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Old 15-08-2009, 04:11 PM
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bojan is offline
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 6,300
Originally Posted by Lumen Miner View Post
Yup my images were not too good, very light polluted. you can barly see the milky way in them, basically black....... I'm not shooting in RAW yet either, just JPEG.... I will try soon though.

Yes, I would appreciate hearing your method... I am not one, to turn down free knowledge... Thanks..
Well, your image is quite good, considering from where it was taken :-)

The method I developed to remove LP background is illustrated below: they are all screenshots from Digital Photo Professional Tool Palette, that I am sure came with your Canon.

1) After opening stacked image from DSS (in Tiff format, darks and flats are applied as well as part of DSS processing), you have the histogram looking like (1).

2) In RGB tab, you form the curve like it is shown on image 2, by clicking and dragging the curve to the desired position. I usually start with lower left side and add 2 or 3 points more.

3) Adjust the individual thresholds for each colour channel (by right-clicking and dragging the left-most edge of the histogram frame in appropriate tab), such that the histograms for colours are more or less aligned, like shown on 3 for Red.
For images from light polluted sites, you will have to do it for Red and Green only.. the Blue one will usually remain as it was. The result is shown on image 4.

4) Then, go back to RGB tab and adjust thresholds as shown on 6.

5) repeat the procedure once more from 3), until you have histogram as shown on image 6.

6) Finally adjust upper threshold like shown on 7.

And this is it.. 5 minutes of work or even less (not counting here the time for stacking, this may take longer, depending on number of images and the speed of your computer). All done without additional expensive software, not a single additional cent spent.. (you should already have DPP, since it comes with each Canon DSLR). You can upgrade it on Canon official site to the latest version for free.
The method also works for dark sites, but then you may have to play with Blue channel as well, and the first RGB curve may not need to be so "curvy".. because there will be much less LP background.
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Last edited by bojan; 15-08-2009 at 06:33 PM.
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