Colour Blotches in background

[Todo43 (Lachlan)]

Hi all,

I have had this issue for a little while now and wondering how I can fix it in post-processing. The attached image is a starless image of the Christmas tree cluster. Taken from a Bortle 1 sky, I had 3 hours and 5 mins of 5mins subs.

Gear is Skywatcher NEQ6, Sharpstar AL90-R scope, ZWO ASI533MC Pro

In the top right and bottom left corners, the image is very splotchy with a sort of mottled colour. Other than possibly having data there that I just don't have enough exposure time to pull out, what could be happening? Is there an issue with my workflow that I might have?

My workflow is as follows. I almost always use Pixinsight for the entirety.

- Stack in WBPP using bias, darks and flats

- Extract Channels from master and linear fit

- Recombine images using Channel Combination

- Dynamic Background Extractor placing reference points on areas with very little nebulosity. In this case, it was the splotchy mottled area. About 10 points per quadrant

- Background Neutralisation

- Spectrophotometric Colour Calibration

- SCNR

- NoiseXterminator

- BlurXterminator

- Stretching using Arcsinh then onto histogram transformation

- Curves transformation for contrast correction and small adjustments

- Star Reduction using EZ Processing Suite EZ Star Reduction

- Saturation Increases using Curves Transformation and sometimes a hue change

- Finishing touches using Curves transformation

Just wondering as it has happened in other images as well. This was taken in B1 so thinking its not caused by light pollution.

Happy to provide examples of other cases that I have had.

Your help is much appreciated!

[oska (John)]

I don't know for certain but to me it looks a bit like over stretch and over saturation? One thing I am pretty sure of is BlurX must go before any noise reduction. Put it just after SPCC.

[alpal]

Hi Lachlan,
Do you have Photoshop? - it's easy to fix.
If you split the channels you'll see that the
blotchy areas are in the Red channel.


Go back to your original picture.
Make a new layer.

Control M will give you curves.
Click on the Red channel.
Sample the dark areas of the blotchyness.
You'll see they are at about 38 bits out of 255.
The brighter Red areas are at around 52 bits.
Adjust the curves to boost the areas around 38 up to about 42.
Adjust the areas that are around 52 down to about 48.
Fix up the other areas that have now moved in the curves
so that they line up with the original curves.
You only want to adjust the areas mentioned.
It's now adjusted - so
select the opacity to change the strength of the changes to your liking.


cheers
Allan

[Todo43 (Lachlan)]

Originally Posted by oska

I don't know for certain but to me it looks a bit like over stretch and over saturation? One thing I am pretty sure of is BlurX must go before any noise reduction. Put it just after SPCC.

Thanks for that tip! I will have another go at processing it and try to reduce the saturation. The thing that intruiges me is that there must be something wrong that I have done earlier because I can only saturate things that are in the image already.

[Todo43 (Lachlan)]

Originally Posted by alpal

Hi Lachlan,
Do you have Photoshop? - it's easy to fix.
If you split the channels you'll see that the
blotchy areas are in the Red channel.


Go back to your original picture.
Make a new layer.

Control M will give you curves.
Click on the Red channel.
Sample the dark areas of the blotchyness.
You'll see they are at about 38 bits out of 255.
The brighter Red areas are at around 52 bits.
Adjust the curves to boost the areas around 38 up to about 42.
Adjust the areas that are around 52 down to about 48.
Fix up the other areas that have now moved in the curves
so that they line up with the original curves.
You only want to adjust the areas mentioned.
It's now adjusted - so
select the opacity to change the strength of the changes to your liking.


cheers
Allan

Hi Allan,

That is an interesting technique. I just tried that and I lost a lot of red in the image specifically around the border between the nebula and the background. It also less of removed the background and just made it a bit more neutral and toned down the colours, except the blotchiness is still evident.

If anyone is keen to try to give the data a process, feel free. Here is the original pixinsight stack from WBPP

[AdamJL]

5 minute edit... might not show up right in a compressed forum jpeg, but just make sure to run some NR early in your editing phase.

It's good data!

[Todo43 (Lachlan)]

Just a quick update,

I ran BlurXterminator before SCNR and NoiseXterminator and then stretched it. Haven't added any saturation and unfortunately, I think it is due to overstretching.

I'm not going to be able to image for a long time sadly which is very annoying and I honestly thought that 3 hours in B1 would give me something slightly better than this.

Another thought is, and I've done it a couple times, use the original image with the weird blotch marks, but then have another image with a better background. And then using pixel math maybe, just pick up the nebula from the original image and place it on the good background image. I just don't quite know how I would do that though.

[alpal]

Originally Posted by Todo43

Hi Allan,

That is an interesting technique. I just tried that and I lost a lot of red in the image specifically around the border between the nebula and the background. It also less of removed the background and just made it a bit more neutral and toned down the colours, except the blotchiness is still evident.

If anyone is keen to try to give the data a process, feel free. Here is the original pixinsight stack from WBPP

Hi Lachlan,
I can assure you that my idea works -

I just tried it again now on your picture.
The border between the nebula will of course change slightly as
I gave you directions for a global adjustment.
In Photoshop you can select just the area you want to work on instead.
That could require working on 3 or 4 areas separately and may
require blurred or feathered layer masks to not look obvious.



You asked -

"wondering how I can fix it in post-processing"
I can't think of a better way.
In Photoshop - you are the artist - you can paint whatever you like


cheers
Allan

[oska (John)]

I don't have PS but in PI I was playing with your data and arcsinh stretch. I could clobber most of bits you are tyring to remove by raising the black point until the clipping basically trod on the dust. Is that cheating?

[Nikolas (Nik)]

If you gathered more data those blotches, which are overprocessed will be better defined, I'm currently playing with your data and there is some detail which is interstellar HA hightened dust and gas


This was with no DBE or background neutralisation.
I did blur exterminator
Noise extyerminator
SCNR
Generalised hyperbolic stretch then into photoshop
Levels and curves adjustments
Star exterminator
extremely little and I mean FADED Topaz denoise
Minor gradient exterminator
Dulled and played around with the stars
Then recombined it all and saved
It isnt perfect but there are no blotches just dust channels that with more acquisition will really stand out


Hope that helps

[Todo43 (Lachlan)]

Those processes all look great. Probably better than I could do. I gathered another hour of data last night as it spontaneously cleared up. Thats from B5 though so nowhere near the quality of data from B1. I think I will try and get as long as I can on this target in the next few months and see if I can resolve the data in the Ha dust clouds around it. I have looked at others images and seen that they have managed to pull it out so it will be really interesting to see how long it takes before my data can do that.

[Nikolas (Nik)]

You don’t need to do a DBE with this data as it doesn’t seem to need it