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Old 16-08-2009, 12:02 PM
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Exfso (Peter)
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Colourless Stars

I have found when doing long exposures with my Modded 40D that even though the images have plenty of color and nice and sharp, that the stars are nearly all just a bland burnt out white color. I have tried various methods in PS to correct this, but nothing seems to work. Does anyone have any ideas on what may be a satifactory solution. The attached Trifid shows this up I think, as I cannot see any blue or orange larger stars.
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  #2  
Old 16-08-2009, 12:13 PM
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renormalised (Carl)
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Smile

I can see plenty of stars in that piccie that show colour....reds and blues mostly. If you burn't the stars out so much that they lost their colours you'd be having problems with fainter details as well.

That's actually a very nice piccie

Bit of compression artifact creeping in but you have to be looking for it to notice it.
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Old 16-08-2009, 12:51 PM
Dennis
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Have a look at the individual RGB Histograms. If they all show 255 then they are saturated so that the Red, Green and Blue Channels will not have any tones other than 255. This will give whites which are burned out; that is, there is no texture or detail in the stellar disc.

An analogy is like trying to take a single photo of a bride and groom on their wedding day in bright sun light, showing the nice pin stripe in the grooms black suit and the fine, lacy detail in the brides dress.

If you expose for the grooms black suit you should see the texture of the material and the pin stripe but the bride’s white gown will be a glowing, saturated white, with no details as it will be grossly over exposed.

If you expose for the brides white dress, so that you can see the lace and texture of the material, then it is likely the grooms black suit will be a single block of black, with no detail as you will have under exposed for the darks.

Cheers

Dennis
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Old 16-08-2009, 01:54 PM
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Thanks Dennis, I can see where you are coming from. Basically it is not really achievable in a one color camera, so there are trade-offs to get a good overall image.
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Old 16-08-2009, 02:22 PM
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you could take shorter images to capture the star colours then mask them in to the final image... thats one way I can think of..

It can also be caused by over stretching data in the higher range... I generally do my final iterations on curves with a backwards S shaped curve, giving the darker tones a bit of a punch, whilst not boosting the the higher ranges, and in turn, burning the star colours out...
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Old 16-08-2009, 03:07 PM
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I have seen some DSLR images posted on IIS where the poster combined several short, medium and long exposures, almost like a high dynamic range image, so that parts of the image did not display burnt out highlights or noisy shadows.

Cheers

Dennis
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Old 16-08-2009, 05:55 PM
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HDR is the only way see here

http://d1355990.i49.quadrahosting.co...5/cardith1.jpg


Bert
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Old 17-08-2009, 05:48 PM
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HDR may well be the way to go here, thanks Bert Dennis and others.
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Old 17-08-2009, 06:33 PM
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multiweb (Marc)
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Quote:
Originally Posted by Exfso View Post
I have found when doing long exposures with my Modded 40D that even though the images have plenty of color and nice and sharp, that the stars are nearly all just a bland burnt out white color. I have tried various methods in PS to correct this, but nothing seems to work. Does anyone have any ideas on what may be a satifactory solution. The attached Trifid shows this up I think, as I cannot see any blue or orange larger stars.
If you lower your ISO it will help keeping star colors. One thing that will make a big difference also is to use median combine rather than sum combine when stacking your subs. By the time you bring your pic in PS for post-processing you shouldn't burn anything.
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Old 18-08-2009, 09:25 PM
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Thanks Marc, I have Noel Carbone's tools and there is an action there which does the job to a certain extent in PS.
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Old 18-08-2009, 10:21 PM
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Its one of those things experienced astrophotographers make look so easy that I suspect is actually quite hard for many of us.

Some suggestions:
- use lower ISO (400 or 800) as suggested to avoid burn out
- isolate (mask) and copy stars early on in processing, so can manipulate and combine and mix back in later
- you need to boost the colour saturation to get the colours you're after, however doing this across the entire picture often accentuates chromatic noise in the background of the image to unacceptable levels
- image from dark sky site with no pollution filter (and no sky gradient)

For quick and dirty in Photoshop:
- Select/Color Range/select:Shadows, tick "invert" box
- Select/Modify/Expand = 2
- Select/Feather = 2
- Image/Adjustments/Hue/Saturation - boost Saturation slider

You need to have the color balance, black points, etc set close to final balanced values for this to work - if you have too much blue or whatever showing at this point, all your stars will come out with that color accentuated - just undo, go back and tweak colour balance, then have a go again as per above.
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Old 18-08-2009, 11:51 PM
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Rob, if that works, maybe you should add it to photoshop tips in the tips forum.

The easiest way to get colour in your stars is to get a 16bit camera.
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Old 20-08-2009, 10:38 PM
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Thanks Rob, much appreciated, might give that a go..
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