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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 18 Oct 2009 (Sunday) 10:43
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First go with a light pollution filter

 
MintMark
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Oct 18, 2009 10:43 |  #1

Here's a first picture with an Astronomik CLS filter. It's one light frame (1 minute, f2.8, ISO800), dark frames subtracted and then just stretched. It's not far above the horizon. I deliberately made it lighter than I normally would so you can see the colours. The sky comes out green blue instead of red brown and I'm sure I can expose for much longer without the sky getting too bright. The brightest stars are blown out though.

What doesn't really show in the full picture is the colours of the stars. At full size they are quite blue and green with red fringes around the bright stars. I've attached a 100% crop. If I try and achieve a neutral colour balance I end up with pink stars. Same thing happens if I try background subtraction.

Any tips on processing these images?


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VIGER
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Oct 18, 2009 11:58 |  #2

Hi Mark

To best help you I would need the RAW file(s)

A+

Michel


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SteveInNZ
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Oct 18, 2009 14:06 as a reply to  @ VIGER's post |  #3

You could start by shooting a white card in daylight through the filter.

Steve.


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troypiggo
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Oct 18, 2009 15:05 |  #4

White balance does get a little funky with these filters, because they remove part of the visible spectrum in the red/orange/yellow area I think. That's why you're seeing green/blue bias. I haven't gone back to really giving mine a workout, but my intention was to do what Steve mentions and do a custom white balance with the filter in the camera while I shoot a white balance card in daylight. I white sheet of paper might be near enough for you, but make sure you don't overexpose it.

If the stars are brighter, ie close to white/saturation, and the bg is blue/green, if you adjust for the bg to be neutral, that will make your stars pinkish I guess. Not sure if you're using curves, but I'd suggest learning them. You should be able to adjust the colour cast of the bg just at the dark/shadow end of the histogram and keep the lighter end the same. Heaps of control.

It's very easy to blow out stars with DSLRs. It's all about controlling your exposures, maybe utilising HDR type methodology.


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MintMark
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Oct 20, 2009 14:54 as a reply to  @ troypiggo's post |  #5

OK, I've had another go at this and I'm much happier with the results this time. This is using pixinsight LE by the way. To start with I generated a backgound using DBE and subtracted it from the image (with rescaling) to leave me with a flat background.

Before, I was trying to achieve colour balance by using the histogram tool and getting all the peaks lined up. I'm sure I saw that in a tutorial somewhere. This time I used the curves for each channel. I found the threshold value between sky and stars for each channel and used contol points there to allow me to adjust above and below that value independently. Thanks for the suggestion Troy!

This let me adjust the bottom end of the curves to achieve a grey background. Then I adjusted the top end of the curve to tone down the red fringes on the bright stars. After that I adjusted the overall RGB/K curve to enhance the contrast.

That's pretty much it. There are still a lot of faint stars that are blue and green, but looking at the pixels there is just very little red information there. I need to work on the star colours some more...

Anyway, thanks for your comments.


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Oct 21, 2009 03:39 |  #6

Much better Mark, well done on the reprocess, looks like DBE is your freind ;)




  
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MintMark
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Oct 21, 2009 14:29 as a reply to  @ DSLR AstroMod's post |  #7

Yes, DBE is good. I was hoping that with the light pollution filter I wouldn't need to use it. Next time I have clear sky I want to do some with and without filter comparisons to see how much improvement there is in terms of reduced sky gradient. I suppose there will always be vignetting from the lens to get rid of... perhaps I should start taking flats.

BTW, it looks like pixinsight LE isn't available any more. That's a shame. I started using it mainly because it could process images in 16 and 32 bit depths.


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Oct 21, 2009 15:42 |  #8
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That's a great improvement, well done!


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08GT
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Oct 22, 2009 10:05 as a reply to  @ Closed 123's post |  #9

Looks real good.. I like it !! What is DBE???? It looks like I need this for some of my pics...


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MintMark
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Oct 23, 2009 07:26 as a reply to  @ 08GT's post |  #10

DBE is a feature in an image processing package called pixinsight. It stands for Dynamic Background Extraction. It lets you interactively indicate areas of the image that are background sky (rather than stars or nebulae) and then it generates a synthetic background from your selected points.

If you subtract the background from your image it removes the combined effects of light pollution (sky gradient) and vignetting (darker corners).


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First go with a light pollution filter
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