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C8 secondary problem

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#1 Don-richardo

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Posted 21 December 2019 - 11:02 PM

hello folk's! I have a 79 C8 and the secondary housing is loos and moves on the corrector plate. 

i'm going to reattach it so i'm looking for some advice on removal and reattachment technics.

centering it isn't a problem, but keeping it centered? help! lol. thanks!!



#2 Eddgie

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Posted 21 December 2019 - 11:17 PM

My guess is that the gasket behind the flange on the front of the corrector has failed.   

 

Starizona used to sell a replacement (I think Sorbothane, so it would not slip.) 

 

To replace it on your scope, If I recall correctly, it is necessary to remove the corrector. 

 

http://www.astronomy...aryremoval.html


Edited by Eddgie, 21 December 2019 - 11:18 PM.

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#3 Littlegreenman

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Posted 22 December 2019 - 04:22 AM

Starizona still sells a Sorbothane gasket but it seems to be for a Faststar accessory. I'd suggest asking them.

 

https://starizona.co...n-kit-c8-gasket


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#4 Eddgie

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Posted 22 December 2019 - 10:43 AM

Starizona still sells a Sorbothane gasket but it seems to be for a Faststar accessory. I'd suggest asking them.

 

https://starizona.co...n-kit-c8-gasket

That is indeed what they sold it for but I pointed it out because it is probably better than the gasket Celestron would sell. I know that a couple of people have used this and it should be a direct replacement for the factory gasket. 


Edited by Eddgie, 22 December 2019 - 10:44 AM.

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#5 Geo.

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Posted 22 December 2019 - 12:39 PM

All cork gaskets shrink with time and 40 years is time. Problem with Celestron's plastic secondary cell is that it's not threaded. The baffle the retains the cell is glue on. Thus a judicious use of heat is needed to free it and replace the gaskets. Last time I face this I used waxed thread wound around the cell to pack it. Seemed to work.

 

BTW,  anytime I remove a corrector from a Celestron SCT I check the ring nut that retains the primary to be sure it's snug. The cork gasket under it tends to shrink too. I usually find I can get about a 22.5 to 45° turn with a gloved hand.


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#6 Eddgie

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Posted 22 December 2019 - 03:09 PM

All cork gaskets shrink with time and 40 years is time. Problem with Celestron's plastic secondary cell is that it's not threaded. 

I did not know this. Important piece of info.  Like your tip on how to patch it too.  Clever idea.


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#7 Don-richardo

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Posted 23 December 2019 - 08:38 AM

All good imfo thank you guys! I wish someone had youtubed a vid on this but i'll just be super careful and get through it. But a good thing to learn.
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#8 Don-richardo

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Posted 30 December 2019 - 08:04 PM

Ok folks. I held my breath and got the secondary housing apart (exhale) that my friends was a little spooky! the gasket seems to be missing as well as any sort of shim to hold the mirror centered in the corrector plate. (I assume they were cork) is it just spacers the keep the housing centered? Or did they really glue it!? HEEELP! Lol.
Thank you!

Rick.
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#9 Axunator

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Posted 31 December 2019 - 02:18 AM

Ok folks. I held my breath and got the secondary housing apart (exhale) that my friends was a little spooky! the gasket seems to be missing as well as any sort of shim to hold the mirror centered in the corrector plate. (I assume they were cork) is it just spacers the keep the housing centered? Or did they really glue it!? HEEELP! Lol.
Thank you!

Rick.


I don’t think Celestron uses any shims nor anything else to center the secondary holder in the hole. The centering of the secondary/corrector system is done by centering the corrector (with shims in the corrector shell in older scopes, with four centering screws in newer models).

I know the above feels a bit weird, since the hole for the secondary holder is quite a bit larger than the holder itself, allowing lateral movement until tightened (Starizona gasket helps in that, as the friction between sorbothane and glass is very high). But the optical tolerance for secondary centering in that hole is actually quite loose, as the secondary is spherical (without optical axis) and a little decentering can be compensated with collimating. Which explains why the design gets away with somewhat ”sloppy” centering of the secondary, as long as actual collimation is done accurately. Even the centering of the corrector plate is not super critical, but that’s another story.
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#10 Don-richardo

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Posted 31 December 2019 - 07:27 AM

WOW! I would have never thought that! So it was literally compression that held it in place!? Sounds as though i can shim to a relatively close center and not worrie if it's a thousandth out long as i have the gasket seated properly on the opposite side from the secondary it should work?
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#11 Axunator

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Posted 31 December 2019 - 07:32 AM

WOW! I would have never thought that! So it was literally compression that held it in place!? Sounds as though i can shim to a relatively close center and not worrie if it's a thousandth out long as i have the gasket seated properly on the opposite side from the secondary it should work?

Correct. One caveat though (perhaps mentioned already): Rotational orientation of the corrector and the secondary should be maintained as they came from the factory.

If you have already lost the track of that, there’s usually some kind of marking at the edge of the corrector, that typically is set at 3 o’clock position (as looked at from the front, dovetail at 6 o’clock).

 

EDIT: I realized only now that you have a kind of older secondary holder that doesn't have threads (as described by Geo above), apologies for that. In more recent ones, it is easy to thread the holder tight enough (especially with the sorbothane gasket) so that the holder can't wiggle laterally in the hole in the corrector, but I'm not sure how you fix those glued-on types. This, however, does not change the bottom line of my original reply to your question about centering the secondary in the hole, i.e. that it's not critical since slight decentering can be compensated for when collimating.


Edited by Axunator, 31 December 2019 - 02:10 PM.

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#12 Don-richardo

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Posted 31 December 2019 - 07:54 AM

Yes sir. I made sure to keep that in mind when i dismantled it. They marked the secondary as well at the same position so i think it should be fine if no one rotated it before i got it. Really appreciate the input on this, i can do the work but with a 40 year old scope it hard to know what the work is! Lol.


Edited by Don-richardo, 31 December 2019 - 07:55 AM.

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#13 Joe1950

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Posted 31 December 2019 - 08:27 AM

I had to check mine of course. The base says 1977, but the number on the secondary holder is more 1981-ish.

 

The plastic housing seems very firm in place.

 

B6CECDDA-AD7C-4B28-88AE-DE341E00A6EE.jpeg

 

There are two gasket type rings, one inside and one outside. They are like a light gray and seem to be more like fiber rather than cork. And, each has some black like adhesive holding it.

 

Any thoughts appreciated!

joe


Edited by Joe1950, 31 December 2019 - 08:28 AM.

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#14 Don-richardo

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Posted 31 December 2019 - 08:36 AM

nice Joe! mine didn't have any type of gasket at all on ether side of the corrector plate. must have dried and flaked away IDK. but it will soon!

I do wish they would have used a threaded cell with a set screw. lol. 


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#15 Joe1950

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Posted 31 December 2019 - 08:45 AM

Closer view...

 

28075612-D934-4411-92C2-AEAC0E4620F3.jpeg

 

So, is this the holder with the cork gaskets? As with the original scopes? Or did they change to something else in the 80’s.

 

I don’t understand why the serial numbers are different, unless the base was made in ‘77 and the tube in ‘80-‘81.

 

Confusing.


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#16 Joe1950

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Posted 31 December 2019 - 08:50 AM

Very good Donricardo!  Even taking it apart would make me jittery!  I’m sure you’ll get it right.

 

I may order a replacement kit, just to have it on hand!  Only $12, very much worth getting before they are discontinued.

 

Thanks!


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#17 Don-richardo

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Posted 31 December 2019 - 08:55 AM

from what I've seen in pics this looks like it might have been refitted Joe. here's a link to a registry one of the members on here is putting together. might find some more info there. wish I knew more about it. sorry. check out the thread "classic telescopes" on here. bet some one there can help.

 

http://www.summers1....erialnumber.jpg

 

rick


Edited by Don-richardo, 31 December 2019 - 08:56 AM.


#18 Joe1950

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Posted 31 December 2019 - 09:15 AM

No, that’s fine! Thank you so much Rick!  I’m sure Ed or someone who has worked on them will know. And I’ll check the registry. Thanks! 

 

joe



#19 Don-richardo

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Posted 01 January 2020 - 11:14 AM

ok folks. I just ordered a set of bob's knobs, and will be resetting the secondary soon as they get here. in the mean time, 

 

step 1

I was thinking of using pin strip tape to center the secondary cell in the corrector plate. rapping the tape around the cell till I get a 

snug but not tight fit? (not really liking the friction will hold it) thing celestron did. was going to use a rubber O ring but couldn't find

one at that diameter with the right wall thickness. (to thick) plus gaskets on both sides of the corrector plate.

 

step 2

would airplane glue work to reattach the baffle to the cell? or is there a better glue to use for this? not sure about out gassing or

chemical effects on the coatings. "if any" i'm a plumber not a chemist lol. thanks everyone for all the great advice! wealth of knowledge 

here that just can't be beat!

 

rick


Edited by Don-richardo, 01 January 2020 - 02:01 PM.

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#20 Axunator

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Posted 01 January 2020 - 02:04 PM

At least you don't want to make the secondary holder so tight that you risk creating some kind of tension/pinching in the corrector plate (but you knew that already, I'm sure).

 

I just wanted to add one disclaimer. When I wrote up there that centering of the secondary in the hole is not that critical, please understand it in the context. I'm definitely not advocating anyone to mess up the factory centering just for the heck of it. At least according to the folklore, some SCT secondaries have been touched up at the factory to correct optical problems in other elements, and may therefore not be fully spherical anymore. What are one's odds to have an intentionally non-spherical secondary, I have no idea.

 

But when one already has a secondary that has become loose in the corrector (seems to happen a lot, and has happened to yours truly), you kinda have nothing to lose (sorry for the pun), and the main point in fixing the issue is not how to hyper-accurately re-center the secondary in the hole of the corrector.

 

If it was really critical, e.g. every sorbothane gasket sold by Starizona would mean an SCT lost, because once the secondary holder has been detached, there's no way for an end user to re-attach it in the exactly same spot it came from the factory. And the way these scopes are built also strongly suggests that centering of the secondary/corrector system as a whole is more important (or at least sufficient - there are built adjustments for that, unlike for secondary holder centering). This article includes excellent discussion about the subject, with instructions to center the corrector plate and secondary holder together, if the whole system is obviously decentered. And the article comes with appropriate disclaimers as well, naturally.


Edited by Axunator, 01 January 2020 - 02:43 PM.

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#21 Don-richardo

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Posted 01 January 2020 - 02:34 PM

yes sir the more I thought about it the more I realized I just need to stop it from moving more than hitting exact center. 

I think I have a basic plan of attack but the glue is still an issue I think. ether way I have time to sort it out before the knobs get here lol. 

you input is really very appreciated! I was at a loss and fretting just a bit lol. thank you so much for your help! i'll post some shots as I go so

maybe it will help someone else.  it is a fun project after all I have enjoyed it a great deal!


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#22 charlesgeiger

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Posted 01 January 2020 - 09:11 PM

All the older (1970's series scopes) had a central screw in the aluminum secondary holder that went through and locked the secondary from falling out if you replaced the allen screws (taking all three out at the same time).  Of course, it was not a good idea to take out the three screws anyway as your secondary can rotate which screws up things.  So at some point around 1980 it appears Celestron went the cheaper route and started to employ plastic secondary holders.  


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#23 Joe1950

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Posted 02 January 2020 - 07:49 AM

That is definitely what is used on my C-8. Plastic holder. 

 

I’m just not sure if there is cork used. All I see is a gray material that is fibrous in nature. But not cork.

Thanks.



#24 Don-richardo

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Posted 05 January 2020 - 09:11 PM

ok guys. any one have an idea what glue I should use to put the secondary back together with?

I used pinstriping tape to center the cell in the corrector plate (worked like a charm) and am ready

to remount it' i'm not sure if fumes from the glue would effect the coatings on the corrector plate or not?

model glue, super glue, PVC glue? last time i'll pick your brain on this i promise lol. thanks!

 

rick


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#25 Don-richardo

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Posted 08 January 2020 - 08:05 PM

well. I couldn't find any info on glue for the secondary best I could do was on a photography sight people were using quick set epoxy so I used it. and it worked like a charm. set it and let it dry/off gas for around 5 hours. reset and I have to say i'm very happy with the way it came out! i'll post a set of pics in a how too post soon. thanks for all the help folks. means a lot! 

 

rick


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