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Old 13-10-2020, 06:49 PM
stephen2615 (Stephen)
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Blue tinge when combining narrowband

Greetings,

For some reason, I have a blue tinge when combining my narrowband images before doing anything to them other than combining. This happens in PixInsight (SHO) or in Photoshop. I can remove it when doing PI DBE but it sort of messes up the colours in general and ends up with more of one colour than there should be. Eg, too much blue in M16 and too much red/pink in Helix Nebula. I am relatively new to Narrowband processing. My filters are Optolong 7 nm and my flats look good. My Sii stack is surprisingly bright and looks blurry (even though it isn't) when imaging the Helix Nebula. The stacked images look good so I just can't determine why this is happening.

Any hints or comments would be very welcome.

Regards

Stephen
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Old 13-10-2020, 09:55 PM
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Harpage (Danh)
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You may want to try doing LinearFit on your narrowband channels before combining them, as per this tutorial: https://www.lightvortexastronomy.com...rocessing.html
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Old 14-10-2020, 08:42 AM
stephen2615 (Stephen)
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Harpage,

Thanks for your help. It did remove a lot of the blue tinge but there is still more blue than I would like. At least I have something better to work with. I might experiment with Pixel Math to see if reducing the amount of Oiii can reduce the blue.

Stephen
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Old 14-10-2020, 09:31 AM
carneb (Barry)
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Did you try doing a background neutralisation to see if that gets rid of the blue tinge?
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Old 14-10-2020, 01:10 PM
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rustigsmed (Russell)
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have you got some examples you can share Stephen.

cheers
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Old 14-10-2020, 06:14 PM
stephen2615 (Stephen)
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Hopefully, you can see this folder. It has three stacked images in PixInsight format.

https://www.dropbox.com/sh/h7ufwy1vq...3oWj5bx0a?dl=0

I used WBPP in PI to get the images that I have cropped a bit. My framing in NINA went pair shaped so that is why it looks the way it is. These images have not had any other work done on them.
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Old 14-10-2020, 06:35 PM
stephen2615 (Stephen)
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Quote:
Originally Posted by carneb View Post
Did you try doing a background neutralisation to see if that gets rid of the blue tinge?
Hi Carneb,

It does work to a certain extent but the problem is that if I don't stretch the image, I can't see it well enough to determine the good dark area. If it is stretched, all I see is blue.

At least I have some other colours with BN.

Thanks

Stephen
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Old 15-10-2020, 08:30 AM
carneb (Barry)
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Quote:
Originally Posted by stephen2615 View Post
Hi Carneb,

It does work to a certain extent but the problem is that if I don't stretch the image, I can't see it well enough to determine the good dark area. If it is stretched, all I see is blue.

At least I have some other colours with BN.

Thanks

Stephen
I've tried all different combinations when processing narrowband, unlinked STF, Linked STF, Linear Fit, and combinations of these and others, but I found the best way of getting the background black (instead of red or blue) was to use unlinked STF to get an almost black background, then choose the preview and do background neutralisation, then do a linked STF to get an idea of what the final streched image will look like before the final stretch.
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Old 15-10-2020, 09:59 AM
stephen2615 (Stephen)
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Problem solved.

Some background might be handy. I have a QHY 16200A CFW7. I installed the filters in the positions numbered from 1 through 7. Ha was 1, Sii was 2, etc.
I use NINA and I (thought) I mapped the same filters to the same positions.

I didn't realise that NINA starts from zero not one. I mapped the filters starting from one (deleting the zero altogether). So each of my filters were one out. I now have Ha mapped to NINA position 0 not 1, Si to 1 not 2, etc, etc.

That is not the first time I have mucked up a zero with a one. I doubt it will be the last.
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