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Meade 8" ACF - optimal backfocus of 110mm - How does your optical train look like?

ZWO ASI120MC-S Equipment Reflectors
Schmidt-Cassegrain Telescope (SCT) Imagers •1 reply•289 views
Gernot Schreider avatar
Gernot Schreider avatar
Hi all,

I have a Meade 8" ACF SC and I am currently using a SONY Alpha 6000 for imaging. I am just about to get operational with my new optical train utilizing an Omega OAG instead of a guide scope for guiding with a ASI 120MC-S as guide camera.
I guess my next step could be to take advantage of a focuser and a reducer. However I do see some challenges with meeting the optimal backfocus of 110mm of my Meade SC. There is a post in Cloudynights where someone shared the information about the backfocus of the Meade 8" ACF being 110mm, see here.

When using the Optec Lepus reducer for the Meade ACF the backfocus will be 105mm, so almost the same situation regarding the optical train as without the reducer, of course different pixel scale and different speed.
So I have currently more questions than answers. Here is what I am asking you.

Do you work at the optimal backfocus of 110 mm ? How forgiving is the system when not meeting this optimal value?

Please share how your optical train looks like and what recommendations you can give.

Thanks for your input.
CS
Gernot
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Björn Arnold avatar
Björn Arnold avatar
Hi Gernot,

The SCT itself does not have a specific back focus as one has it for reducers since you have a movable mirror. So the position of your imaging plane can be shifted (which also changes the focal length of the OTA)

For the ACF, I am using an AstroPhysics CCDT67 which has a nominal back focus of 85mm which can be changed as needed. I am using it at 105mm but you would need to check if vignetting starts to be an issue there for you sensor size. In my case I have a 1” sensor and an APS-C.

To connect the reducer and all the attached stuff, I use a SC-thread to 2” barrel clamp from Baader.

CS!
Björn
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