I was wondering if anyone had any experience with an issue I am facing. My imaging rig consists of a C8 edge HD with the f/7 reducer and an asi294MC pro. Notionally my camera sensor fits just within the supported image circle of the reducer, but I notice the stars are beginning to look elongated even in the centre of the FOV, pointing outwards towards the corners.
My total backfocus distance is 105.3mm and I just recently recollimated the scope so I'm really scratching my head as to how this could be happening.
Now that J look at it I haven taken into account the extra distance the whole train has moved inwards by threading onto the front of the reducer. Looks like I should play around adding 1mm at a time to see how that changes things.
Just wanted to com back to this thread after doing some testing moving the camera out in mm increments. Unfortunately, the problem is still present, mostly in the top right and bottom left corners, less so in the others
The thread depth of the reducer, I’m assuming it’s 9, or 10mm? Has that been added to the image train?
Honestly, I would recalculate everything, paying particular attention to the threads. Select a zero reference point and begin from there adding back to your camera sensor, taking into account filters and the depth the sensor sits within the camera body.
I have tested the FOV starting from 1 to 11mm extra in 1mm increments. There didn’t seem to be any change. I might try and take off the OAG and use the T adapter to make sure it is flush with the reducer. Unless I need to add even more backspacing?
Very puzzling. Agreed, rearrange the image train to something basic, I can’t help but think there is still a miscalculation somewhere between the reducer and camera sensor (105mm).
I haven taken into account the extra distance the whole train has moved inwards by threading onto the front of the reducer.
Not quite sure what that means.
The reducer must be the first thing screwed onto the OTA. After that it's 105mm from the very end of the reducer to the camera chip. Attached is an example from Cloudy Nights.
Sorry, I should clarify. The SCT adapter part of the Celestron OAG threads onto the end of the reducer. I had counted 105mm total back focus including the full length of that adapter piece. But, because it threads onto the reducer, it is effectively brought “inwards” because the end of the reducer is now inside that adapter piece.
Hence why I tried adding a couple mm, however that didn’t seem to work. The next clear night I will try removing the oag and using the t adapter. I also need to find a way to upload my Fit files to illustrate the issue better, but they are 22MB each.
Certainly worth trying straight trough as per the pic below. I don't have the reducer myself but do have an 8" Edge which I set up my focus positions during the day - distant TV antennae are great for showing up issues!
You can use the snipping tool to grab a screenshot of your FITS image for sharing, they save down nice and small.
Update: after getting a new reducer from my retailer under the warranty, I've gone back out to test the new reducer. Unfortunately, the elongation is still there.
I've attached some screenshots of what the frame looks like. The first two are with 0mm added backfocus, and the last is with 6 mm extra added, since the OAG adapter has gone in by about that much on the threads of the reducer. Even adding the extra 6mm seems to have done nothing. I doubt this is caused by tilt since it is in multiple corners, and when shooting at f/10, the stars were perfect from corner to corner.
My last idea is to bring the main camera out to 133.35mm like at f/10, and see if the pattern goes the other way, indicating the sensor is "too far out", which means there has to be a turning point in there somewhere, as clearly 11mm extra wasn't enough from my previous testing.
