Whatever I do, I end up with coarse details in galaxies - where is my process failing?

Hi Arny,

Not a galaxy imager here, but to me it seems that you are clipping the blackpoints too much. I would look into your GHS processing step, as too aggressive adjustments can cause these issues.

Also color calibration seems a bit off. Maybe try to play around first with RGB only and then add Ha at the end when you are happy with the RGB processing. 

If you want to add Ha, use minimally processed raw and linear files. This means, only Gradient experimination, no blurx or noisex. Then do a continuum subtraction and proceed with the processing of the continuum subtracted HA.

I hope that helps. I learned that in processing, less is more. Better invest in more and cleaner data.

Christian Koll:
Arny,

I don't see the point why you would do these two steps:
- Autohistogram on to red channel for RGB subframes
- continuum extraction in Ha channel (Toolbox Script)

Usually, RGB and Ha are stretched on their own and combined later in their non-linear form.

As allready suggested, try to process an RGB image withouth Ha and without the two steps mentioned above.
In the first step you should manage to get a propperly colour corrected RGB image, before thinking to add Ha.

I also suggest to use a mask when adding Ha, not to change the colour of the galaxy core (in case the continuum subtraction doesn't perfecly cancel out the core, which it probably woun't).

All the best!
Chris

Arny:
Autohistogram serves to align the RGB channel to the same median value in statistics - that usually allows for better color balance in RGB. But I will now try without!

I agree with Chris here. If you want to have a better color balance, you could either use SPCC or similar on the combined images. Or if you really want to pre-calibrate your channels, you could try LinearFit with your weakest channel. I dont see the point, though. As you are processing a broadband target.

Cheers, 
David

Arny:
I have tried several routes processing 17h of integration time, but never end up with a satisfying result, as the image below shows:
the fine details in the galaxy arms are always coarse and somewhat bloated. I tried reducing denoising to a minimum without effect.

As I have had that with most of my galaxy images I wondered whether I have a flaw in my process or setup.

What could I do better?

Setup:
- EdgeHD 11, well collimated
- Asi2600mm pro camera at -10 degree cooling
- EAF for focus and EFW 7x2“ with Zwo RGB and Ha filters
- AsiAir plus controlling the setup
- 200 images of 180s and 300s in RGB and Ha
- bortle 4 sky at 2-4 degree celcius
- excellent guiding of 0.2-0.3“ on my AM5
- good automatic focus usinf EAF with fwmh of 1,31“
- raw images look ok, even though not too strong of an SNR from what I can tell visually …

Process in Pixinsight
- thorough deleting of suboptimal frames before stacking
- WPBB stacking with 100% images being processed
- Autohistogram on to red channel for RGB subframes
- continuum extraction in Ha channel (Toolbox Script)
- Channel Combi RGB
- Graxpert gradient reduction for RGB and Ha
- BlurxTerminator for RGB and Ha
- NoisexTerminator for RGB and Ha
- StarsXTerminator for RGB and Ha
- LRGB Combi of Ha into RGB
up to here everything looks fine, the issue start during stretching when trying to eliminate background brightness
- GHS stretching of starless and stars RGBHa
- Curves adaption increasing color contrast and making the image pop
- screen blend re-integration of stars

Here some revisions of the image representing several processing alternations, where I changed
- the order of when to integrate Ha (early vs late)
- stretching simpler with autohistogram
- integrating Ha only towards final end of process
https://www.astrobin.com/wrouwh/

Any ideas or recommendations of there the cause might be in the process or where I need to look in the raw images?
Especially @Gary Imm never seems to run into these problems even taking shorter exposures in a similar bortle 4.5 sky … :-)

Arny

• Don't combine luminance and/or narrowband in a linear state.
• With regards to narrowband addition, I strongly recommend reading this guide as it delivers by far the best results: https://www.nightphotons.com/guides/advanced-narrowband-combination
• Don't bother using linearfit or similar process for RGB. The first thing I do with RGB after stacking is to combine the three channels then run DBE or some other gradient extraction technique. Then I will run SPCC to colour calibrate. Then BlurX, noise reduction, etc. all in linear state.
• On the topic of noise reduction, I recommend running the AI tools before removing stars. It's not strictly necessary but I prefer this methodology because these tools were trained with stars and I want the noise profile between stars and starless to match.
• Ensure that nonlinear intensity matches roughly before attempting to combine luminance.
• Don't stretch in such a way as the clip blacks. Contrast should be low until very late in the process. Usually one of the last things I'll do is increase contrast with curves.
• Don't combine star and starless with a simple addition. This will create all kinds of artifacts. I strongly suggest reading this guide: https://www.nightphotons.com/guides/star-addition

Yes. Personally I've tried every method of stretching and the very best results by a long shot come from either GHS or HT (or a combination of both). I recommend reading the documentation/watching the videos on how GHS works and how to use it. It's extremely powerful and flexible once you understand how to use it.

Timothy Prospero:
Please let me know if you're open to sharing your data for checking. 😉

Jon Rista:
I've read a bit about GHS, and it sounds really interesting. I'll be done with some alternative palette processing with my latest image, and will move onto reprocessing another old image, and I'll give GHS a try.