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#1 snowboycosmos

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Posted 12 March 2013 - 07:34 AM

as I'm looking through my CPC
at the moon for instance
the image never seems to 'focus'
as i return to my little nexstar 5se
the image is clear and crisp and exciting.
anyone know why this might be ?

#2 Tapio

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Posted 12 March 2013 - 07:41 AM

What CPC do you have ?
If it's big 8,11,14" might take long time to cool.
Other reason - collimation.

#3 snowboycosmos

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Posted 12 March 2013 - 03:20 PM

11...

#4 mclewis1

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Posted 12 March 2013 - 04:21 PM

Match magnifications (remember the 11 has more than twice the focal length of your 5), make sure the 11 has completely cooled (may take more than an hour or so). If your collimation is good then you should have views just as crisp and brighter than with the the C5.

#5 Sorny

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Posted 12 March 2013 - 06:30 PM

Having both a C5 and C11, I can't say I've had your issue.

As others have said, the 11 will take FAR longer to thermally stabilize; my 5" takes about 30 minutes most of time and my 11" takes a LOT longer, usually about 2 hours. The 11" will also have considerably more mag from each eyepiece. Remember, 2800mm focal length on the 11" and 1250mm focal length on the 5". Figure 2.24x much mag from any given eyepiece. An 8mm in the 5" is equivalent to about an 18mm eyepiece in the 11".

As an example, an 8mm eyepiece in my NexStar 5SE produces about 156x. The same 8mm eyepiece is at least 350x in the CPC1100 (I figure about 375x in my particular setup due to longer focal length of 2" diagonal with power switch). Make sure you are using equivalent magnification in the scopes, and verify the collimation on the 11" is good. The 11" should be quite superior to the 5" in views on anything that will fit in the field of view of both.

#6 Steve Daniel

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Posted 13 March 2013 - 02:06 PM

I don't see how cooling issues could cause no focus at all.
Is every surface in the optical path clean and unsmudged?
Have you tried racking the focus in and out all the way with a low power (i.e., long FL) eyepiece?
Will the scope focus on a distant terrestrial object, such as a distant horizon or tower?

#7 mclewis1

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Posted 13 March 2013 - 05:39 PM

I don't think the OP means no focus at all but that the image doesn't seem to ever come to a crisp focus.

You're right Steve, not cooled down won't prevent coming to focus at all but will often prevent you from coming to an accurate focus.

When an SCT isn't cooled down (or at ambient temperature) it will produce lousy images. The air currents within the tube (from the hot/cold air rising/falling) really affect the image quality. This effect seems to be much more pronounced for folks in colder climates.

#8 Edward E

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Posted 13 March 2013 - 07:04 PM

It could be the seeing. I have a C11-GPS and have seen nights where nothing would come to a sharp focus. I waited a few days and the next night out everything was fine.






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