How far should a camera be mounnted behind a CPC925?

[Packen104]

My ASI 1600mm camera is mounted 90mm behind the CPC925 and I seem to be experiencing vignetting, the outer corners of my images are much brighter.
Focus is fine at that distance but would a longer of shorter distance produce a flatter field?

[Zuts]

Please someone correct me if i am wrong but

1. If you are not using a flattener or reducer then there is only one place to put the camera and that is at the focus point. You may need some adaptors to reach focus.
2. If you are using a flattener/reducer then the chip surface must be 55mm behind the back surface of the glass. Chip surface to top of camera is 6.5 mm and ZWO provides enough adapters to make up the 55 mm.

An edge 9.25 is different as it has a build in corrector so you need to get the celestron recommended spacing from back surface of corrector to chip.

[Outcast]

Stumbled across this... you may (or not) find it helpful...

A few pages in it has recommended spacing of 146.05mm... has a bunch of other interesting & potentially useful information both in the table & throughout the article..

https://www.firstlightoptics.com/user/EdgeHDWhitePaper_2012-09-25-Final_.pdf

[Merlin66]

Carlton,
The Celestron paper refers to the EDge design not the CPC design.
The standard backfocus would be 125mm from the rear cell.
If a x0.63 reducer is used the backfocus is 110mm

[Outcast]

Doh!!... my mistake... for some reason I thought I read he had the EdgeHD... possibly I conflated the OP & the first response...

Thanks for pointing that out Ken... I'd hate to steer someone down the incorrect path..

Cheers

[Packen104]

I am not currently using a flattener, I also have trouble with that getting the appropriate distance when trying to use it.


Thanks for the info. Looks like the 90mm is a bit short.


I was a bit confused in that it does seem to focus OK using the CPC focus knob.
I assumed that that would provide the correct path distance from the main mirror to the sensor.

[The_bluester]

Within reason on the vanilla C925 (As in not an Edge and not using a reducer/corrector) the exact camera spacing is less important. If you apply too much spacing then you will have to move the mirror further up the tube and might introduce vignetting from the baffle tube cutting light off. Another effect is that if you change the distance between the mirrors, you actually change the effective focal length somewhat. I would try it with the camera mounted as close as is practicable to the rear cell and see how it looks, but the focus knob will need quite a bit of twirling to get in from 90mm to say 45mm.

What I can't quite work out though is the description of vignetting making the corners brighter, vignetting should see the light levels fall off as you get further off center in the image. So long as vignetting is not severe you should be able to largely correct it with flat frames though.

I am using a C925 (Which used to be a CPC925 until I removed it from the fork mount) with a Celestron 0.63 reducer/corrector and an Astronomik L3 luminance filter (With resolved a huge reflection issue I used to have) and a ZWO ASI294MC pro. It is producing results the really are pleasing for what was supposed to be a visual instrument. This is one I am still working on the processing of from my C925.

https://www.astrobin.com/full/v74z82/0/?nc=user