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Image Train and focus with ADC

ZWO EAF Celestron EdgeHD 8" Planetary Camera Prime Focus Help Needed
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Adrian Webster avatar
Adrian Webster avatar

Afternoon all.

I’m after a bit of help.

I am just setting up my first SC type scope (Celestron EdgeHD 8) for planetary.

I can get focus with the following image train:

OTA>visual back>Camera (ZWO 679MC with UV filter) via nose piece.

📷 IMG_4471.JPGIMG_4471.JPG📷 IMG_4470.PNGIMG_4470.PNG

When I fit a x2 Barlow, I can also get focus.

OTA>visual back>Barlow>Camera (ZWO 679MC with UV filter) via nose piece.

📷 IMG_4473.JPGIMG_4473.JPG📷 IMG_4472.PNGIMG_4472.PNGBut for the life of me, I can’t get even close with I fit the ADC.

OTA>T piece >ADC>Camera

📷 IMG_4474.JPGIMG_4474.JPGI can see the T piece and ADC push the camera back a good 2” difference in back distance, which i’m guessing is the issue.

So my question is, what do I need to be able to focus with a ADC fitted?

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David Foust avatar
David Foust avatar

It looks like the ADC corrections are set pretty high (the two levers (one pointing up and one pointing down). This may be giving you a result that looks out of focus. I think the ADC neutral position is when the levers are in the same position, and parallel to the horizon. Maybe try resetting the levers to the neutral position?

Otherwise, you may have run out of back focus with the ADC. Have you measured the backfocus of each setup?

Most folks will put the ADC behind a telecentric Barlow, which means backfocus and Barlow effect are less sensitive to backfocus issues.

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Adrian Webster avatar
Adrian Webster avatar

I’ve retested with the ADC correctly set up (I had just thrown that on for the photos) but I still can’t focus on far away objects. I’m guessing the back focus is wrong, but everything indicates I should be reducing the back focus distance, which I can’t actually do, as the visual back is a set length.

andrea tasselli avatar
andrea tasselli avatar
Your T-adapter is seriously long, you need to buy a shorter adaptor..
Robin Bosshard avatar
Robin Bosshard avatar

Is that the t-adapter at full lenght? The one I use is a two-part design so it can be shortend for use with the reducer. Hard to see in the pic…

Robin Bosshard avatar
Robin Bosshard avatar

Robin Bosshard · Oct 11, 2025, 05:36 PM

Is that the t-adapter at full lenght? The one I use is a two-part design so it can be shortend for use with the reducer. Hard to see in the pic…

Had a chance to check my setup, and, yup, that’s the t-Adapter already at half length.

I did a quick comparison with the visual back combined with the universal t-adapter from Celestron, but that makes no real difference. So, maybe some other, shorter visual back that allows for a shorter t-adapter? I think Celestron doesn’t offer something like that, though. I will have to look into that myself since I can just about fit my rotator and filterdrawer in when using the shortened t-adapter with the C8 without the reducer on, but with the reducer I will have the same problem. So if you find a nice solution, let us know!

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andrea tasselli avatar
andrea tasselli avatar
T-adaptor is just a glorified name for a M42 threaded adaptor. Just get one of the suitable length.
Robin Bosshard avatar
Robin Bosshard avatar

andrea tasselli · Oct 11, 2025, 07:46 PM

T-adaptor is just a glorified name for a M42 threaded adaptor. Just get one of the suitable length.

With Celestron/C8 it’s the other end that complicates things…

Craig Towell avatar
Craig Towell avatar

Stick the barlow lens cell on the nose of the ADC, should hopefully push the focus point further out to allow you to focus? Or if you don’t want to use the Barlow you could remove the top part of the ADC and screw the camera directly to the prism module.

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Adrian Webster avatar
Adrian Webster avatar

OK, problem solved!

I could have just deleted the thread, but i’m going to put my hands up here, and tell the truth. I was using a EAF to focus, and like a biff, when it got to the end of the run, it didn’t dawn on me that the C8 mirror still had space to move.

I took of the EAF, manually focused with the ADC, and then put the EAF in the middle of its run, and refitted it.

Job jobbed!

Sorry for being a biff and wasting everybody’s time!

Still, lesson’s learned!

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David Foust avatar
David Foust avatar

No worries. Glad you got it sorted out!