Request you to provide your critique - It is my first attempt to post-process galaxy data. I obtained the data from TAIC workshop.
The LRGB subs were already stacked and registered, even cropped to good extent. I proceeded with these steps, providing my rationale to get your critique and feedback. There are some subs before and after Nova in the original data, but I did not bother to go into those details for now!
Bckground removal of each channel individually
Used RGB channel recombination to get a colour image. I tried to use LRGB recombination directly as well, but my images with that workflow would come out to be lacking colour. I did a few youtube videos - especially PixInsight which informed that the âlightnessâ of RGB and L channels have to be comparable. So i followed the following seuqence:
2a - Linear fit using the L channel as reference
2b - RGB combination followed by SPCC
2c - Stretched the RGB channel (without removing stars yet, but ensuring stars dont become white). In this case, I just used the STF and applied it to local histogram and got a decent image. I have struggled with GHS usually, and at times when I have attempted to use Seti Astro statistical stretch, I get carried away with details like Target Median and Curves Boost. In my âgreedâ to get maximum detail, I would end up using lot of curves boost that usually ruined the image. Finally I chose to just apply STF to HT and get the stretched done
2d - Stretched the L channel similarly
2e - Since the stretched L channel was too bright compared to RGB, I applied HDRMultiscale transformation (9 layers) to reduce the core and arm brightness
2f - With the lightness (brightness) matching now, I injected L into the RGB channel and got my LRGB image.
[Question]: I have seen tutorials suggesting to use CIELab (perceptive brightness) - what is the generall recommendation that you follow?I applied a galaxy core mask, and increased the saturation of galaxy (while avoiding the dark areas) - done in ~3 steps. Applied some S curves stretches as well.
This gave a decent enough image. Since I had Ha layer as well, decided to play around with Continuum subtraction and CombineNBwithRGB also. This was a long trial and error. I actually started from the raw basics - using ChatGPT to get the raw pixel math as a step-by-step process. While this really helped me understand âwhyâ and âhowâ the script does it, it was a very painful (but enjoyable) learning process. The first image was horrid, but I gave another pass using the scripts. While I do not remember the thresholds I used, the outcome was far easier than raw pixel maths.
4a - Applied BlurXterminator and Graxpert Denoise to linear layer
4b - Stretched the Ha channel by applying STF to HT (did not bother with GHS or scripts for stretching)Again applied galaxy core mask (the mask did not have âscreenâ turn on, i.e. it was of uniform brightness), did Local Histogram Transformation - once with kernel radius ~60 and 15% strength, and again with kernel radius ~160 and 15% strength.
With galaxy core mask still on, applied Unsharp Mask (default Std. Dev = 2, reduced the amount to 0.65, deringing turn on, no changes to dynamic range extension)
Some last touchups using S curves and color saturation, boosting very very mildly the blue channel also.
Would appreciate your critique on:
The overall workflow - what looks OK and does not look OK in the image
BlurXterminator and Graxert DeNoise - I applied this only on the L channel, assuming that this is the channel that lends to all details in the image. Is this correct? Somehow I overdo NoiseXterminator and have since moved to Graxpert DeNoise - maybe a personal bias!
I struggled a lot with LRGB initially - what is the ârightâ way to do this? The challenge with going through too many videos is that I am now confused whether it is done on stretched or unstretched image? Finally I went with PixInsight tutorial - performed linear fit on RGB channels, combined them in linear state and then stretched it. Then stretched L channel also. Did some HDRMultiscale Transformation to get the brightness down for L channel. I am sure there will be nebuale and galaxies where I cant make this brightness match - what is the right thing to do here?
Whether any linear fit is to be used on the Ha image also (using RGB or LRGB as reference, etc)? I feel there is a need, because I had to end up stretching the Ha image when my first output of continuum subtraction seemed to be having less brightness - just not sure what is the right thing to do here.
Anything related to Continuum Subtraction and HaRGB combination that I should know?
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