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How to fix mirror flop?

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15 replies to this topic

#1 avodcap7  Happy Birthday!

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Posted 05 December 2020 - 05:54 AM

Hello,

 

I've been doing astrophotography for a year now with the same old telescope - a Celestron C8 (from SE series).

 

This telescope had some issues, and focusing was one of them, but now, every time I move my scope to a new target, it defocuses as much as me turning the focus knob twice. Completely out of focus. There are no mirror locks on this telescope, so I wanted to ask for a solution to fix this issue.



#2 MJB87

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Posted 05 December 2020 - 07:16 AM

Moving to Cats and Casses forum for a better fit.



#3 Sacred Heart

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Posted 05 December 2020 - 08:18 AM

I have a C14,  I had the mirror lock installed by Mike Rice at New Mexico Skies about 15 years ago.  It is a collar that rides on the baffle tube, to lock just turn two set screws with an allen wrench.     http://nmskies.com/newmexicosite.html

              Joe



#4 WadeH237

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Posted 05 December 2020 - 09:48 AM

That doesn't sound like typical mirror flop.  Typical mirror flop will shift the image by up to a couple of arc minutes.  If you are completely losing focus, then something else is going on.

 

It sounds like this was working fine for you, and now it's not.  What changed?



#5 avodcap7  Happy Birthday!

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Posted 05 December 2020 - 10:02 AM

That doesn't sound like typical mirror flop.  Typical mirror flop will shift the image by up to a couple of arc minutes.  If you are completely losing focus, then something else is going on.

 

It sounds like this was working fine for you, and now it's not.  What changed?

I changed nothing, it just decided one day to start defocusing a lot. It's not THAT bad, but if I move the telescope 20-ish degrees it completely defocuses. When it is focused, the focus doesn't change though,



#6 Sacred Heart

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Posted 05 December 2020 - 10:12 AM

You move the scope 20 degrees in dec and it moves??  My Mirror flop mainly moved when I was refocusing and I could see where I lost it on the screen.  My neighbor has a Meade 12 & 10 both shift when focusing.  Joe



#7 jgraham

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Posted 05 December 2020 - 10:14 AM

I have seen similar behavior from several Cats including my C11. So far the fix has been the same; snugging the lock ring that holds the mirror to the baffle. This ring should be snug, but not tight. In each case I found that the lock ring had become completely loose. In the case of my C11 it turned about 1-1/2 turns before it felt about right. I’ve got a C9.25 that’s showing the same problem and I’ll be opening it up today to check the ring.


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#8 bobhen

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Posted 05 December 2020 - 11:29 AM

You move the scope 20 degrees in dec and it moves??  My Mirror flop mainly moved when I was refocusing and I could see where I lost it on the screen.  My neighbor has a Meade 12 & 10 both shift when focusing.  Joe

Mirror flop is different than image shift when focusing.

 

The poster has 2 choices. As others have pointed out you can pull the scope apart and fix it yourself. There are some YouTube videos on how to take apart an SCT. If you are uncomfortable with disassembly, you can send the scope back to Celestron and they will fix it. If the scope is under warranty, they will make the repair and collimate the scope at no cost.

 

My (bought new) year 2000 C11 had mirror flop and I sent it back to Celestron. They fixed the mirror flop at no cost, and since the repair, that scope held collimation for months/years at a time.

 

Bob



#9 WadeH237

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Posted 05 December 2020 - 04:00 PM

I changed nothing, it just decided one day to start defocusing a lot. It's not THAT bad, but if I move the telescope 20-ish degrees it completely defocuses. When it is focused, the focus doesn't change though,

Did this problem occur suddenly, or did it slowly get worse over time?

 

John's solution in post number 7 is a possibility, but if it were that, I would expect it to not be something that shows up all at once (unless the OTA experienced vibration, like being transported over a washboard road).  If it worked one day, but not the next (ie. it started suddenly), then I would have to wonder if the RTV glue that bonds the primary mirror to the carrier assembly has come loose.  That's a pretty rare problem, but I've heard of a few cases of it and it manifests like your symptoms as well.



#10 jgraham

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Posted 05 December 2020 - 05:04 PM

Bingo! I just finished putting my new to me C9.25 back together. It had the classic symptom of not holding collimation and not holding focus after slewing. I suspect that this scope has never been taken apart as there were spots of fungus on the inside of the corrector and the corrector was really stuck! I marked its orientation, sprayed around the edge with Windex, and carefully worked it loose. I reached down inside and engaged a wooden dowel with one of the spanner holes in the lock ring a nudged it. Sure enough, it was completely loose. I could turn it about 1/2 rotation without any resistance. Just a tad more, and it was snug. While I had it open I removed a few tiny spots off of the primary and secondary with lens tissues .(I _hate_ touching the mirrors, but sometimes you get lucky) The secondary also have a bit of fungus, now clean.  I carefully cleaned the inside of the corrector with Windex followed by breath and lens tissues. Before closing the tube I inverted it and used a puffer bulb to blow the dust out, and then carefully lowered the corrector back in place. I didn't like the original tiny spacers, so I cut ribbons of black construction paper and layered them evenly around the corrector until the gap was full (about 5 layers total all the way around). I remounted the rail, reinstalled the finder, touched up the paint, and now it's ready for testing.

 

So far I'm 3 for 3 finding loose lock rings. Tomorrow I'm going to open up a 12" that's showing the same symptom. Gotta do something while its cloudy. :)


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#11 Andrew Brown

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Posted 05 December 2020 - 05:13 PM

If it's an old scope out of warranty then why not do some learning first then open it up?

 

One thing you have to be aware of;  the advice you get on here ie doing this and that, is meant with the best of intentions, but can also leave one high and dry when "something" else happens as a consequence of say, disassembly.



#12 jgraham

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Posted 05 December 2020 - 05:27 PM

Absolutely! The corrector on this C9.25 was really stuck and I was scared to death it would crack or clam-shell getting it loose.

Proceed with caution!

#13 MarMax

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Posted 06 December 2020 - 02:36 AM

Absolutely! The corrector on this C9.25 was really stuck and I was scared to death it would crack or clam-shell getting it loose.

Proceed with caution!

I like the windex "liquid wrench" method. Will have to keep that in mind if my corrector is stuck.



#14 jgraham

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Posted 06 December 2020 - 06:51 AM

Heh, heh, yeah, I have refurbed several classic SCTs and the trickiest part is usually removing the corrector for the first time, particularly the old orange tube Celestrons. The corrector that they used was very thin compared to the correctors in modern SCTs. It’s a lot of fun getting these old scopes reconditioned and back out under the stars.

 

Gotta enjoy the little things. :)



#15 Chucke

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Posted 06 December 2020 - 05:46 PM

Back in the day folks would fix their mirror flop by putting a large diameter spring over the baffle tube behind the mirror.



#16 moonrider

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Posted 06 December 2020 - 10:18 PM

One thing on the larger Celestron SCT's corrector removal. On my 9.25 there are no spacers like on the older scopes. Mine has 4 very small set screws with nylon tips that center the corrector to the primary. Upon re collimating if you loosened up these screws you will have to re center during collimating.




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