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 Remove light pollution from night sky 
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Joined: Fri Jun 15, 2012 3:22 pm
Posts: 12
Location: Belgium
Post Remove light pollution from night sky
Hi everyone,

I've been fascinated with star photography and time lapse.
But every time i face a lot of trouble with removing noise, and light pollution from nearby villages.

Can anyone give some hints on how they overcome these problems? Or a tutorial, piece of software that can remove it i would really appreciate it.

My shots are made with a canon 5D II + 24mm 1.4
usually with:
aperture: 1.4
Shutter: 25 - 30sec.
ISO: 250 - 400

(this image is 30sec. ISO 250)
These are the results i'm getting. This image is straight out of the camera (only converted from raw to .jpg)
Image

The same image but after i tried some things in Photoshop mostly in the camera raw screen.
Image

Any feedback is appreciated .
thanks


Sun Jun 30, 2013 6:45 am
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Joined: Fri Oct 21, 2011 3:25 pm
Posts: 153
Location: North Bend, Washington
Post Re: Remove light pollution from night sky
David Kingham has some excellent free (donationware) night sky presets for Lightroom and ACR that may help:
http://www.davidkinghamphotography.com/ ... om-presets

Also, it may sound counter-intuitive, but to combat noise you might try higher ISOs - the idea being that noise is much worse with under-exposed images. I usually use at least ISO 1600 for night skies.


Sun Jun 30, 2013 10:18 am
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Joined: Wed Apr 03, 2013 7:20 pm
Posts: 340
Location: Brisbane, Australia.
Post Re: Remove light pollution from night sky
Agreed re the ISO - I never shoot astro below 1600, and often I'm up at 3200 or 6400 depending on the shoot.

Rather than spend time in post trying to remove light spill, I find myself driving further and further away from the cities to find darker sky. Even at almost 2 hours away I can still make out Brisbane, a city of maybe 2 million including surrounding suburbs, if the camera is pointing in that direction.

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Sun Jun 30, 2013 3:26 pm
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Joined: Tue Jun 15, 2010 8:21 am
Posts: 22
Location: Ann Arbor, MI
Post Re: Remove light pollution from night sky
kmatm wrote:
Agreed re the ISO - I never shoot astro below 1600, and often I'm up at 3200 or 6400 depending on the shoot.


Not sure about other cameras, but on the 5D2, 6400 is not a native ISO. If you're shooting RAW, you'll end up with better dynamic range when using 3200 and lower (at full stops).


Sun Jun 30, 2013 5:35 pm
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Joined: Fri Jun 15, 2012 3:22 pm
Posts: 12
Location: Belgium
Post Re: Remove light pollution from night sky
@jimre-temp: wow, looks amazing. Definitely going to try that. I just hope it will also work on image sequences.

Yes, it also seems higher ISO is probable needed to go further... Though most times that feels wrong.

@kmatm: wow, sounds great to be able to go out that far into untainted nature.
I would probably need to go out of the country for this. Since I live in a country with an overly eager public service company. They install street lights kind of the same way a farmer plants corn! :roll:

The above image was taken inside a nature reserve in a very rural location, with only a handful of homes/ farms within 10min. Driving distance,
But every road with a house has street lights every 100-150meters. :cry:

Anyway, thanks for the feedback. if anyone has some more, feel free to share, and I'll try it all the next time I'm out shooting.


Sun Jun 30, 2013 10:43 pm
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Joined: Fri Apr 03, 2009 6:15 pm
Posts: 832
Location: Middle Earth
Post Re: Remove light pollution from night sky
On a 5DII I use 3200 iso but somewhere I read that the best Sweet spot is found at 1250 iso.

For one of my starlapse I opened the window in the bathroom and pointed at the southern cross and milkyway and went to bed :-)

The main reason was that a heavy frost was due that night and I wanted about 400 shots= 4 hours shooting at continuous 32 secs exposure while keeping the gear above the dew point.

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Mon Jul 01, 2013 2:10 am
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Joined: Wed Apr 03, 2013 7:20 pm
Posts: 340
Location: Brisbane, Australia.
Post Re: Remove light pollution from night sky
Well it's not quite 'untained nature' - still a lot of farmland but yes you can find places with no close light sources.

As far as removing noise there are a lot of solutions, I've trialled a few and currently use Noiseware by Imagenomic.

I'm sure there are tutorials around about removing light, Ben Canales provides a paid tutorial here: http://bencanales.wordpress.com/. You could also try playing around with layer masks in PS, but different coloured light spill can be a real pain to work with.

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Mon Jul 01, 2013 3:07 pm
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Joined: Tue Jul 16, 2013 7:48 pm
Posts: 4
Post Re: Remove light pollution from night sky
Depends on your particular problem. The trick is you don't want to remove real nebulosity!

If it is a "sky only" shot, you might be able to find a plug-in for setting white balance by clicking on the background sky. In ImagesPlus [not free], I click on "Neutralize Background" and poof it's gone, easy to apply in batch mode to hundreds of images. This software also lets one remove gradients (more light pollution and sky glow towards the horizon).

For a wide angle that includes horizon and fixed light pollution, I can only think of using a mask or an "inverse bias" field (instead of brightening the edges due to vignetting, one dims the zones where there is LP).

Everything is a trade-off... driving to a darker site may actually cost less in time than trying to post-process the LP out. There is no substitute for a dark site for some things, but you can get amazing results with narrow band filters and oodles of exposures to beat back the noise. But even absolutely dark skies get aurorae (for us northerners), and I've seen pictures of natural airglow at Texas Star Party, Mauna Kea, Chille. Oh, and the gegenschein/zodiacal band... be careful not to remove that, or shoot at another time of the year. And there is always latitude, if what you're trying to get lies in the lowest 5 degrees above the horizon, stop wasting your time, save up and travel to where it is high in the sky. If that's out of the question you will have to accept limitations.


Thu Jul 25, 2013 10:50 am
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