C8 Edge HD + f/7 reducer coma

[EC48675 (Euan)]

The centre stars are kind of normal. Again, with the top right corner they start to elongate pretty much at the centre but the others start a little further out

[bojan]

I may have something similar in the past with my Meade .63 FR...

It turned out one of the elements was assembled upside-down, see below:
https://www.iceinspace.com.au/forum/...ght=meade+4000

[EC48675 (Euan)]

Really? An improperly-built reducer? I would like to avoid taking it apart if I can.

Maybe my next port of call could be to have a more earnest attempt at adjusting the tilt. I had a quick go but was honestly fed up with tinkering that night as I had just spent 45 minutes collimating.

[ChrisD (Chris)]

Originally Posted by EC48675

The centre stars are kind of normal. Again, with the top right corner they start to elongate pretty much at the centre but the others start a little further out

ok, then it's not tilt, I agree, it's backfocus.

I have an older C8 (not Edge) but the optics have the same characteristics except for the inbuilt flattener. I've had 3 different field flatteners/reducer on the C8, and original Celestron, an Orion and currently I'm using a Starizona SCT corrector IV.

One thing that I've learned is that the generally accepted "star coma pointing to the center means you need to increase backfocus" is true for most telescopes, but not for SCTs.

If I have stars with a coma pointing to the center I always move IN. This has been consistent with all the reducers used on my C8.

Another thing I've found is that the manufacturer recommended backfocus has never been correct for me, in fact with the Starizona I'm 5mm in from published backfocus, and I'm getting near perfect stars in the corners of a APS-C size sensor (IMX571).

Chris

[EC48675 (Euan)]

That is very helpful as well, thank you very much!

I shall try again but with less backfocus. Hopefully there is an inflection point where the star pattern reverse so I have a ballpark idea where the optimal backfocus lies.

Interesting that you find moving IN helps, Chris. Given I use the Celestron OAG, the sensor is technically already a bit too far in, as the adapter threads onto the front of the reducer and isn't technically flush with the last piece of glass.

However, I will try adjusting backfocus again using shorter exposures.

Cheers

[ChrisD (Chris)]

Originally Posted by EC48675

That is very helpful as well, thank you very much!

I shall try again but with less backfocus. Hopefully there is an inflection point where the star pattern reverse so I have a ballpark idea where the optimal backfocus lies.

Interesting that you find moving IN helps, Chris. Given I use the Celestron OAG, the sensor is technically already a bit too far in, as the adapter threads onto the front of the reducer and isn't technically flush with the last piece of glass.

However, I will try adjusting backfocus again using shorter exposures.

Cheers

Yeah, I would move the camera in by a large amount initially and hopefully see the coma change direction. After bracketing the correct backfocus make small moves until you get it right. It takes time but it's worth it, and you only have to do it once.

I've attached a single frame from my C8 with the Starizona. It is just a single sub, no processing or crop. Way outside the recommended backfocus and achieved by moving opposite to the recommended directions for coma. It has a small image tilt but I use the camera on a refractor as well so i keep a flat tilt and just have to put up with it.

Chris