[RCC] Crescent Nebula - Forums

Topic starter

Jun 10, 2025

Hello everyone,

I'd like to get some feed back on my take on the Crescent nebula.
My workflow for the image was to first discard bad frames (I culled close to half of them because of smoke from the wildfires out West).
Stack them with Siril then move over to Seti astro for processing before going to Gimp for final edit.

At this point, I'm not 100% satisfied with the result but I'm having trouble putting my finger on what need improving the most.

Thanks in advance

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andrea tasselli

Frequent contributor

Jun 10, 2025

Not enough time for one, poor color calibration for the other. Additonally, over the board denoising (which ties to the lack of signal).

Médéric Hébert

Topic starter

Jun 10, 2025

When you say not enough time, should I double, quadruple the time? Would it be better to focus on the NB data or try to go for all filters?
Do you have a suggestion on how to color calibrate narrowband data? I know how to do it for the RGB, but I'm having a hard time for NB.

Thank you very much for your feed back!

andrea tasselli

Frequent contributor

Jun 10, 2025

Médéric Hébert:
When you say not enough time, should I double, quadruple the time? Would it be better to focus on the NB data or try to go for all filters?
Do you have a suggestion on how to color calibrate narrowband data? I know how to do it for the RGB, but I'm having a hard time for NB.

Thank you very much for your feed back!

*Think 40 hours and you'll be in the right ballpark. That's NB. I use PI and I calibrate either with PhotometricColorCalibration (using the appropriate filter curves) or automatic calibration (CC) which I'm not sure you have in Siril. This is for the NB-only image. Star colours came from normal RGB balancing (though I find them pretty lacking in your image, too).

Ashraf AbuSara

Jun 10, 2025

I agree this needs quite a bit more time. Don't spend any time on Sii for this target, and throw all that extra time into Oiii to get better definition on that outer shell.

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TiffsAndAstro

Jun 10, 2025

Spectrophotometric colour correction is in siril 1.4

Ty siril devs

Tony Gondola

Frequent contributor

Jun 10, 2025

More time for the Ha and perhaps some time on a good L frame as it all seems a bit low in resolution considering your aperture.

andrea tasselli

Frequent contributor

Jun 10, 2025

Ashraf AbuSara:
I agree this needs quite a bit more time. Don't spend any time on Sii for this target, and throw all that extra time into Oiii to get better definition on that outer shell.

I'd tend to agree but more on a 1/3 - 2/3 split between Ha and OIII, so that you get a nicer (and redder) background. L won't do anything good here.

Rene Matjanec

Jun 10, 2025

Médéric Hébert:
When you say not enough time, should I double, quadruple the time?

I was doing a lot of research on that very question recently. The thing that helped me visualise that the best was a youtube video I found that compares what images of the same target look like with same processing, but varying total integration time. (I am pretty sure the video was from lukomatico, but I cant find it at the moment)
In that video and in some other resources I have found, the conclusion seems to be that if you want to see a noticeable improvement on your final image that you should double your total integration time.

Clear Skies!

Tony Gondola

Frequent contributor

Jun 10, 2025

Rene Matjanec:
Médéric Hébert:
When you say not enough time, should I double, quadruple the time?

I was doing a lot of research on that very question recently. The thing that helped me visualise that the best was a youtube video I found that compares what images of the same target look like with same processing, but varying total integration time. (I am pretty sure the video was from lukomatico, but I cant find it at the moment)
In that video and in some other resources I have found, the conclusion seems to be that if you want to see a noticeable improvement on your final image that you should double your total integration time.

Clear Skies!

That's right, to get the same improvement in S/N you have to double the integration time. 1 hour, 2, 4, 8....you get the idea. Of course the deeper you go in this progression the harder it is to accomplish to the point of insanity so it also pays to know how to get the absolute best from the data you have, be it 6 hours or 500.

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Arun H

Frequent contributor

Jun 10, 2025

Others have commented on noise, color etc. But in looking at integration time, also consider the effect of aperture/f#. So an image taken with a large aperture scope at the same image scale as a small aperture one will need much less integration time. Below is a crescent with about 11 hrs of integration time taken with a 200 mm f/4 (with corrector, actually f/4.6).

https://www.astrobin.com/v6xbe3/D/

I had two things which I think helped:

Lack of smoke which can significantly degrade signal
Use of 3nm filters for OIII which can boost SNR. I believe you are using 7.5nm filters

Both of the above can be overcome with increased integration time, but how much time is hard to say.

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Médéric Hébert

Topic starter

Jun 11, 2025

Thanks everyone for the feed back!

The consensus is very clear: More OIII and Ha data.
Overused denoising will go down with more data.

Arun H:
Below is a crescent with about 11 hrs of integration time taken with a 200 mm f/4 (with corrector, actually f/4.6).

That look stunning! While I have a faster system (254 mm f/4.72, f/3.54 with the corrector), I don't think 11h will be enough for me because as you said, I used "nominal" 7nm filters and the smoke really do a number on the images.

Tony Gondola:
More time for the Ha and perhaps some time on a good L frame as it all seems a bit low in resolution considering your aperture.

The lack of sharpness is most likely due to the smoke, I'm having a lot of trouble getting good tracking.

I'll also try lowering the sub lenght, maybe that'll help too.

Joe Linington

Jun 11, 2025

I wouldn't change the sub length. It would make integration a PITA and reduce the amount of faint detail that you are trying to capture. Just my thoughts.

Médéric Hébert

Topic starter

Jun 21, 2025

Thank you very to all of you for your help!
Here's my second attemp:
https://app.astrobin.com/u/sgthebert?i=f67nr3#gallery
Is the denoising too much again?
Are the color better this time? If not any suggestion on blending? (Pixelmath equation would be appreciated)
Should I still pursue more data?

Thanks!