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Removing the secondary on a C11

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#1 gfeulner

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Posted 21 November 2012 - 02:34 PM

Hello everyone. I don't post much but enjoy reading the posts on this forum and have learned a lot. My question is how do I safely remove the secondary housing from the corrector plate. Is it just a matter of removing the Bob's Knobs? Also does the small baffle just unscrew from the rest of the housing? I know I have to use the usual precautions when removing the corrector plate etc. I have a problem keeping the scope collimated and have narrowed it down to a possible loose secondary and would like to possibly modify the way the secondary is mounted onto the plate by using small springs like some posters have done with their scopes. My C11 is the grey tube with "Celestron" on the seconday mount and the serial number beneath it. Thanks for any help. Gerry.

#2 Steve OK

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Posted 22 November 2012 - 06:46 AM

On my C-11, a 1980's black tube, the baffle unscrews from the rest of the housing. The rest of the housing + secondary lifts out from the front of the corrector. Do not undo Bob's knobs, as that would allow the secondary to fall inside the tube.

#3 gfeulner

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Posted 22 November 2012 - 10:43 AM

Thanks Steve.

#4 Starhawk

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Posted 22 November 2012 - 10:52 AM

Hold on- please stop.

The secondary is held against a pivot by the three screws. All of them should be snug in use. Are they?

If you put springs in there, it will be impossible to collimate.

If you take it apart, you need to preserve the clocking of the secondary to the corrector.

Please explain exactly what it's doing to make you think the secondary is loose.

-Rich

#5 mich_al

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Posted 22 November 2012 - 01:20 PM

I've heard bad things about Bob's Knobs and holding collimation. Ya might want to do a search on the subject, or maybe someone can address it here.

Al

#6 gfeulner

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Posted 22 November 2012 - 01:35 PM

There is at least one post of mods made on different forums that show how 3 springs were mounted over the Bob's Knobs and attached to the back plate eliminating the need for the pivot point. There is also one that shows a small spring mounted over the pivot point. These aren't mounted on the back of the mirror itself but on the metal backplate which would create pressure on the screws and allow for adjustment of the secondary.
The reason I've narrowed it down to the secondary is because there is no main mirror shift that I can see, no movement of the corrector plate (I carefully put shims between the plate and corrector holder)and the Bob's Knobs are snug and not overtightened. I have to collimate the scope everytime I observe because I lose collimation sometimes by simply going from something like Cassiopia to Jupiter. I have no plans on taking the secondary apart (I've taken the corrector off several times for cleaning) unless I'm certain that this is the problem. I know how risky that can be and the last thing I want to do is send it back to Celestron. I honestly don't see what could be causing loss of collimation except a loose secondary. I can live with this problem but it's frustrating. Btw, I'm aware that the primary, corrector and secondary have collimation marks on them for realignment.

#7 rmollise

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Posted 22 November 2012 - 04:36 PM

If the secondary is loose, it is because the collimation screws are not snug. Leave the baffle (which is often glued in place) alone. Collimate by tightening screws. Only when one is snug should you loosen an opposite number to continue in the proper direction.

Bob's Knobs hold collimation just fine, BTW. ;)

#8 gfeulner

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Posted 22 November 2012 - 07:00 PM

Thanks Uncle Rod-I've been doing that and I suspect the secondary is actually pulled away from the adhesive backing on the plate. I forgot to mention in my original post that I have a small crack in the plastic housing where the "Celestron" logo and serial number are near the pivot point. Originally I think I had the knobs a little too tight causing this crack. I'm going to play with the tension on the screws some more before playing with the secondary housing. I have some experience going inside the tube. I flocked the tube and reset and cleaned the corrector but would rather not mess with the secondary if I don't have to. If it ain't broke, I don't want to fix it!

#9 mayidunk

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Posted 22 November 2012 - 08:34 PM

Slightly OT... did flocking the inside of the OTA make a noticeable difference?

#10 gfeulner

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Posted 22 November 2012 - 08:46 PM

Bob- I really didn't notice any difference but I took the corrector off to clean it and while I was at it I flocked the inside. I read different opinions about it and figured what the heck it can't hurt.

#11 Steve OK

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Posted 23 November 2012 - 07:00 AM

The crack in the housing may be the root of the problem. I had the same issue with my C11. Collimation was difficult to achieve, with screw-turns not having a predictable effect and collimation not holding. The cracked housing was to blame. I removed the housing from the corrector, and removed the secondary from the housing. I put the housing on my lathe and carefully turned a flat spot on the front about 1" in diameter and 1/16" deep. I then used three small machine screws and cap nuts to attach a 1" steel disk (actually a punch-out from an electrical box) into the milled recess. I also put some JB Weld in the recess under the steel disk. The disk covers part of the Celestron logo on the housing, but after being painted black it is not horrible looking. My C11 is nearly 30 years old, was purchased used from a guy who used it, and has had a lot of in-the-field use since. In other words, it has character. I was less interested in cosmetics than function. Since the repair, collimation is easy and holds very well, even after a 5 mile trip in the back of my truck up a bumpy mountain road to my dark site. If you find that the secondary is still firmly attached to the backing plate, I'd suspect the crack.

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#12 gfeulner

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Posted 23 November 2012 - 10:12 AM

Hi Steve- I agree. I completely forgot to mention the crack in my original post (advanced age, etc). The collimation is unpredictable and doesn't hold. Your fix would probably do the trick. My scope is only about 6 yrs old and as long as I can get the housing off the corrector I should be able to fix it. Anyway, I'm sure I can work something out. I'm going to leave it alone for a while. I had the collimation spot on last night when I viewed Jupiter and it held very well as long as I didn't slew around viewing other objects. I want to thank everyone for their help. Gerry

#13 Starhawk

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Posted 23 November 2012 - 12:00 PM

Or you can get a Starizona fastar kit. The parts are all metal, and it just takes the place of the plastic housing. You'd also be able to do hyperstar imaging if you got into AP.

-Rich

#14 rmollise

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Posted 23 November 2012 - 02:45 PM

Slightly OT... did flocking the inside of the OTA make a noticeable difference?


I can use my POWERFUL REMOTE VIEWING SENSE to answer that: "no"...

;)

#15 bremms

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Posted 23 November 2012 - 04:06 PM

I had a similar problem when I picked up my 88 or 89 C11. Fastar kit cost half as much as my scope.. My secondary holder wasn't craked, just distorted enough and the mirror was nearly 1mm higher on one side of the holder. So there wasn't enough adjustment. I made a little ring that fit over the button on the inside to space the secondary out about 0.9mm. It would be nice if the cell was a little stronger from the get go. Guess you can get the fastar if you want something better made...

#16 gfeulner

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Posted 23 November 2012 - 05:29 PM

I have to be honest and say that sometimes I wish I had gotten one of the "old" classic C11 orange ota's from the 70's. I think they were made better. I had a C8 I bought from Roger Tuthill in 1975 and I'm sorry I ever sold it. That thing gave me a diffraction image that was close to the one I get with my 6" AP refractor






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