removing light pollution from photographs of the night sky

<p>Is there a way to remove light polution from photographs of the night sky without making a photo so dark or altering the color so much, that the details and colors of the stars are not ruined. For example, this a copy of an original raw photo, that I took of the night sky. How do I get rid of all of the brown light pollution, while keeping the rest of the skys color and detail looking as natural as possible. The editing softwares that I have are Photoshop Elements 6, and Canon Digital Photo Professional. Thanks for any advice.</p><div></div>

<p>I also have an edited copy of this image posted in my portfolio, but I am not to happy with the results, it doesnt look natural enough to me, like I said any advice would be appreciated. </p>

<p>Exactly how do you want the finished photo to look? Do you want the brown tone color-corrected to natural bluish skylight, or are you trying to "remove the light" altogether?</p>

<p>Thanks Rick for your quick response. I want the sky to look more natural, more bluish. In my edited version in my portfolio the sky is bluish but the colors in the stars were lost, and I think I may have made the upper portion of the photo to bright, but the details of the milky way show up more. I just need some advice on how to get the most out of editing, these types of photos where light pollution is a problem, and advice in general about editing photos of the night sky.</p>

<p>Christa:<br>

I don't think you can accomplish what you want without using better tools than Elements and DPP. With PSCS 3 or 4 you can do exactly what you want, but it's a long post to explain how, especially if you have not used Photoshop before. And it takes some experience. Essentially, it involves the use of color range selection tools, carefully constructed masks, and I would estimate a dozen or more careful steps to transform the original. With Photoshop, you will get a good result.<br>

I know that's not what you want to hear, and maybe someone else has a simpler solution, though I doubt it. Let me know if you want to learn more, as I don't want to clog up your thread unnecessarily.</p>

<p>Christa, I took a look at this photo in your portfolio and I think you did a very good job with it. You don't want to over-process it to the point it's unreal. The only suggestion I'd have is to turn-off your car headlights next time so you don't light-up the foreground; that was more of a distraction for me than the brownish light pollution.</p>

<p>Another thought would be to literally get further away from the city and aim at the sky in the opposite direction of the offending light source. And take a couple friends with you. </p>

<p>You have some nice night/sunset images in your portfolio; you have the skills and don't need to rely on over-processing your images.</p>

<p>Thanks Bill for your very nice compliments! I agree about the foreground being a distraction, but at the time I thought it would look good to light the foreground. I live 30 miles from cincinnati, and still get alot of light polution when pointing in that direction (west).</p>

<p>Hi Rick,<br>

Sorry I didnt see your response a little while ago, I have been thinking about getting one of the better editing programs, hopefully within the next year. But right now I am stuck with what I have......... thanks for your suggestion.</p>

<p>The orange glow near the horizon is almost certainly due to the use of sodium vapor lamps in the nearby city. These lamps emit almost all of their optical power in a narrow band around 590 nm. The amount of light they emit in the green and blue portions of the spectrum is many stops less. Thus, it is almost a hopeless cause to try to change the color in post processing by using PS tools such as "photo filters or curves to try to pull down the orange part of the spectrum and boost the remaining colors. Rather, one should use a PS tool such as hue/sat with the colorize box checked which can, by brute-force, shift the orange hue into the more visually pleasing blue part of the spectrum and not just change the relative intensities of these two parts of the spectrum of the original image.<br /> <br /> OTOH, the narrow band output of these lamps, particularly, the low pressure variants, makes it very easy to remove this color of light by using a traditional glass filter in front of your lens. Such filters are commonly available (check any amateur astronomy supplier). Unfortunately, because they are narrow band, not a usual wide band photo filter, they are not cheap. <br /> <br /> Using the hue/sat (with "colorize" checked), I did a quick retouch of your image to demonstrate what can be done in PS. The first step in the process was to be able to separately process the sky from the rest of the image. I didn't have the time to generate the accurate mask needed to do this (ie, get all the jigs and jags of the trees on the horizon), but you will get the idea from my rough mask. The second step was to use a vertical B&W gradient in an adjustment layer (in overlay blending mode) to roughly equalize the brightness of the sky as you move upwards from the horizon. At this point, I inserted the hue/sat layer to change the color. The final step was to use a curves layer on the foreground to bring out a bit of detail.<br /> <br /> I have no experience with Photoshop Elements so I don't know if the tools I used are available in that version of PS, but I can tell you that in PS, it took no more than 5 min. to generate the tweaked image.<br /> <br /> HTH,<br /> <br /> Tom M<br /> Washington, DC</p><div></div>

<p>PS - I forgot to mention a couple of other processing steps: I used Neat Image to clean up some of the noise in the image, and I also used a curves layer on the sky to adjust the contrast and brightness.</p>

<p>PPS - I mentioned that I hardly spent any time developing a mask to separate the sky from the vegetation. As an example, look at the very tip of the nearest plant. ;-)<br>

T</p>

 

<p>Thanks Tom,<br>

I think I can do most of this in photoshop elements, although I dont have layers mask, I did find a plugin download for layers mask that I will try. What are the glass filters called? I would much rather deal with the light polution, in camera. Thanks for taking the time to help me.</p>

<p>Hi Christa -</p>

<p>You're quite welcome. Glad to add my $0.02.</p>

<p>You can find a lot of info on these filters by Googling {sodium filter astronomy}. Below are a few links that (on a quick perusal) look useful:</p>

<p>1) http://www.astronexus.com/node/4<br>

2) http://www.nezumi.demon.co.uk/nonad/nonad.htm<br>

3) http://www.scopecity.net/astronomy_optics.php?fnm=Nebular&sch=filter&nst=Nebular+Filters&title=Nebular+Filters (scroll down to the section titled, "Anti-Light Pollution Filter").<br>

4) http://books.google.com/books?id=l2TNnHkdDpkC&pg=PA238&lpg=PA238&dq=sodium+filter+astronomy&source=bl&ots=Sqlv2IGQl1&sig=Yjp8lxpUuKgcx4q84mTJkrwcxIE&hl=en&ei=isyZSsmCG6WCmQfOyYHCBA&sa=X&oi=book_result&ct=result&resnum=8#v=onepage&q=sodium%20filter%20astronomy&f=false<br>

5) http://www.modernastronomy.com/filtersAstronomik.htm</p>

<p>Note the comment in the last sentence of the 1st link: <em>"... The stronger filter (which is too strong for any decent astronomy sites) but may be of interest to city dwellers cuts the D-line by about a factor of 100000. This is sufficient to look up at a street lamp and see no yellow light! ..."</em></p>

<p>My guess is that there probably are quite a few filters of this type available used on e-bay.</p>

<p>BTW, obviously, you will have to be sure that the size of the filter matches your lens. In addition, as I recall, the screw threads used to mount filters for astronomy are not the same as the screw threads to mount normal photographic filters. However, with a bit of Googling / improvisation, you should be able to get a functional setup.</p>

<p>Good luck & report back with some nice pix!</p>

<p>Cheers,</p>

<p>Tom<br>

Washington, DC</p>

<p>PS - Since there are many broadband sources of light pollution, eg, tungsten lights, don't expect miracles, but they help out tremendously if a city has gone over to using Na street lamps.</p>