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How to remove purple stars

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#1 space(man)

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Posted 20 August 2018 - 12:43 PM

After years of not seeing the north sky - my house is in the way - I've managed to establish a temporary set-up at the end of the garden which now provides a decent view over the roof of the house = hooray!  Just starting to image some of the popular features in the north + my first time out for many weeks due to absence of astro darkness managed to get some good narrowband subs last week of the Heart Nebula. 

 

As would be expected the Ha-channel was good and surprisingly so was SII.  For a first time effort I'm quite pleased with the resulting SHO image (see attached file) except for the purple stars and wondered if anyone could provide (a) an explanation and (b) a processing solution - I'm using an old CS2 version of PS and a recently acquired and excellent Affinity Photo.

 

Thanks, Graham   

Attached Thumbnails

  • SHO Final (Medium).jpg

Edited by space(man), 20 August 2018 - 12:44 PM.

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#2 pyrasanth

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Posted 20 August 2018 - 01:14 PM

I think the issue with the colour is that your mapping narrow band wave lengths to get simulated RGB which can only give an unusual colour spectrum. A simpler solution to correct the purple stars would be to use an adjustment layer in PS and change the colour to anything you might like. This entails creating an accurate mask which protects the areas you don't wish to change. There are a few places in the image which would benefit from a dynamic range adjustment but apart from that its a nice image.

 

Did you map RGB to SHO?


Edited by pyrasanth, 20 August 2018 - 01:19 PM.

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#3 cfosterstars

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Posted 20 August 2018 - 01:20 PM

One option is to take some RGB data directly and use it to just replace the stars only. I know how to do this in Pixinsight but not with photo shop. You should just google "replace stars with RGB stars". I know you will get answers with PS, because I had to weed through them to get an answer for Pixinsight. You can also do color mapping and that is what I do, but again I use pixinsight.



#4 Dan Crowson

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Posted 20 August 2018 - 01:27 PM

Taking some RGB for the stars could be accomplished in PhotoShop by just pasting the RGB layer on top and selecting 'Lighten' for the blending mode. At this point, use the levels tool and pull it from the left to the right (cutting/clipping into the data) until only the stars are impacted. Finally, flatten the image.

Dan



#5 gardenfish

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Posted 20 August 2018 - 03:30 PM

In LR if you have it use the HSL tab and you can remove the purple in Saturation Hue and another with the sliders. I have done this a couple times and it worked well. 



#6 schmeah

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Posted 20 August 2018 - 06:33 PM

You can get natural appearing stars without obtaining RGB. If you use just Ha and OIII and process speparately as a bicolor image first ( I use Cannistra's method ) the star color  can be very close to natural. Here is an example of such bicolor processing:

http://www.pbase.com...image/164236118

You can use this starfield to replace the magenta tricolor narrowband starfield that you get later.

Here are a couple of examples using bicolor stars in a tricolor NB image:

http://www.pbase.com...image/149679628

http://www.pbase.com...image/160607382

 

This will save you from having to shoot an additional night of RGB.

 

 

 

Derek


Edited by schmeah, 20 August 2018 - 06:44 PM.


#7 space(man)

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Posted 21 August 2018 - 02:31 AM

Thanks all for the comments which are much appreciated and demonstrate a number of possibilities for correcting the star colour.  I'll take a look today and may get back with further questions - yes this was RGB mapped to SHO.  Not sure I understand the bi-colour route, but have already done HOO (see below), what next to get to SHO then?  

 

Despite my best efforts, I still find processing something of a dark art which I continually battle to understand and conquer, as I'm often sure I'm not getting the best from my data. I have the 'how to' book on PixInsight and might need to take the plunge this coming winter - the results look good for others but the learning curve is scary - oh well, that's astrophotography!      

 

Graham

 

HHOOx (Medium).jpg



#8 rigel123

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Posted 21 August 2018 - 08:14 AM

One other method in PS is to go to Selective Color, pick Magenta and just move the slider for Magenta all the way to the left



#9 cfosterstars

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Posted 21 August 2018 - 12:47 PM

For my NB SHO images in Pixinsight, I am able to get rid on the Magenta star and get them to look very blue without going to RGB star replacement. I have done it both ways. This is the wall in NGC7000 without star replacement but with color mapping:

 

NGC7000LSHODRZ NR


#10 adamphillips

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Posted 21 August 2018 - 12:54 PM

my method is almost too easy, its for photoshop.

simply go into adjustments, selective color. select magentas. and then pull the magenta slider down to zero.

hope this helps

 

oh, I see someone else already posted this method, but seriously, I do it every time and it works great.

 

and your first SHO image up there, you can probably play around in selective color and get the colors even better. I always experiment with it


Edited by adamphillips, 21 August 2018 - 12:57 PM.


#11 Ladyhawke

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Posted 21 August 2018 - 12:56 PM

Thanks all for the comments which are much appreciated and demonstrate a number of possibilities for correcting the star colour.  I'll take a look today and may get back with further questions - yes this was RGB mapped to SHO.  Not sure I understand the bi-colour route, but have already done HOO (see below), what next to get to SHO then?  

 

Despite my best efforts, I still find processing something of a dark art which I continually battle to understand and conquer, as I'm often sure I'm not getting the best from my data. I have the 'how to' book on PixInsight and might need to take the plunge this coming winter - the results look good for others but the learning curve is scary - oh well, that's astrophotography!      

 

Graham

 

attachicon.gif HHOOx (Medium).jpg

Now that your stars are green and there's no green on the rest of the image, just use Hasta La Vista Green to turn the green to white.



#12 space(man)

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Posted 21 August 2018 - 02:07 PM

One other method in PS is to go to Selective Color, pick Magenta and just move the slider for Magenta all the way to the left

OK but that affects the entire image - how do I restrict it to the stars only?



#13 terry59

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Posted 22 August 2018 - 07:03 AM

OK but that affects the entire image - how do I restrict it to the stars only?

Go to select, then color range and select highlights. Next, go to “Select,” then “Modify,” then “Expand,” and put in a value of 6 pixels. Go to “Select,” then “Modify,” then “Feather.” Choose a value of 3 pixels. Typically, you want a value for this that’s half the amount you expanded.

 

6 and 3 are arbitrary numbers. Use values that work for you



#14 Mike7Mak

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Posted 22 August 2018 - 09:16 AM

Or, take the original posted image and invert it, then run HLVG on it and invert it back.

Attached Thumbnails

  • post-263196-0-42436900-1534.jpg

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#15 buras

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Posted 23 August 2018 - 02:17 AM

Or, take the original posted image and invert it, then run HLVG on it and invert it back.

this ^. inverted magenta is green, so running green reduction on inverted image will result in magenta reduction. But be aware that it will reduce magenta/purple in all image, not only in stars, so if your wish is only to reduce it in stars- use masks. In PixInsight this workflow is as simple as "ctrl+i -> SCNR Green -> ctrl+i". There are also some other methods which might work better or worse on some images, e.g. ColorMask with magenta selected, then applying that mask to image and doing manipulations with colors in CurvesTransformation. Then theres this PixelMath formula as well:

R: $T[0]
G: iif(min($T[0],$T[2])>$T[1],min($T[0],$T[2]),$T[1])
B: $T[2]

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#16 Mike7Mak

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Posted 23 August 2018 - 05:19 AM

 

this ^. inverted magenta is green, so running green reduction on inverted image will result in magenta reduction. But be aware that it will reduce magenta/purple in all image, not only in stars, so if your wish is only to reduce it in stars- use masks. In PixInsight this workflow is as simple as "ctrl+i -> SCNR Green -> ctrl+i". There are also some other methods which might work better or worse on some images, e.g. ColorMask with magenta selected, then applying that mask to image and doing manipulations with colors in CurvesTransformation. Then theres this PixelMath formula as well:

R: $T[0]
G: iif(min($T[0],$T[2])>$T[1],min($T[0],$T[2]),$T[1])
B: $T[2]

Yes agreed. The only time the simple 'invert, hlvg, invert' technique works in a subtle way is when the star halos are the dominant, or only, magenta in the image. I've had it turn entire 'magenta-ish' ha nebulas from red to a sickening pumpkin yellow brown. HLVG is gonna affect anything that appears green-ish in the inverted image.



#17 GrandadCast

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Posted 23 August 2018 - 08:57 AM

Getting back to PS, I use this plugin for years as it has many useful function. 

https://www.prodigit...ll_Version.html

 

Once click action 'Reduce Small Blue/Violet Halos' and it was done in a second (i7 processor). About as easy as it gets. 

 

Jess

 

post-263196-0-42436900-1534787060 copy.jpg




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