C2023/A3 - Bad Data - How to improve? [Solar System] Acquisition techniques · Jean-David Gadina · ... · 9 · 391 · 4

I photographed comet C2023/A3 on October 27, but I was quite disappointed by the quality of my images after stacking.
I'm looking for recommendations on how to improve.

Unfortunately, I overexposed the core on my previous attempt, so I changed my settings this time to avoid this.

I use a Celestron NextStar Evolution 6 with a HyperStar (300mm, f2) and a ZWO ASI585MC Pro.
I set the gain to 0, offset to 6, and took 15-second exposures.
I took 200 flats and dark flats and used 200 darks and bias from my library.

This time, I checked the histogram, and everything looked good to me.
Overall, I was pretty happy with the session. I had a few clouds passing over, but not as much as the previous attempt.



I used PixInsight to stack my best 100 sub-frames from the beginning of the session, used the Comet Alignment tool on the registered frames, Star Exterminator on the aligned frames, and integrated the starless images.

Then, I used DBE, SPCC, Blur Exterminator, and Gradient Correction, and finally, I processed the image using the HDR script.
This is what I got:


Needless to say, I was expecting something a little bit better.
Now I have a few questions.

First, I have a nasty circular gradient at the center of the image that wasn't corrected by my flats.
It also doesn't really show on my flats:



Why is that? I took the flats at the end of the session when the telescope was almost pointing at the horizon.
Should I have raised the telescope to where to comet was at the beginning of the session to take my flats?

Second, it is pretty blurry. I used NINA's autofocus, but this was probably a mistake when I think of it now.
Should I have focused manually on the comet, like with a Bahtinov mask? How do you folks do it?

And last, I have a lot of horizontal lines in the final integrated image:



I guess they are leftovers from Star Exterminator since they are following the comet's motion.
This is more of a processing question, but how can I avoid that?
I used the same techniques in my previous attempt but didn't end up with such stripes.

C2023/A3 is the first comet I have tried stacking, so I'm looking for advice and recommendations on how to get better data on the next comet.
Also, I would appreciate any tips anyone has on how to salvage this data, if possible.

Thanks a lot, clear skies!

The issue with the stars' leftovers can only be fixed by integrating the same field for the same length of time and then remove the stars (from the re-shots) from each star-aligned frame and only then comet-align them. As for the central shadow showing up it could be due to a number of things one of which is your flat frame has a small light leak (another is the mirror flopped between the position where the scope where pointing at when taking the shot and when taking the flats). You can test whether the issue is with the flat by integrating the frames without flat-fielding and see whether the shadow shows up. Focusing shouldn't be an issue but Fastar systems are notoriously finicky with focusing. Obviously things can be improved once the issue with the central shadow is addressed. The rest is processing.

Johannes Maximilian Möslein:
Autofocus should be fine. 
Concerning the stripes: They are pretty normal, especially when the seeing conditions aren’t a 5/5 with perfect clarity. StarX won’t remove the slight haze around the stars. You can try to integrate the lights again and play around with Pixel Rejection. You have to stretch carefully to bring out the comet’s details but not the stripes. Maybe the HDR script pushed them?

Thanks for the suggestions!
I'll try to play with pixel rejection and see how things go.

Tony Gondola:
Since you're shooting with a 585 sensor you should set your gain to no lower than 252. This triggers HCG mode where read noise is drastically reduced. That should help with your walking noise streaks as well. 252 or slightly higher is where you should be set for most sessions with this sensor.

Thanks!
I forgot about HCG mode; that's true. I should have kept the gain to 252 and played with the offset instead.
I'll keep that in mind for the next one! : )

Same field, same length of exposure and same gain. Take as many as reasonable, pre-process them and get the average of the stack (you might need to experiment here to leave a little residuals as possible). One other technique is to interleave each exposure by a pause so that there is clear separation between each shot once aligned with the comet. Alternatively, you might want to guide on the comet (with short exposures) and then proceed as usual and then re-image the stars of the same field your comet image is centered on. Hope it is clear.