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RWC
member
Reged: 04/15/08
Posts: 14
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I just got an old Super C8 Plus that I mounted on a CG-5. Last night I put some Bobs Knobs on it and attempted my first collimnation. I ran into a problem, one of the knobs tightened to the point that the thumb wheel was flat against the cell face. Yet I still needed to move the cell in that direction. Do I need longer screws or have I damaged the cell?
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1653
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Quote:
I just got an old Super C8 Plus that I mounted on a CG-5. Last night I put some Bobs Knobs on it and attempted my first collimnation. I ran into a problem, one of the knobs tightened to the point that the thumb wheel was flat against the cell face. Yet I still needed to move the cell in that direction. Do I need longer screws or have I damaged the cell?
Don't panic. If you need to keep going in that direction and the screw is snug, it is OK to loosen it's opposite numbers.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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RWC
member
Reged: 04/15/08
Posts: 14
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Sorry, I was afraid I had not described the situation correctly. The screw is not snug against the mirror. There are not any threads available left to turn. The knob is cranked down flush against the face of the cell. The only thing I can do is back it off
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David Pavlich
Postmaster
Reged: 05/18/05
Posts: 6542
Loc: Mandeville, LA USA 30.38 X 90....
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I think you misunderstood Rod's explanation. You have to compensate with the other screws by going in the opposite direction. That way, you can back off the screw in question.
You may have to do it more than you like to get it back to where you can collimate with all three screws again. That will take some time and patience, but you'll eventually get it back to where it's easier to work with.
David
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A few scopes and mounts.
Proud Member; PAS NOLA,
Life expectancies would go WAY up if green vegetables smelled like bacon...
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RWC
member
Reged: 04/15/08
Posts: 14
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Thank you all. I had reached the end of my rope last night. The seeing was very bad tonight. But as best I could see it all seemed to come together. I think I am very close.
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2820
Loc: Fayetteville, NC
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sound like your on your way to getting it sorted.. post back and let us know that you did get it .
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group scope pic
my refractors
LX200 10"-St120
LX200 8" f6.3-Orion 80ed
LX200 8" f10-Orion 90 Mak setup pic
Meade 102ED LXD650
Sky Watcher 100 ED Triplet prototype
Nexstar 5
etx125
etx70's
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1736
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I think that perhaps the others HAVE mis-understood.
When you tighten one screw completly, lossening the other screws will NOT help you. Once a screw is bottomed, loosening the other screws will simply cause the secondary to become loose.
There are two circumstances that can cause this..
The first is internal damage due to over-tightening the secondary. It is VERY hard to damage the steel housing for the secondary, but it can be done.
There is little protrusion on the back of the secondary mirror mounting plate (a thick disk of steel) that sits on the bottom of an inverted steel cup (the baffle assembly).
When you tighten the screws, the little protrusion rocks at this point like a fulcrum on a see-saw.
If you get too overzealous in tightening these screws, it can deform the cup that the secondary holder. The little projection puts a dimple in the cup meaning that the pivot is lowered. YOu can run out of travel before reaching your collimation point.
The other condition that can cause this is that the corrector is not centered. This is most likely. Because the secondary is a -5 mirror, if it is not pretty perfectly centered over the central baffle, to "OVERCORRECT" this, you wind up cranking one screw down further than you normally would to get enough tilt to the secondary.. If it is bad enough, you simply run out of adjustment screw.
My guess is that you have a secondary centering problem. You MIGHT have a situaion where the cup is damaged, but this is unlikely.. It would be very hard to tighten the collimation screws this tight using the small Phillips head screws that came in the scope...
But it is entirely possible and even likely that over the years, the corrector has moved.
Using as accurate a ruler as you have, measure the distance out radially from the edge of the corretor to the retaining ring at several different places around the diameter of the tube.
If you find that the secondarly cell is not dead center, this is most likely your problem...
Regards.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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David Pavlich
Postmaster
Reged: 05/18/05
Posts: 6542
Loc: Mandeville, LA USA 30.38 X 90....
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Eddgie's right...I missed the bottomed out part of the post. 
David
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A few scopes and mounts.
Proud Member; PAS NOLA,
Life expectancies would go WAY up if green vegetables smelled like bacon...
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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I talk about this problem in another post on this same page...
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Number=2579945&page=0&view=collapsed&sb=5&o=all&fpart=1&vc=#Post2579945
Only difference is....my standard Meade collimation screw has reached its end exactly as you describe what has happened with your bobs knobs. Its the same situation i think.
I haven't had a clear sky yet to tweak it some more, but soon as these clouds are gone ill be out there.
Let me know exactly what you did to correct the screw that was screwed in to its end. Bottomed out as people call it.
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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