Crescent Nebula raw data available for processing - 21 hours collected

Fabian Butkovich:
I did manage to extract quote a bit of the Oxygen and make it stand out quite a bit, but still not very happy with it, mainly the color hue, is still too close to green or cyan and not purely “blue”

Hi Fabian!

Thanks for sharing your data, I’m really glad I got the chance to work on this target since I’ve never imaged it myself.
I started from your raw frames and here’s the result of my interpretation.
I’m definitely not an expert, but I tried to keep a natural look while bringing out the OIII a bit more, and keeping the Ha a bit more restrained for balance.
My workflow is a bit messy and I didn’t write down everything, but if you have any questions feel free to ask and I’ll gladly answer.

Great job on the acquisition, especially under Bortle 8!

📷 NGC6888.png

@Fabian Butkovich No, I never mix spectral bands into the RGB channels. My workflow depends on the target and on the signal quality, but for your image I mainly worked with masks.

Here’s how I worked on your image. I didn’t save my workflow, so this isn’t an exact step-by-step copy of what I did, but more of a general overview of my workflow.
I redid the steps to take some screenshots, but I spent less time than when I processed your image.

• Preparing the image for mask creation
  I started by removing the stars, then did the usual stuff (DBE, denoise, etc.) while still linear. After that, I did a pretty strong stretch to make all the colors pop out.

  
  📷 stretched.png

• Creating color masks
  From that stretched image, I used PixelMath to generate R, G, B, C, M and Y masks.
  For a narrowband image, I usually end up using the R mask for Ha, the G/B/C masks for OIII, and the Y/M masks to possibly bring out some specific regions.
  I tweak them depending on which areas I want to highlight.
  Also, the choice of which masks to use depends a lot on how the image is processed (like background neutralization, stretching, etc.). So for the same image, the OIII signal can show up only in the green channel, or sometimes in both green and blue channels.
  
  In my case, in the green mask, the OIII presence was clearly visible, but the background was too bright. So I created a nebula-shaped mask to darken the background in the green mask.
  
  In the end, I got things like:
  • an OIII mask for the nebula,
  • an Ha mask for the nebula,
  • an Ha mask for the background,
  • a yellow mask for areas that contain both Ha and OIII (red + green = yellow).
  

  📷 masks.png

• Selective corrections
  Then I went back to the stacked color image, and after stretching it I used the SelectiveColorCorrection script with my masks to boost the areas I wanted.
  

  📷 stretched_colorCorrected.png

• Stars
  I also process the stars separately, usually with the Seti Astro NB to RGB Star Combination script for narrowband images, and I add them back at the end of the workflow.
  
  Of course, the more signal you have in the data, the easier it is to reveal the colors in a clean and natural way. But this workflow can help rescue images with low signal, especially when OIII is weak. I’m not saying it’s the best method, just the one I used for your image.
  
  Hope this makes sense! Let me know if you want more details.

Hi @Fabian Butkovich

I saw your post and thought, why not...

📷 Crescent nebula PI_astrobin.jpgWell, I'm a complete beginner with Pi and processing, but you have some great information! I hope you like it.

JuanM.

Hi Fabian,

thank you for sharing your great data!

Here’s my quick attempt on processing it. Will perhaps need some more color correction.

📷 NGC6888_1.jpgCS
Andreas

Cédric PASTOR · Jan 11, 2026, 11:22 PM

@Fabian Butkovich No, I never mix spectral bands into the RGB channels. My workflow depends on the target and on the signal quality, but for your image I mainly worked with masks.

Here’s how I worked on your image. I didn’t save my workflow, so this isn’t an exact step-by-step copy of what I did, but more of a general overview of my workflow.
I redid the steps to take some screenshots, but I spent less time than when I processed your image.

• Preparing the image for mask creation
  I started by removing the stars, then did the usual stuff (DBE, denoise, etc.) while still linear. After that, I did a pretty strong stretch to make all the colors pop out.

  
  📷 stretched.png

• Creating color masks
  From that stretched image, I used PixelMath to generate R, G, B, C, M and Y masks.
  For a narrowband image, I usually end up using the R mask for Ha, the G/B/C masks for OIII, and the Y/M masks to possibly bring out some specific regions.
  I tweak them depending on which areas I want to highlight.
  Also, the choice of which masks to use depends a lot on how the image is processed (like background neutralization, stretching, etc.). So for the same image, the OIII signal can show up only in the green channel, or sometimes in both green and blue channels.
  
  In my case, in the green mask, the OIII presence was clearly visible, but the background was too bright. So I created a nebula-shaped mask to darken the background in the green mask.
  
  In the end, I got things like:
  • an OIII mask for the nebula,
  • an Ha mask for the nebula,
  • an Ha mask for the background,
  • a yellow mask for areas that contain both Ha and OIII (red + green = yellow).
  

  📷 masks.png

• Selective corrections
  Then I went back to the stacked color image, and after stretching it I used the SelectiveColorCorrection script with my masks to boost the areas I wanted.
  

  📷 stretched_colorCorrected.png

• Stars
  I also process the stars separately, usually with the Seti Astro NB to RGB Star Combination script for narrowband images, and I add them back at the end of the workflow.
  
  Of course, the more signal you have in the data, the easier it is to reveal the colors in a clean and natural way. But this workflow can help rescue images with low signal, especially when OIII is weak. I’m not saying it’s the best method, just the one I used for your image.
  
  Hope this makes sense! Let me know if you want more details.

Thank you so much @Cédric PASTOR for providing your detailed workflow! Iv’e never used PixelMath for creating masks before, it’s something I will definitely have to explore! Usually I just use the range mask tool, and only more recently I started experimenting with the color mask script utility.

Also only more recently I started getting the idea of just using the image itself (usually starless) as a mask for another window in my workflow.

“So I created a nebula-shaped mask to darken the background in the green mask.” - This was the smart move, as it’s what really makes the Oxygen POP against the background Ha.

Thank you again for processing my image and inspiring me with some new editing techniques and tips!

CS

Fabian

Andreas · Jan 12, 2026, 02:29 PM

Hi Fabian,

thank you for sharing your great data!

Here’s my quick attempt on processing it. Will perhaps need some more color correction.

📷 NGC6888_1.jpgCS
Andreas

Thank you @Andreas for processing my data! Lovely edit you have there. I especially like the added star spikes? Was this done in PI? if so which module or tool is this?