Imaging with C8, 0.63x, OVL OAG - help requested!

[Padraic M]

Hi all, I'm having trouble finding the right configuration for my Celestron classic C8 (non-EdgeHD) with the Celestron 0.63x FF/FR and the OVL OAG, and I'd love to get some opinions on what the correct set-up is. Also, if I'm asking too much of a non-EdgeHD SCT then please let me know so I can stop wasting clear skies!

It's very difficult to find a definitive answer in Celestron documentation or online, defining the back-focus distance required between the FR and the camera sensor, and from where to measure that distance.

The most common answers are either 105mm or 85mm, measured from the male threading on the reducer. No mention of whether it's at the camera end of the male thread (also the back edge of the last glass element) which incidentally is hidden when all connections are in situ, or whether it's from the scope end of the thread. There's 10mm difference between these two planes.

I've taken some test images at varying distances as shown below; the distance is measured from the scope end of the reducer male thread as shown in the picture. I'm not happy with any of the results; it's very hard even to pick a favourite, or to match the images with the OVL diagram.

I have collimated with Bob's knobs and I believe it's reasonably well adjusted.

The components, from scope to camera are:
Celestron 0.63x reducer/flattener; WO SCT-M48 adapter; various M48 extensions if needed; OVL OAG (16mm depth); various M42 extensions if needed; ZWO EFW (20mm); filter adjustment .67mm; ZWO ASI1600MM Pro (12.5mm).

Also fyi, guide camera is ZWO ASI290MM Mini. I can get good focus on imaging camera and guide camera.

 

Sample images and associated distances:

85mm - 1420mm focal length; f/7.1

 

100mm - 1409mm focal length; f/7

 

115mm - 1344mm focal length; f/6.72

 

128mm – 1224mm focal length; f/6.12

 

Finally, a helpful image from OPT Corp giving guidance for back focus:

 

 

[Merlin66]

Should be close to 105mm from the rear edge of the reducer body. As shown with your dimension "D"

(the 85mm was for the x0.33 reducer - and a one off crap batch of x0.63 reducers)

Edited September 25, 2021 by Merlin66

[Padraic M]

Interesting - that's a distance I haven't tried. It's close to the 100mm above, which looks almost reasonable.