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C8 corrector rotated - help me fnd the right posit

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#1 kosmokratur

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Posted 01 December 2005 - 11:50 AM

Hello -

I believe that the corrector plate on my C8 was rotated out of position by the previous owner. Is anyone familiar with the procedure to determine the correct rotational position of the corrector? Is there an "easy" way, such as positioning it so that the Celestron name on the secondary holder is horizontal? Thanks for any help - GD

#2 garyc11

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Posted 01 December 2005 - 12:05 PM

how are the views thru the scope?

#3 BluewaterObserva

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Posted 01 December 2005 - 12:37 PM

I have rotated them a few times on SCT's without consequence... Are you having an issue with the OTA?

#4 kosmokratur

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Posted 01 December 2005 - 04:39 PM

Yeah, it seems to collimate OK but there seems to be a bit of "coma" effect that I can't quite get dialed out. I'm using mostly TV eyepieces so I know they're not the problem. Maybe the corrector shims didn't all get put back, so the axis is off?

#5 BluewaterObserva

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Posted 01 December 2005 - 05:09 PM

Hmm coma in a c8??? F/10?? Ok not good, agreed.

You would have to remove it to see what is going on. That is a bit of an odd issue for sure.. Coma... Meaning field curvature correct?

I'm not good enough with SCT's to advise you further in all honesty. I'll bet some of the real SCT guru's will chime in here given some time.

#6 wh46gs

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Posted 01 December 2005 - 08:09 PM

there seems to be a bit of "coma" effect that I can't quite get dialed out



If it's off-axis, that's normal. The secondary is spherical or near-spherical, so those SCTs are not corrected for coma.Roughly, for the linear field, it should be similar to the coma level in f/6 Newtonian. If it is in the center and can't be removed by collimation, then the corrector is either decentered or tilted. Orientation doesn't matter.

Vlad

#7 kosmokratur

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Posted 02 December 2005 - 01:19 AM

Any suggestions on how to proceed? It is present even on-axis. I am thinking of taking it off and re-installing it with shims to be sure it is pressed in perpendicular to the tube. This seems reasonable, but is it the right way to go? Thanks for all help so far and for any more suggestions.

#8 jason_milani

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Posted 02 December 2005 - 07:49 AM

There should be a mark on the corrector and ota so you can line them up. If you remove the retainer from the corrector you should see them and also, any shims. Hope this helps.

#9 kosmokratur

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Posted 02 December 2005 - 10:06 AM

Thanks to everyone for your help. It looks like I have a project to keep me busy while it snows for the next few days...

#10 wh46gs

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Posted 02 December 2005 - 02:08 PM

Making sure side shims are of even thickness and properly placed
is the easy part. If that doesn't do, I'd check out how much of a play there is in positioning of the corrector in the opening and the secondary on the corrector. It shouldn't be much, especially the latter. If the secondary is in its proper place at the corrector, it still shouldn't be too hard. In that case center coma is likely caused by decentered corrector/secondary, and they are decentered in the direction of the comatic extention. They should be moved slightly in opposite direction, by manipulating edge shims.

Some units have corrector/secondary purposely decentered in the tube to compensate for not quite centered primary. Maybe the pervious owner had this and tried to have it "fixed". Whichever position results in no center field coma should be good. Hopefully, everything is OK mechanically.

Vlad


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