# Light pollution removal



## pursuer (Sep 27, 2005)

I have had a few people ask me about this so I decided to make up a tutorial. I will be using adobe photoshop CS2 but most of this should be relevant for earlier versions as well.

 Below is an image of the peiades star cluster taken by doenoe.
Instead of a nice dark sky the background is saturated by light pollution, ay attempt boost the levels of the stars will also amplify the light pollution.







Open the image in PS and duplicate it.





apply a dust and scratches filter just strong enougth to remove all the stars to the copy of the image.
I used a filter with a radius of 16 and a threshold of 7. 





This should leave you with just the light pollution.





So right now we hve the original image plus a copy with just the LP.





The next step is remove the isolated LP from the original. There are a few ways to do this but the best is to invert and color burn.

select the origional image and then choose apply image from the menu





The apply image dialog box will appear, 
select the copy as the source
set blending to Color Burn
check the box that says invert
hit ok





You should now have and image of just the stars with the LP removed.





Now you can boost the levels to taste and apply a usharp mask, I used 0.5 at 100%

The finished product




origional


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## Meysha (Sep 27, 2005)

Great tute Pursuer! Once again shows there's a million ways to skin a cat.
I did it a slightly more simplified way (for me) by using layers, instead of creating a whole new document. But whatever works best.

I tried the way you explained here on one of my shots and it worked a treat! But it also lost the tree detail in the foreground. Is there any easy way you know of keeping the tree visible?


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## pursuer (Sep 27, 2005)

Thats a tougth one Vicky, I tried using the tree itself as a mask





But I realised the tree is so dark that it just won't show up against the black sky, so the best I cloud come up with is this






I used the same method as above, except I set the opacity to 60% so the sky would not be completely dark. That left me with a slightly redish sky, So I used the hue and saturation dialoge to selectivly edit the reds and setting the lightness to 0. This is new for me, my astro pics never include a foreground.


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## doenoe (Sep 27, 2005)

wow, thats a great tut. Cant believe thats the same image. So many stars
Thanks alot, ill probably gonna use this alot


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## Onyx (Sep 28, 2005)

im too lazy to go through every step so heres the psd http://www.jimkvavle.com/startree.psd

the bottom layer i created by using a dark blue gradient map.


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## PlasticSpanner (Sep 29, 2005)

Here is a page that links to more processing tutorials. Hope they are of some use!


http://www.madpc.net/~firmament/astro/main_screen.html?http://www.madpc.net/~firmament/astro/software.html&2


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## ShaCow (Oct 10, 2005)

minted. very sweeeet


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## vonnagy (Oct 10, 2005)

nice work mate - thanks for the tutorial!


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## Bettybooty (Oct 12, 2005)

Is Photoshop CS2 pretty much the same thing as version 7?


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## toruonu (Oct 12, 2005)

The order should be: 
Adobe Photoshop 7.0
Adobe Photoshop CS (actual version 8.0)
Adobe Photoshop CS2 (actual version 9.0)


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## JohnMF (Oct 12, 2005)

Bettybooty said:
			
		

> Is Photoshop CS2 pretty much the same thing as version 7?



bettyB, Its similar. I just upgraded from 7 to CS2 and found that most of the new features are aimed at photograhy rather than web/graphics. More so than recent previous versions. Getting back to what PS was originally for!   

but if you still have 7, you will be able to get by no probs


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