image processing questions

I’m trying to understand the basics of image processing in Siril before finishing off in an image processor. From various pages, I see that background extract, colour calibration, histogram stretching, green noise reduction seem to be the common steps, but the order seems to be different in different tutorials. Does the order matter, and what should it be?

Background Extraction
For background extraction, where should I put the green squares? I see tutorials mentioning that they should go where there aren’t stars, but then the videos place them in areas with stars. My squares seem to be quite small too. Should I use “generate” to get a grid of squares and delete the ones that are not my subject? Does it matter if I use the red, green, or blue channel to add the squares?

Colour Calibration
Do we need to apply background neutralisation separately to the R, G, and B channels? I seem to end up with splotches of red, blue, etc. Do I need to repeatedly neutralise the background on each channel until there’s none of the corresponding colour in background areas? If I apply background neutralisation and white point calibration, the image tends to have very little colour… almost becomes black and white. I think I must be doing something wrong… any pointers?

As an example, here’s my first attempt at Andromeda. You can see that there’s red and blue tones in various places that looks like background.

You have to put your samples in the background. I mean the real background. If one sample is on a star it is not a big deal (if the stars is not too big). If all samples are on stars. So it will be a problem.

Why don’t you use the photometric color calibration?

Thanks, @lock042 .

I’ll try the photometric calibration. So would the workflow be like this:

  1. Background extraction
  2. Green noise reduction
  3. Stretch the histogram (via auto?)
  4. Photometric colour calibration.

Would you recommend a different order? Anything else you think I should be doing before exporting to darktable to finish off?

The colour calibration must be done before green noise reduction and before any histogram modification of the image. The first will modify the colour balance in a way that will not help the colour calibration, the second will make it completely fail because stars will not have a real relative magnitude anymore.
So your order should be 1, 4, 2, 3.

Thanks, @vinvin

Should I be completing all background extraction before colour calibration? Or can I also do some afterwards (if I realise there’s more left to clear up)? And should I be doing background extraction once on red, once on blue, once on green, or does doing it on one channel cater for the other two?

I think it can be done before because it will not change star magnitudes and I think running it extracts the background on all channels. You may run it at the end too if you like, you may also run it after preprocessing on each image of the sequence if you have a long acquisition time with varying gradients, it’s easier to remove then in the stacked image.

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For what it’s worth, using Lights, Darks & Biases my workflow is:
Change directory to where the three folders are.
Under Scripts click Processing-no-flats.
When script finishes, OPEN “” in the folders where the files are.
Colour Calibration - select BLACK area and then WHITE area - apply
Remove Green noise - average neutral
Change Auto Stretch back to Linear
Histogram Transformation - drag black pointer to the right.
Asinh Transformation - drag black pointer over to the right
Background Transformation - remove any spots that are on any part of the image you want (right click) - Apply
Repeat Asinh and then,
Colour saturation.

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