Backfocus Solution for Celestron 9.25 SCT (Non Edge Version) - Forums

Topic starter

Mar 7, 2023

Hello,
I am new to astrophotography and have come across a problem that has taken me weeks to find a solution to. I have a Celestron 9.25 SCT and have begun taking images. I have found a backfocus solution that works for me, but it requires using the Celestron 0.63x reducer to achieve the recommended 139.065mm of backfocus. Does anyone know of a way to bypass the reducer to still achieve the same backfocus length? Thanks for any help or suggestions!

The picture below is my exact setup (56mm) + 50mm T-Adapter + 34mm Reducer = 140mm backfocus

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Tim Ray

Mar 7, 2023

If I understand the 9.25 correctly, (my friend has same ota) the 139mm distance is from the rear of the reducer or scope. I believe the celestron camera adapter is expecting 55mm of back focus after the adapter. So…

scope - reducer - celestron camera adapter - (optical train you have pictured in your post)

If you don't want the reducer in the optical train, simply remove the reducer and re-attach the camera adapter to the rear plate of the celestron and refocus primary mirror.

That is the way it works on my C11EdgeHD…. I hope I am correct and this helps!

CS - Tim

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Caleb Evans

Topic starter

Mar 7, 2023

Thanks for the response. I will give it a try Tim and see if it can still focus properly. If so, this is such a simple solution that I severely overlooked.

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Tim Ray

Mar 8, 2023 · Edited: Mar 8, 2023

Caleb

Please PM me with your result. I don't want to give bad recommendations to you or anyone else. I am curious how it turns out…

Of course always focus the 533 first then your OAG camera without touching anything else… If possible, tell your acquisition software to disable guiding while autofocusing (assuming you have a focus motor on the Primary mirror).

CS - Tim

Caleb Evans

Topic starter

Mar 8, 2023

I will let you know, Tim. Looking like the next few nights will be cloudy, but whenever that clear sky comes I'm going to try it out.

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Matteo Beretta

Mar 8, 2023

If I may make a suggestion, once you find an approximate backfocus, you can use N.I.N.A.'s Hocus Focus plugin for fine-tuning.
I don’t use N.I.N.A., but I’ve installed it just for the Hocus Focus Aberration inspection tool so I can fine tune the sensor tilt on the camera attached to my Celestron C8 XLT.

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Caleb Evans

Topic starter

Mar 11, 2023

Thanks for the suggestion @Matteo Beretta . Unfortunately I use the ASIAIR for my workflow, so I’m unable to hook up my device to NINA. However, I do have an EAF that is extremely easy to use in order to change and find the perfect focus.

@Tim Ray I had an opportunity to try your method and I did not have any luck with it. I guess I need to find some other sort of spacer that can either replace the t adapter and reducer, or can simply add another 34 mm (or close) to my t adapter.

Thanks for your suggestions.

Lynn K

Mar 11, 2023

Caleb, as Tim explained, the back focus is associated with the reducer. All reducer/flatteners have a focal length for maximum performance. If you do not include a reducer in the imaging train, that is nolonger an issue. As Tim explained, focus will be achieved by standard moving the primary mirror. SCT have a lot of focal travel and are able to usually achieve focus. Imagers talk of the sweet spot that is achieved when the primary mirror is at a particular distance from the secondary. I have a C9.25, C11 and Edge 8 and never concerned myself trying to fine the sweet spot. Back focus is of particular importance with the Edge scopes, due to the built in flattener, and can be difficult to get right.

The Celestron 6.3 reducer was designed pre 2000 when CCD chips where smaller than most guide chips today. I have always used Starizona's reducer/flatteners. Mine is the older F7.5 version. But the new F6.3 version is designed to accommodate today's larger chips.
Starizona will design/make an adapter for your particular imaging train. Back focus will no longer be an issue.

Lynn K

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John Hayes

Frequent contributor

Exemplary contributor

Mar 11, 2023

Lynn is correct. The back working distance is critical with the Edge systems in order to achieve good imaging performance over the field with a larger sensor. The BWD is not so critical with the non-Edge systems mainly because they don't have very good field performance in the first place. They work fine with a small sensor but the image quality will not be very good into the corners of a large sensor no matter what BWD you use. Having said that, you should try to get it close to the recommended distance but I'm guessing that the tolerance might be in the range of maybe +/-5mm.

John

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Matteo Beretta

Mar 11, 2023 · Edited: Mar 11, 2023

Lynn K:
The Celestron 6.3 reducer was designed pre 2000 when CCD chips where smaller than most guide chips today. I have always used Starizona's reducer/flatteners.

After some months with a Celestron corrector/reducer, replacing with a Starizona SCT Corrector IV was a big step ahead for me.

John Hayes:
image quality will not be very good into the corners of a large sensor no matter what BWD you use […] I'm guessing that the tolerance might be in the range of maybe +/-5mm

You are right, I’ve never got good stars at the corners. In my case, I think Starizona SCT Corrector is rather restrictive as a tolerance. If I get the backfocus wrong by even 1 mm, it greatly accentuates the defects in the corners. The collimation must also be OK. With the old Celestron there was worst performances but a larger tolerance.

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Rick Veregin

Mar 11, 2023

I have a C925 standard version. Celestron suggests the T-adapter# 93633-A for prime focus photography. It screws to the SCT threads and the other end has a 42 mm T-thread. It then expects that the focal plane will be at the usual 55 mm further away. As long as you are reasonably close to that additional 55 mm you will have no problem focusing, As explained by others, the exact spacing is critical for the standard C925 at prime focus, lots of tolerance. As mentioned, the field is not flat without a reducer, so only stars near the center are any good.

For good stars with APSC, I find the Starizona f6.3 reducer is way better than the Celestron, though tolerance is smaller if you go that way.
Rick

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