t make sense, because it doesn’t make contact with the primary in any way. And also the spider alignment didn’t change. Only the light annulus was affected. So now I’m wondering if my baffle tube isn’t square or not seating properly or something. It was off in a way 90 degrees to gravity, so it’s not sagging or loose. Very strange.

Edited by Monkeybird747, 20 January 2020 - 04:18 AM.


#4 charlesgeiger

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Posted 20 January 2020 - 07:40 PM

Well, if you can again remove the baffle tube you may want to roll it on a flat surface and see if it oscillates as it moves.  It might simply be out of round.  If it is, than you can possibly shim it.  If it is not out of round, then threads may be out of wack.  Just a thought.



#5 Monkeybird747

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Posted 20 January 2020 - 09:14 PM

Well I think I'm on to the problem. I can add some pictures later to hopefully help someone else out. If I collimate with baffle tube removed everything looks great. Concentric rings through Tak scope for primary, spider vane reflections lined up perfectly, secondary dead on, and focuser firing dead center of secondary. Several iterations of laser/tak scope/laser to be sure. Then baffle tube goes on and it looks way off.

 

If I collimate with baffle tube installed the spider vanes won't line up when the rings are concentric. 

 

I started examining and measuring everything I could. What I noticed is that one of my primary mirror push screws is almost twice as far out than the other two (+2mm or so). So I marked the axis of error of the baffle misalignment, and what do you know it is directly along the axis that collimation screw controls. 

 

What I think is going on is the mirror is not sitting in the 16 point floating cell mechanically aligned with the collimation plate that is is connected to. The baffle is mechanically connected to the rear collimation plate (not the back plate that the focuser mounts to) and makes zero contact with the primary. So when you adjust the collimation bolts to align primary you are essentially correction for this mechanical misalignment. 

 

In short (well, short-ish), the baffle and mirror are not mechanically aligned. To fix this is a slightly scary prospect, as it would require shimming up the primary or making adjustments to the floating cell so that is is mechanically aligned with the baffle tube. This would be extremely iterative, as you have to fully disassemble and pull the primary, make adjustment, reassemble, and then check with laser rings or tak scope. I'm not sure this is a road I want to go down, although I've gotten proficient at disassembly after going round and round with the clips on and off. My concern would be causing the mirror to not be supported evenly. I estimate the error between baffle and mirror to be roughly 2mm.

 

I did look at possibly shimming the baffle, but there isn't enough play to center it, and it is a tight fit in the central ring it threads in to. No gap to wedge a shim. I have the scope with good garage collimation with baffle removed, and will star test tomorrow night. 




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