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ZWO ASI1600 MM Pro with Celestron Classic 8 SCT

astrophotography CMOS imaging Celestron
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#1 astroap


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Posted 04 August 2020 - 10:05 PM

Hello everyone,


I recently bought ZWO ASI 1600MM Pro kit with EFW and LRGB filters. I want to attach the CCD on my old celestron classic 8" SCT. But I'm unsure how is the attachment done to avoid back focus. Also, I recently got interested in astrophotography over visual astronomy.

#2 kisstek


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Posted 04 August 2020 - 10:37 PM

SCTs have huge focus distance ranges. If you're not using a focal reducer or corrector, you can use just about any spacing that you have the spacers for.


If you're going to use a FR/FC sometime in the future, you may want to match the back focus distance that FR/FC will require. Then you can swap the FC/FR in and out without redoing all of the spacers. The Celestron f6.3 reducer/corrector wants a back focus distance of 105-115mm. Unlike a lot of other FR/FC, there seems to be some variation in what works best depending on the individual FR/FC and SCT.

#3 tjz


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Posted 04 August 2020 - 10:45 PM

I have my ASI1600MM-Pro setup two different ways on my Celestron SCT. One "straight through" at f/10 which will have artifacts at the edges/corners of the frame, and one with an Starizona SCTIII focal reducer / flattener (pricey!). I tried the Celestron f/6.3 focal reducer but could never get good stars across the field, no matter how I adjusted the backfocus.


First for the SCTIII, I have an imaging train like this:




Then for the "straight through" option I have an imaging train like this:




After considerable tinkering with various connection schemes involving visual backs, clamps, etc, I am now a firm believer in fully-threaded imaging trains. Since I took up this practice, I've had much fewer incidents of sag/tilt. Between Amazon, Agena Astro, and what comes with your ZWO camera, you should be able to get the miscellaneous pieces to put it all together.


I've also found a spreadsheet and a digital calipers to be invaluable to double check my math.


Happy imaging!



#4 bobzeq25



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Posted 04 August 2020 - 11:39 PM

The SCT is a difficult scope to use to start in DSO AP, as many have discovered.  This is an absolutely typical experience.


"I regret spending the first 6 months trying to learn imaging with an 8" Edge, with that scope it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"


It's simply not intuitive how difficult the long focal length, heavy, slow scope makes things.  DSO AP is nothing at all like visual.


Good book about how to do DSO AP, there's a lot to learn, more than you can get from short posts here or websites.  The best $40 you'll ever spend in DSO AP.



Edited by bobzeq25, 04 August 2020 - 11:44 PM.

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#5 Ron (Lubbock)

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Posted 05 August 2020 - 08:55 AM

I started off the same way: imaging with my 1997 vintage C8.  There is an f/6.3 focal reducer that will help a lot, but I never really got results that made me happy.  In the end, you will probably have issues with ugly stars in the corners because these telescopes were not really made for imaging.  My stars were shaped like seagulls in all four corners.  I have since bought a refractor and a C11 Edge for imaging purposes.  Just FYI.


I'm not sure how to do the back-focus calcs. for your system, but you are going to  want to get it exactly right.  I would suggest calling a vendor who has a lot of experience with older telescopes, and letting them figure it out and sell you the right adapters/spacers.  I just asked Starizona to help me with a camera coupling and backfocus calcs. for my refractor, and they did it perfectly.

#6 bobzeq25



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Posted 05 August 2020 - 11:24 AM

Note that backfocus is really only an issue if you use a reducer nad/or an Off Axis Guider.  With neither, the SCTs focus range should be more than adequate.

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