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Fuzzy stars in my CPC1100 last night. WUWT?

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#1 spongebob@55

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Posted 15 November 2012 - 11:51 AM

Hey now......Was out last night for 5-6 hours. Cold, 32 degrees. No wind. Proper cool down. No dew, using dew shield and checked. No wind. Needed to do a star test; couldn't use polaris so I used Betelgeuse. Mind you, first time doing this, outside of reading about it in books. The donut hole was a little off center, so I used Bobs Knobs to center it. I got a very 'hairy' but centered donut hole. Couldn't see to many concentric rings clearly, because it seemed to be very solid of a donut. Does that make sense? I mean it wasn't the concentric black circle, space, black circle space........now when I first got the scope, I put on the Bobs Knobs and I might have tightened too much on all of them, or maybe they're too loose. Could that be it, or just poor seeing. Help me out, I'm still a newb and I don't have anyone to ask in person who's knowledgeable. Seemed like the stars just weren't pinpoints as they have been.
thanks
SBob

#2 HeyJP

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Posted 15 November 2012 - 12:00 PM

The most common cause of fuzzy stars is poor seeing which can happen on the clearest and calmest of nights. Just not that calm 20,000 or 30,000 feet up!! Sounds like you probably took care of the collimation okay.

Jim

#3 MikeBOKC

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Posted 15 November 2012 - 05:03 PM

Good rule of thumb on an SCT is don't fiddle with collimation unless you have pretty good seeing. Poor seeing can lead you to make adjustments that are not needed.

#4 Tel

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Posted 16 November 2012 - 06:55 AM

Hi SBob,

Personal opinion of course, but for collimation purposes Betelgeuse is too bright at its variable magnitude range, (+0.2 to +1.2), for an 11" 'scope. Normally a second magnitude star is ideal for an 8" SCT so look for nothing brighter than this: somewhere around magnitude three being probably more suited to the CPC1100.

Secondly, and given the appropriate atmospheric stabilty necessary for accurate collimation, you don't state what focal length of EP you were using. Ideally this should be slightly shorter than that which your 'scope will accept on the best of seeing nights. I would think anything in the range from 4mm to 6mm appropriate.

Thirdly, collimation screws or "Bob's Knobs" should not be overtighten. In tightening one, a corresponding knob should be slackened; not so far as to become loose but just enough to take up the tilt on the secondary mirror produced by the other's tighting action.

Lastly, if you haven't seen this already, here's a copy of Steve Walter's excellent aid to SCT collimation published in 1999. In it you will note that his diagram illustrates adjustments made when looking at the corrector plate. However, our good CN friend and colleague, Art Dent, made this somewhat easy by reversing it so that adjustments can be made as seen from the EP end of the OTA.

here's the link to Steve's instructions:

http://www.starrynig...Collimation.htm

Art's modified diagram Post.


Hoping this helps,

Best regards,
Tel

#5 spongebob@55

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Posted 16 November 2012 - 10:56 AM

Hi Tel,
Wow, that's super information. I'll get a star on my charts that's the right magnitude, and use a high powered EP. I'm sure I wasn't using one, maybe the highest I had with me that night was a 6.7mm. I have a 5mm that I'll use....don't have a 4mm. And all I did was tighten one a tiny bit, but I'll look to make sure the others aren't too tight. Going out tonight, so after cool down, I'll try it. Thanks for the great information and links!
Bob

#6 Tel

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Posted 16 November 2012 - 12:20 PM

Hi Bob,

Glad to help.

Make sure though, as Steve's text states, to re-centralise your collimation star each time after making any adjustment in order to assess the effect.

This is very important where SCT collimation is concerned.

Best regards,
Tel






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