Processing the nebulae in orion - wipe and colour

Questions and answers about processing in StarTools and how to accomplish certain tasks.
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EmanueleDM
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Joined: Sat Sep 19, 2020 9:59 am

Processing the nebulae in orion - wipe and colour

Post by EmanueleDM »

Hello, having recently acquired a new PC I am finally processing various pictures from this winter.

This one in particular has been giving me some headaches, it's a stack of about 70 120" lights, 20 flats and biases shot at ISO 400 with a modified Canon 100D and a Samyang 135 at f2 on a star adventurer from a bortle 4 location. Images were captured directly in .fits with astroberry and then stacked in siril. As can be seen from the severe walking noise I did not dither, I am looking into a DIY motorization of the dec axis to deal with this in the future.

Here is the stacked file if anyone would like to give it a spin.

And this is my definitive attempt at processing with the related log.
m42.jpg
m42.jpg (387.15 KiB) Viewed 190 times

Removing the gradient caused by a streetlight proved tough, I had to push aggressiveness to 98% and there still appears to be some light pollution around the upper right corner. Is it bad to bring that parameter almost to the max? Would it be better to leave it at a lower value and crop out remaining gradients? I masked some dark portions of the image (the horsehead and the dark clouds of M78) as I had the impression that they were messing with the result, I am unsure whether it's actually something I should be doing, or perhaps the dark anomaly filter is sufficient?

Usually to colour balance I sample from a star mask with very good results, but this time in several attemts it has always resulted in a strong green cast which I tried to deal with in "Max RGB" mode. I imagine it has do with chromatic aberration and fringes around the stars, but balance was still off after shrinking the mask to exclude them.

There is also some banding, how should I remove it?
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Re: Processing the nebulae in orion - wipe and colour

Post by admin »

Hi,

There is definitely a lot of fast undulating unevenness going on that is hard to discern by people (and algorithms) with 100% certainty.
That gradient is indeed pretty bad, but I cannot be completely sure your flats are perfectly working either (you will know from your other datasets - do they perfectly work there?). The "hole" at 8 o'clock is somewhat suspicious, with a uneven lit border around the edges.
Vertical banding (I would have though would be caught by your bias frames) conspires with the above to make very fast undulating uneven lighting differences, which cannot be removed by modelling a slowly changing gradient.

Dithering would help a lot to mitigate all these issues. Any location-based unevenness, will be spread out, making it easier to reject by the stacker.

With regards to the gradient, I think you did a decent job in Wipe (you could also try the Uncalibrated 1 and Ucalibrtaed 2 presets). Gradient removal will only get you so far; once the low frequency gradients are removed, any remaining unevenness and uncertainty in the signal will remain visible if stretched a lot - Wipe's Temporary diagnostics stretch does precisely that (e.g stretching too far on purpose).

If your dataset exhibits strange behaviour around the edges, you can try a different Gradient Edge Behavior mode (I tried "Grow opposite axis", which makes some assumptions about the nature of the gradient near the edge being of artificial origin rather than celestial).

The vertical banding can be quite effectively mitigated by selecting Adaptive Vertical Bias for the Synthetic Dark/Bias setting.
StarTools_544.jpg
StarTools_544.jpg (412.57 KiB) Viewed 174 times
If I cannot be 100% sure all faint detail in my entire image is real, then, out of an abundance of caution, I would just specify a Region of Interest over the object exhibiting the greatest dynamic range (e.g the object that runs from darkest to lightest the most), which in this case is M42;
StarTools_545.jpg
StarTools_545.jpg (286.47 KiB) Viewed 174 times
Jumping to the Color module, I found it came up with a decent default color balance for the nebulosity. The only exception, as you point out, being some of the stars. This could be due to alignment issues or stacker misconfiguration.

The quick and dirty way to address this is to use the Legacy preset, which simply desaturates the highlights like other software does, conveniently "hiding" the issue. You can add Cap Green and Highlight Repair to that for good measure if you wish.

You will end up with something like this;
Image
Hope this helps!
Ivo Jager
StarTools creator and astronomy enthusiast
EmanueleDM
Posts: 11
Joined: Sat Sep 19, 2020 9:59 am

Re: Processing the nebulae in orion - wipe and colour

Post by EmanueleDM »

That gradient is indeed pretty bad, but I cannot be completely sure your flats are perfectly working either (you will know from your other datasets - do they perfectly work there?). The "hole" at 8 o'clock is somewhat suspicious, with a uneven lit border around the edges.
Vertical banding (I would have though would be caught by your bias frames) conspires with the above to make very fast undulating uneven lighting differences, which cannot be removed by modelling a slowly changing gradient.
I am not sure what's wrong with this data. I have never noticed banding in calibrated images with this camera, could it have to do with the flats? I tried stacking without them and although less evident it looks like it's still there. Here is the flatless stack for reference.

As usual thank you for your help.
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Re: Processing the nebulae in orion - wipe and colour

Post by admin »

EmanueleDM wrote: Sun Apr 11, 2021 9:26 pm I am not sure what's wrong with this data. I have never noticed banding in calibrated images with this camera, could it have to do with the flats? I tried stacking without them and although less evident it looks like it's still there. Here is the flatless stack for reference.
As you get deeper into the data, which proper calibration and dithering will let you do, then minor signal aberrations will start to stand out more.
The banding is visible in the stack without flats as well. Dithering would probably take care of a lot of this.
Ivo Jager
StarTools creator and astronomy enthusiast
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