Small stars, big fuzzies, a 'How to...'

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Small stars, big fuzzies, a 'How to...'

Postby ollypenrice » Sat Oct 05, 2013 8:54 am

In galaxy widefields you need to stretch very hard to catch the faint fuzzies but this tends to mean you'll stretch your stars as well. I find star masks only semi effective so here's an alternative approach for galaxy images. It won't work for stars set against nebulosity but it will give a nicer emphasis to the faint galaxies since the big stars won't dominate the image. This works for one shot colour, though I'll explain it for LRGB.

1) Prepare your hard LRGB stretch as usual, accepting that the stars will grow at least as fast as the galaxies.

2) Set your background sky to the value you like using the Colour Sampler Tool in Ps. I like 23/23/23 or thereabouts. The flatter your background sky the better. DBE in Pixinsight is surely the best background tool but you can use Gradient Xterminator etc.

3) Return to your RGB only linear image. This will almost certainly give you the best stars (unless you've binned the data in which case this probably won't work!)
Give it a stretch in Curves which rises steeply as usual but then flattens early. Your objective is to brighten the background till it has EXACTLY the same values as your main image but has small colourful stars. Ignore the galaxies. You won't be using them. Once the background reaches a flat 23/23/23 (or whatever your main image was) stop and save the 'Starfield' image.

4) Copy and paste the Starfield image on top of the Main Image and set it's opacity to Zero in the Layers palette. So now you can't see it at all. Leave it as the active layer, though.

5) Take the Eraser, choose zero hardness to give it a soft edge, and make it the right size for being able to run over your galaxies without going too far beyond them. You'll need to change its size as you go round the image.

6) Now simply run the eraser over all the galaxies you can see. Nothing will seem to change because you are erasing an invisible layer. But then...

7) Lift the opacity of the top layer and your small stars will cover your overstretched big ones but the galaxies will show from below. Provided you got the backgropund skies dead equal the result will be seamless. Flatten and save.

If you binned the stars just do a separate starfield stretch of the LRGB data but follow the same principles. Ditto One Shot Colour /DSLR.

A note on DSLRs; these burn out star colour easily so maybe this idea could be used to restore it?

Some examples of this technique;

http://ollypenrice.smugmug.com/Other/Be ... WEB-X3.jpg

http://ollypenrice.smugmug.com/Other/Be ... ORE-X3.jpg

http://ollypenrice.smugmug.com/Other/Be ... ial-X3.jpg

http://ollypenrice.smugmug.com/Other/Be ... IDE-X3.jpg

Olly

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Re: Small stars, big fuzzies, a 'How to...'

Postby hemihaggis » Sat Oct 12, 2013 7:55 pm

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Re: Small stars, big fuzzies, a 'How to...'

Postby Wide-Field » Sat Oct 12, 2013 8:53 pm

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