Can you remove magenta star halos in StarTools?

Questions and answers about processing in StarTools and how to accomplish certain tasks.
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PaulInNorthMichigan
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Joined: Sat Aug 08, 2020 5:44 pm

Can you remove magenta star halos in StarTools?

Post by PaulInNorthMichigan »

1st of all I realize where my issue is coming from, but if this problem can be solved it will open the window too numerous opportunities in the future.

A while back an individual from another forum challenged me to step my older 180mm lens to the smallest focal ratio which is 2.8 as it would greatly reduce the amount of necessary integration time over the next step at 4.0. Of course this would naturally increase vignetting but this specific lens also has another greater challenge. In getting the tightest star focus it adds a magenta hue to the stars.

Jerry Lodriguss does an exceptional review of the Nikkor 180mm ED AI-S lens. He specifically addresses the focus issue in using the 2.8 focal ratio.

https://www.astropix.com/html/i_astrop/ ... 180mm.html

He discusses the challenge and the potential payback in using the lens at it's optimal focal ratio and focus.
It would come down to the question of whether you want to waste clear dark-sky time shooting more frames stopped down to improve the signal-to-noise ratio, or spend the time in the electronic darkroom fixing the halos on a cloudy night. Personally, I would prefer an image with a higher signal-to-noise ratio because there is no substitute for this in terms of image quality, and clear dark-sky time is precious, whereas I know I can improve the halo problem later in processing.
A couple nights ago I had the great opportunity to image closer to the southern horizon while I camped on a Michigan lake. Still I had a very limited window of time to shoot the sky so I am trying what Jerry suggests and am hoping that StarTools has an antidote to the magenta halos.

I can show you what the results appear like. I know that I can't just increase the red rejection because there is some red nebulosity that I'd like to preserve.

Here is the base Fits file prior to the StarTools import in case someone would like to look at the image.

https://www.dropbox.com/s/fgp6nvqj0nc3y ... .fits?dl=0

Here is the resulting image that I processed in StarTools.
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admin
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Re: Can you remove magenta star halos in StarTools?

Post by admin »

Hi Paul,

It's a super easy one.
After running the Color module, run the Filter module.
Create a "Fat Stars" mask and make sure all the stars with the halos are selected in that (e.g. Grow once or twice).
Then back in the Filter module, set the Filter Mode to "Fringe Killer". Now zoom into one of the stars and keep clicking on the purple halo pixels around the stars. They should start to disappear;
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Hope this helps!
Ivo Jager
StarTools creator and astronomy enthusiast
PaulInNorthMichigan
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Joined: Sat Aug 08, 2020 5:44 pm

Re: Can you remove magenta star halos in StarTools?

Post by PaulInNorthMichigan »

Ivo,

Excellent! This shows that it is possible to rid the image of magenta halos.

The steps that you took to accomplish this appear to be fairly easy.

The hard part will be with understanding the interface in applying masks appropriately. Once that is grasped it should be easy.

I have been going through some of the user notes associated with masking and brush application and will give this a try this afternoon.

Thank you so much for taking the time to show me that this is indeed possible and the steps necessary to achieve this.

I anticipate that I'll use this technique frequently when using my 180mm lens at 2.8 focal ratio.
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Re: Can you remove magenta star halos in StarTools?

Post by admin »

PaulInNorthMichigan wrote: Sun Sep 13, 2020 11:31 am The hard part will be with understanding the interface in applying masks appropriately. Once that is grasped it should be easy.
The Auto Mask generator will do most of the heavy lifting for you;
Mask > Auto > Fat Stars > Do, then, back in the mask editing interface, Grow, Grow.
That's the mask I used.
Ivo Jager
StarTools creator and astronomy enthusiast
PaulInNorthMichigan
Posts: 12
Joined: Sat Aug 08, 2020 5:44 pm

Re: Can you remove magenta star halos in StarTools?

Post by PaulInNorthMichigan »

Ivo, I did figure this out. It was tricky to understand how to get to the correct sequence with masking but from now on this should be easy.

It also was a bit tricky in screen selecting for the removal of the magenta coloration, but it did apply correctly. Once again knowing how to do this will make it easy the next time.

Thank you for spelling the order out clearly for me; without knowing this I doubt I would have stumbled on the answer.
happy-kat
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Re: Can you remove magenta star halos in StarTools?

Post by happy-kat »

I used another tip IVO has shared before, it was this one:
'if you find purple stars offensive, launch the Layer module, choose "Invert foreground" as the Layer Mode, Keep, launch Color module, set Cap Green to 100%, then launch the Layer module, choose "Invert foreground" as the Layer Mode.'
resized for upload
Though it looks to be an over correction as I lost the pink bit, though it was useful to me to try that on your image.
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PaulInNorthMichigan
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Joined: Sat Aug 08, 2020 5:44 pm

Re: Can you remove magenta star halos in StarTools?

Post by PaulInNorthMichigan »

happy-kat wrote: Mon Sep 14, 2020 2:14 pm I used another tip IVO has shared before, it was this one:
'if you find purple stars offensive, launch the Layer module, choose "Invert foreground" as the Layer Mode, Keep, launch Color module, set Cap Green to 100%, then launch the Layer module, choose "Invert foreground" as the Layer Mode.'
resized for upload
Though it looks to be an over correction as I lost the pink bit, though it was useful to me to try that on your image.
Trifid_Region invert fg.jpg
Thank you, it's good to know that there are different ways to attack a problem within the software.

In seeing how easy a mask can be deployed to allow fine tuning of undesirable image aspects it appears that you can greatly improve an image in StarTools.
happy-kat
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Re: Can you remove magenta star halos in StarTools?

Post by happy-kat »

The tip IVO shared was used on a narrow band image where the colours are different anyway, but I thought to try it on your RGB image, though I guess a mask could have been created when in the colour module in the middle step on the fat stars grow x2 and then the cap green before going back to the layer module
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