Good day to everyone.
I’ve recently purchased an ASI 294 MM, for imaging with my C8 HD, with the 0.7x focal reducer, Celestron OAG and ZWO filter wheel 7x36.
Tonight I’ve been setting up everything. Did the maths to reach the right backfocus (105 mm with the focal reducer):
SCT Adapter: 25.3 + OAG Body: 29 + Male M42 adapter: 12.5 + Filter wheel: 20 + Camera backfocus: 6.5 = 93.3 mm.
So I added the (included with the OAG) Medium T-thread Spacer Ring: 11.55mm
93.3 + 11.55 = 104.85 mm
After attaching everything, I took the first images to have a glance. What I saw shocked me: awful, elongated stars, and what’s more: some of them showed the central obstruction of the optical tube.
I assumed the backfocus distance was wrong (sometimes reality differs from paper) and added and substracted amounts o backfocus (+/- 3 mm +/- 6 mm, +/- 10 mm…). Elongated stars everywhere, repeating outcomes. I’ve discarded tracking or guiding as the problem, since I was taking 2 – 5 seconds frames, with a good PA (made with Polemaster), and also the stars, yet oblong and awful, didn’t show trails or switched positions. Also, focus was made with Bahtinov mask everytime, carefully, so I don't think it's a focus issue.
Intentionally, every time I added or substracted backfocus, I’ve placed the camera with the same orientation. And what I’ve found is that depending on the backfocus distance, stars elongation seem to rotate (with no camera rotation). The optical gear seems solid to my eyes, with no sagging.
I’ve downloaded the trial versión of CCD Inspector to check if something is wrong with my camera’s sensor, but to be honest, I don’t know how to use it and get lectures from it.
As I don’t know what to think about this, any advice woul be welcome. Might be collimation? My camera is damaged with a non ortogonal sensor? Am I doing wrong with backfocus (never saw such a critical difference with +/- 3 mm)?
Tomorrow I might try removing the focal reducer or any other part from the optical gear, but if I could get any piece of advice before doing any operation, would be great.
I’m sharing link to the images (in fit) in case someone wants to have a look and / or inspect them.
Thanks in advance.
Edited by Tuant, 20 September 2021 - 10:33 PM.