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SCT collimation screw not turning smoothly


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Hi all,

Many hits on SCT collimation, but I can't find any info about the possibility to break the mirror bracket by over-tightening one of the collimation screws. With my scope (C5+), one of the three screws doesn't turn smoothly, and I wonder if this is caused by over-tightening in the past.

Would be interested in a description or proper drawing on how exactly the secondary mirror is mounted, and how the screws tilt the mirror.

Regards,

Han

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I can't help with drawings, but I took a snap of my secondary when I took it out a while back. As you can see, the mirror back is pretty substantial.

If I were having trouble with one of the screws I'd buy a set of replacements and get them swapped over ASAP - one at a time obviously.

image.png.dbcd6ab717172f5a951614c24ebfe375.png

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Thanks a lot for the picture. Is that from a Celestron SCT as well? Looks pretty straight forward. A pivot in the center and three bolts just threading into the aluminum base of the mirror bracket.

Could be the thread is slightly damaged. I may have a look at it when I replace the bolts with Bobs knobs. Then again, I can still collimate the scope, so it’s not really broken, and if it’s not broken, don’t fix it.

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When you say it doesn’t turn smoothly, is that just when tightening, or also when loosening? The secondary pivots around a central pivot point. If you overtighten the screws it is possible to buckle the pivot as it’s plastic. I have seen a thread with some pictures so will try and track it down.

When I collimate my SCT, if the screw I need to tighten is already tight, I loosen the opposite screws slightly so that I can tighten the one I need to. I then nip them all back up tight after. I have read some instructions that say you should only ever tighten the screws but can’t see how that could work and eventually something will break.

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Not sure this is the thread I was thinking of, but it does sound like a similar issue and there are some photos of the holder and adjustment mechanism, as well as how it can buckle. 

https://www.cloudynights.com/topic/465107-c8-collimation-at-limits/

I’m not suggesting you have a problem with yours, it might just be a case of loosening one of the others to allow you to tighten the one you need. Just be mindful that the screws are the only thing holding the secondary on, so only loosen one at a time and make sure you haven’t unscrewed it fully.

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On my C8 I loosen some and tighten others keeping the tightness of the three screws even and not too tight.

As well as breaking something if they are too tight I worry about potentially deforming the secondary and I introducing abberations.

 

Edited by Paz
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Thanks all. Indeed, you can't keep tightening them without loosening others. I think maybe there's just a little burr on the thread, it's not going smooth either ways. But like I said, collimation isn't a problem so I'll just keep it as it is.

If I ever take it apart I'll fix the thread by tapping it again, or putting in a helicoil.

But thanks for all the replies it at least clarifies the mechanics.

Han

Edited by Hanah
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