Back focus help with Celestron OAG

[LindV]

Hey everyone,

Hope someone can help me with achieving the correct back focus on my image train.

I am trying to fit a Celestron OAG onto my Esprit 120 using the stock field flattener but it has been quite challenging so far. I believe the distance the guide cam sensor to the prism needs to match the distance the main imaging camera's sensor to the prism as well. I seem to have the correct distance for the imaging cam sensor to the flattener at 75mm, but main sensor to the prism is 25mm, while the guide cam sensor to prism is 55mm.

Does anyone have any thoughts about how i should go about achieving the correct distance and what adapters i will need (I'm mainly using the adapters that came with the OAG and cameras)?

Thanks!

 

Edited September 2, 2023 by LindV

[Elp]

It's likely you can take all those adaptors off. Using the zwo 1.25 inch nosepiece adaptor instead of the 11mm female to female ring will get the camera closer a bit. May have been included with your camera (check dimensions first):

https://www.firstlightoptics.com/zwo-accessories/zwo-125-t-mount-photography-camera-adapter.html

 

Edited September 2, 2023 by Elp

[Anne S]

I've just invested in the t2 extension tube set. At the moment I'm trying the 7mm extension on the ASI533 instead of the 11mm one supplied. Mind it helps that my SX fw as a T thread female adapter. I could just attach the camera with no extensions but there's a screw that gets in the way!

I need 55mm backfocus so it's really tight if I use a OAG.

[LindV]

2 hours ago, Elp said:

It's likely you can take all those adaptors off. Using the zwo 1.25 inch nosepiece adaptor instead of the 11mm female to female ring will get the camera closer a bit. May have been included with your camera (check dimensions first):

https://www.firstlightoptics.com/zwo-accessories/zwo-125-t-mount-photography-camera-adapter.html

 

I gave that a try, but the nose piece doesn't stay secure in the helical focuser of the OAG. I also tried to remove the helical focuser and directly attach the camera that way, but doesn't seem like any of my spacers fit the threads where the focuser went.

 

EDIT: I completely forgot i removed the thumb screws from the focuser hence why i couldn't secure the nose piece adapter. Let me retry again and see if i can also adjust the prism height to achieve proper distance.

EDIT 2: Unfortunately didn't help much in distance. I will try again after work and experiment some more.

EDIT 3: Does anyone know what size male threads is used to attach to where the helical focuser goes on the OAG? I may need to order in a step-down adapter to go from M42F to whatever size male threads to directly attach the camera to the OAG without the focuser for it to work out.

Edited September 2, 2023 by LindV

[Elp]

2 hours ago, LindV said:

what size male threads

Measure the diameter with a rule, it's usually metric and quite accurate.