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skj
super member
Reged: 04/13/07
Posts: 118
Loc: McKinney, Texas
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Hello experts,
A friend of mine asked me to look at his C6 because he claimed the view was not as good as what he sees through others'. Of course I figured it was most likely collimation as he bought his scope fairly recently and has never attempted to adjust it.
I've had fair (at best) seeing conditions lately, but after collimating as best I can, I see what appears to be an evenly distributed diffraction ring that is "broken" into 3 equal length arcs of light instead of a uniform circle. The corrector plate does not seem overly tight in the cell and the secondary screws do not feel over tight either.
I've not seen this before, and wonder if it's pinched optics or poor seeing or cool down issues or what. If it's pinched optics, where is the problem since the cell and secondary do not seem invoved?
Any advice appreciated!
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Steve Johnson
Celestron C100ED Refractor
Celestron C6 SCT
SkyView Pro Mount
Free access to God's amazing creation
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sang33ta
professor emeritus
Reged: 07/08/08
Posts: 539
Loc: UK
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Focusing problems are nearly always just poor seeing conditions and I'd advise anyone to wait for clear skys before messing with collimation.
I'm no colimation expert but I'd guess if you see 3 veins when there should be none then it's possible someone has over collimated and cracked a mirror.
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Hioptic 152mm f12.5 Maksutov
Celestron Advanced CG5-GT Mount (Mr Noisy!)
Meade 4000 Super Plossl Set
Casio QV-2900UX
Got fed up of waiting for Meade ETX-150 so put this together for £500/$1000 
Edited by sang33ta (07/14/08 02:05 PM)
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1723
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If the seeing was good enough that you could see a STABLE first diffraction ring, and you see three STATIONARY LOBEs on the ring (equally spaced brighter segments and dimmer segments) then there are several possibilities.
First, this is traditinally the sign of "Pinched" optics. It is hard to deform the secondary of an SCT though, and since the primary floats, it cannot be pinched.
That leaves some other possibilities though
First is surface roughness. The easiest way to check for surface roughness is to defocus. If when you defocus so that only four or five rings are visible, and you see that the same pattern of brightness is visible, and if you see what appear to be any "Spikes" or "Fattening" of the outside ring, or if the rings look unevenly illuminated as you go around the pattern, and this is STATIONARY, then this is typical of "Dog Bisquit" error in the mirror. Unlikely to be present in a commercial SCT, but it does happen. Could also be the corrector, but again, not all that common to be so bad as to show up in a focused diffration ring.
The next possibility is astigmatism in the mirror. Roll through focus. If the lobes shrink in on one side, the expand out on the OTHER side in different positions, this is astigmatism. It is a saddle shaped lobe in the mirror.
Search the web for "Star Testing" and you can find many different pictures of various abberations. Perhaps you can find one that matches your friend's scope.
If ANYTHING is MOVING, then that is most likely caused by seeing conditions. So if the bright lobes move around or change in brightness, that is seeing. If they are relativly fixed in orentiation, even if the image is otherwise wavering, then you still could have trouble with the optics.
Defocus and tell us what you see... If the rings are not evenly illuminated around the circumference and the pattern seems more or less "Fixed", then there is something wrong with the telescope most likely.
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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mclewis1
Thread Killer
Reged: 02/25/06
Posts: 2434
Loc: Colorado
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Take anything off of the back, and in daylight have a look up the baffle tube from the back. Notice that the top inch or so is a different texture. Look to see if there are "breaks" in the textured area.
On my C6 the texture is applied as an adhesive flocking and it doesn't completely cover the inside of the baffle. The ends of the flocking don't quite match up and that creates a bit of distortion in the diffraction rings. On my scope it's only one spot so the rings only show a bump in one place.
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Mark
C11, C6, APM/TMB115, and AT80ED
Tandem mount CGE and CG-5A, WO EZ-Touch and AT Voyager
25x100s, 8x56s, T-Mount Light, and Mark 1 eyeballs
Modded 350D, SPC900, Mallincam
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skj
super member
Reged: 04/13/07
Posts: 118
Loc: McKinney, Texas
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Thanks for the detailed responses.
I'm a little more convinced that there are no physical defects now, but I'm going to have to wait for good seeing to confirm. The arcs don't seem to move MUCH but they are moving... and they are definitely 'shimmering' or changing slightly in intensity as the atmosphere fluctuates.
On the other hand, it's a suspiciously long way from a uniform diffraction ring! Fingers crossed for good seeing soon... last year this time I had only 1 or 2 really good nights all summer.
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Steve Johnson
Celestron C100ED Refractor
Celestron C6 SCT
SkyView Pro Mount
Free access to God's amazing creation
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1723
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Trust me, to see a perfect diffraction ring is rare. To see one formed enough even to see that it is a RING is rare in big scopes.
If the ring was not 100% stable, then everything you saw could easily be attributed to seeing.
Good luck.
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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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