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Why does my M33 have a lack of blue?


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Thanks for the input all. Well, I've had a few more attempts at processing thanks to the input of everyone, and this is the best I have come up with using Siril and GIMP.

103_stretches_and_gimp_bluemask.thumb.jpg.b007b4ff5ab453cb4215e01b689d0254.jpg

I've had to do several more steps compared to most, partly as I was being a bit more careful with the steps, but noise levels remain the biggest detractor when zooming in, however I feel this is definitely a step forward compared to what I had before. Without PI the only way I seem to be able to reduce noise effectively is more subs ! GIMP's noise reduction always destroys the stars when I use it, even masking them leaves them looking somewhat fake against the background with the overlap between them.

But the only way I could bring out this level of blue in the galaxy was a separate layer with a blue curve adjustment, and then mask and brush it. If I applied it to the whole image, it looked very cold and the blue star halos were too obvious.

Comments / observations / criticisms welcome 🙂

Siril: Crop (x=259, y=0, w=5363, h=3579)
Background extraction (Correction: Subtraction)
Photometric CC
SCNR (type=1, amount=1.00, preserve=true)
Asinh Transformation: (stretch=1000.0, bp=0.00000)
Histogram Transf. (mid=0.035, lo=0.332, hi=1.000)
Generalised Hyperbolic Transformation
Saturation enhancement (GLOBAL amount=0.5)
Cosmetic Correction
Deconv. (iter=1, sig=1.522)
Saturation enhancement (RED amount=0.42)
-- EXPORT TO GIMP
New layer with sharpen + contrast boost. Mask and brush to core.
New from Visible. S-curve.
Colour select for background "green noise". Noise reduction 7 and de-saturate. Darken shadows.
New layer for contrast boost to core, mask and brush.
New from visible. Darken levels.
New layer. Increase blue curve, mask and brush to spirals.

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Looking good! On my IPad there looks to be a hint of a green gradient running left to right so toning the green down a bit with SCNR might help. More subs will always help with noise reduction, along with everything else.

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1 hour ago, tomato said:

Looking good! On my IPad there looks to be a hint of a green gradient running left to right so toning the green down a bit with SCNR might help. More subs will always help with noise reduction, along with everything else.

Thank you 🙂 Currently on my iPad too and I agree, there is a greener cast on the right side of the image. Didn’t notice that, if I recall that’s the side that had more LP, will definitely take a look at it. 

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2 hours ago, WolfieGlos said:

Didn’t notice that, if I recall that’s the side that had more LP, will definitely take a look at it. 

Then it doesn't have to be a gradient. More light pollution means more noise. And noise is a random pixel to pixel intensity variation. There are more bright and dark pixels where there is more noise, but we only notice the brighter pixels. Therefore, the noisiest channel can appear as a colour cast, while it isn't. Anyway, Tomato's suggestion to use scnr should solve the problem.

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