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Rear Opening Sizes of Celestron SCT OTAs

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#1 kenn

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Posted 23 February 2008 - 11:47 AM

I would swear someone just posted this a few days ago but I can't seem to find the thread. I've searched all the recent threads but perhaps it was in another forum and I'm searching in the wrong place. It was basically a small clip of the rear opening sizes on the celestron SCT OTAs from 6" to I believe 14". Someone asked me this morning the difference between the 6" and 8" and I wanted to be sure before I answered.

#2 Randy Roy

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Posted 23 February 2008 - 12:55 PM

C5 25mm
C6 27mm
C8 38mm
C9.25 48mm
C11 54mm
C14 54mm

Sorry, I only have Celestron.

Randy

#3 mclewis1

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Posted 23 February 2008 - 01:03 PM

"Visual back" usually refers to the add on component that adapts the threaded opening on the rear of a scope to something that eyepieces and diagonals can be mounted in. Scope vendors include them, usually in 1.25" barrel size and there are a bunch available from 3rd party vendors in 1.25 and 2" sizes with a variety of clamping mechanisms (set screws, compression rings, etc.).

Large aperture SCTs (11 and 14" Celestrons, 12" and up Meades) have a large baffle tube and rear opening and also come with a threaded adapter that reduces that large (3"+ threaded opening) down to a more common 2" SCT threaded opening (48mm) so that SCT threaded accessories can be attached directly to the scope without the need for a visual back.

The rear opening of a C6 is 27mm, the thread around the opening is the SCT 48mm standard.
The rear opening of a C8 is 38mm, the thread around the opening is the SCT 48mm standard.
The rear opening of a C9.25 is XXmm, the thread around the opening is the SCT 48mm standard. I have seen 45mm published as the diameter of the rear opening ... can someone provide a measurement and maybe a picture?
The rear opening of a C11 is 54mm, the thread around the opening is 3.25", an SCT threaded adapter is included.
The rear opening of a C14 is 54mm, the thread around the opening is 3.25", an SCT threaded adapter is included.

#4 kenn

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Posted 23 February 2008 - 01:46 PM

Sorry, I should have used rear opening. I'll correct the thread title so others can find it.

#5 mclewis1

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Posted 23 February 2008 - 08:07 PM

Ken,

I should have added another comment about the term "visual back". It seems that at various times the SCT vendors themselves have used the term to describe that threaded adapter that they include with the larger scopes. It has confused many folks. I don't believe that they officially use that term for those adapters any more.


I'd also like to clarify my comments about the rear opening of a C9.25. I've seen quite a few different published numbers and something has always concerned me. Everyone says the C9.25 has the 48mm SCT threads on the rear opening. Those are obviously 48mm in diameter. There needs to be some material to support the threads, say 2-3mm at minimum. That means the opening is 48-6=42mm at the best, which is smaller the published figures that I've seen. In fact most when there are the 48mm threads on the outside most openings on accessories like diagonals and visual backs are only between 38 and 40mm.

I've never seen a picture of just the rear opening of a C9.25, in every picture there seems to be a diagonal, focuser or dust cap over the opening. Any C9.25 owners want to publish a nice picture of the rear of their scope, maybe with a metric ruler across the opening?

#6 Randy Roy

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Posted 23 February 2008 - 08:31 PM

Mark,

I measured the rear ID of my CPC9.25 at 46mm which is 2mm smaller than the rear of the baffle tube itself.

Randy

#7 jbredmo

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Posted 19 February 2012 - 12:47 PM

Are the adapters mentioned above for the C11 and C14 obtainable? My C11 is equipped for ab Astro-physics compression ring style reducer, but I would like to use some thread-in components on occasion. I can't find the original threaded adapter to handle the 2" Celestron diagonal, or reducer corrector.

#8 DanaJ

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Posted 19 February 2012 - 05:17 PM

Nice info, Mark and Scott. I got my CPC1100 used, and it came with an unmarked lightweight adapter (3.29" to 48mm SCT). I put an A-P compression ring unit on.

You have an interesting point about finding one of these adapters -- they don't seem to be easy to find (C11 back to SCT). It's easy to find ones that give you a T-ring, 2" compression ring, or various custom port size.


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