Dark halos around stars - what are your best techniques to remove them? - Forums

Topic starter

IOTD Staff

Jun 11, 2022

Alrighty team - An image popped up recently that was very well done, but - there were dark halos visible around the stars, which was a distraction.

This led me to ponder…hmmm - what's the best way to remove these?

Specifically, we're talking about images where the stars are removed and replaced ie: replace NB stars with RGB etc.

I'm primarily a photoshop user and my usual method is to shoot short 2min RGB subs just for the stars, stack & stretch then paste into the respective channels of a new RGB document, then layer these stars over a starless image using 'Lighten" mode.

Works pretty well … most of the time!

So I'm curious as to what other methods are in use, particularly if you have managed to avoid creating those annoying dark halos.

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Jacob Heppell

Jun 11, 2022

For me, I've experimented heaps with my Photoshop stretching techniques and have settled on a method that avoids forming dark halos all together.
I also add RGB stars to a starless narrowband image but I use "linear dodge" and not the "lighten" blend mode.

Carastro

Jun 11, 2022 · Edited: Jun 11, 2022

Jacob Heppell:
For me, I've experimented heaps with my Photoshop stretching techniques and have settled on a method that avoids forming dark halos all together.
I also add RGB stars to a starless narrowband image but I use "linear dodge" and not the "lighten" blend mode.

Please share your method. I use Photoshop CS3 and have this problem.

l have in the last week been shown how to use Images Plus for star reduction which works well but would like to know how to do it in Photoshop.

Thanks

Bogdan Borz

Jun 11, 2022

Hi Andy,

I think the best way to treat dark halos is not to produce them in the first place. I never used Photoshop for that, I always use Pixinsight. The dark halos are provoked by the mask used for adding the stars back in. Usually because the mask was created for stars of a different size (usually more stretched ones on the main image for example). So you have to be careful which one you use. A method for bypassing this issue completely is to combine only the color from the RGB stars in the Cie Lab mode in Pixinsight (a*b applied on the masked stars). Of course, if your star luminance is burned and pixels are > 1.0 value, no color information can be inserted. But this avoids the creation of the dark halos in the first place.

CS,
Bogdan

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Jacob Heppell

Jun 11, 2022

Carastro:
Jacob Heppell:
For me, I've experimented heaps with my Photoshop stretching techniques and have settled on a method that avoids forming dark halos all together.
I also add RGB stars to a starless narrowband image but I use "linear dodge" and not the "lighten" blend mode.

Please share your method. I use Photoshop CS3 and have this problem.

l have in the last week been shown how to use Images Plus for star reduction which works well but would like to know how to do it in Photoshop.

Thanks

I'll pm you some links to a few video tutorials I did regarding my stretching and processing in Photoshop. I have CS5 but I expect the features I use are also in previous version. I have made some changes since so should probably do an update.

Carastro

Jun 11, 2022

I think what may be causing the halos is the method of Select color range, select expand and select feather being too big. I have in the past used expand by 4 and feather by 2, then I reduced that to expand by 2 and feather by 1, but even that still gives halos.

So today i expanded by 1 and feathered by 0.5 and that seems to work better.

Carole

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David Koslicki

Jun 11, 2022 · Edited: Jun 11, 2022

My approach is to use StarNet2 to remove the stars in the NB image, then edit that starless image. I then take the stacked RGB stars and use the Photoshop "difference" blend mode between the (unedited) starless NB and RGB image to get just the stars (on a pure black background). I then paste this over the starless image with screen, linear dodge, or lighten; whichever looks best for that particular image. Since the background of this layer is pure black, this results in basically the best star mask you could hope for when using one of the PS lighten modes.

Example of the result of the process:

The Crescent Nebula

I'm still learning PI, so have been sticking with PS for now. Likely there's a better method in PI, but that works for me for now.

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Andy Wray

Jun 11, 2022 · Edited: Jun 11, 2022

David Koslicki:
I'm still learning PI, so have been sticking with PS for now. Likely there's a better method in PI, but that works for me for now.

I've been doing exactly the same in PixInsight. We have very different setups, however we ended up with similar results (albeit yours are better from a resolution and colour point of view). I'm actually OK that my £380 OTA is a bit worse.

See my image for comparison:

Quick and dirty Crescent

Bob Lockwood

Jun 11, 2022 · Edited: Jun 11, 2022

Andy 01:
I'm primarily a photoshop user and my usual method is to shoot short 2min RGB subs just for the stars, stack & stretch then paste into the respective channels of a new RGB document, then layer these stars over a starless image using 'Lighten" mode.

Hi Andy,Not sure just what you are doing from the beginning, but what I do for my NB images is similar to what you do. I take a set of short rgb’s like you, typically 2 min maybe 6 to 10 each, they are just for stars, don’t need a ton of data. From the start I get the combined NB image, now an SHO, and the combined image of the rgb stars and open them in PS, I’m using PSCC. From there before any processing is done, and you don’t want to do anything to either image before you remove the stars, I use both the new starNet and StarXTerminator depending on the image, sometimes one works better than the other. I do the subtract on the rgb star image so you have just a star field and I save it as ‘Just Stars’ makes it easy to find. I may tweak the star image just to up the color or do any minor adjustments and save it. With the stars removed from the SHO image, and still no processing has been done yet, right, there should not be any halo artifacts to begin with, so do whatever processing you do and what I do to add the stars back in is using Layers ‘ Linear Dodge (Add)’ It seems to work better, leaves all the stars where ‘Lighten’ cuts some out, also leaves the stars a little brighter. That’s what I do, hope it helps.

Forgot to mention that all my pre-processing, calibrating, darks and so on to get to the SHO or LRGB tif is done using CCDStack.

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