We don’t all have the luxury of an observatory at the top of a dark mountain, miles from the nearest city lights. For the vast majority of budding astrophotographers, the most convenient and accessible location is from their own backyard. Unless you live in the country already (and most people don’t), there is likely a largish city nearby lighting up the night sky and washing out all those beautiful faint fuzzes you want to image.
There are two obvious options. The first is to go remote, and those with the means can set up a remote scope on a mount some place like New Mexico skies, and have some automation system such as ACP farm the heavens for photons all night long most nights of the year. To me, this feels like cheating… I still think I need to “earn” the data. I’ll likely change my mind one day, but that’s where I am right now. The second option is to go narrow band, a topic I’ve written about before, and a practice I often employ from outside Orlando Florida. But I’m not going to discuss that either.
I want to talk about good old fashioned RGB “Pretty Picture” imaging from outside a city, with a sky that is somewhat less than pristine. Here’s a hand full of tips to help you make the most of light polluted skies.
1. The Sun, Moon, and Planets are virtually immune to light pollution. Okay, maybe I’m cheating with that one… but it’s worth pointing out that these are interesting targets, and shooting the sun doesn’t even require staying up all night.
2. Pick bright objects. Open clusters, globulars, asterisms… all these things stand up exceptionally well to light pollution. Bright emission nebula are also fair game, I’ve seen some stunning images of M42 shot from right outside a major metropolis. Large galaxies with low surface brightness like M101 or M33… not so much, but brighter galaxies like Andromeda are certainly approachable.
3. You need LOTS of subs. Maybe three times more than from a dark sky. Because the background is bright, it produces a lot of signal for the background, lowering the contrast of the object your shooting. More subs will bring down the noise, allowing you to more aggressively stretch your data.
4. The longer you can go the better. Under light polluted skies, most CCD’s and DSLR’s will saturate very quickly. This is where the extra money for those CCD cameras with more full well capacity pays off. The signal on your chip increases with time, and the light of the object is actually the light from the object itself + the sky glow. You have to go a long time to bring out the fainter areas against that sky glow. More subs will not help for this, you have to go long to get the fainter signal out of the noise. Another thing to watch for is to keep your histogram within the linear range of your camera. Even if your not saturated, most (anti blooming) cameras start to compress their dynamic range somewhere after the 2/3 mark of their full well values (typically 65,535).
5. YOU MUST TAKE FLATS. Seriously… so many people I’ve talked to think flats are just for vignetting or dust bunny’s. Instead of taking flats, they’ll use some gradient tools in Photoshop or Pixinsight to get rid of the light fall off. However, flats do more than that, they get rid of “fixed pattern noise”. Fixed pattern noise technically isn’t really noise at all (noise is uncertainty of signal), but rather unwanted signal. It’s almost undetectable at low signal strengths such as when your imaging a faint object in a dark sky. But under urban skies with ample sky glow, the over all signal is very high, and the fixed pattern noise soars. The proper name for fixed pattern noise is PRNU, or Photo Response Non Uniformity. It is the small variation from pixel to pixel in the sensitivity of the imaging chip. This peppering of your image will completely mask fine details if you don’t eliminate it by taking proper flats.
6. Use a light pollution filter. Note this is near the bottom of the list. I would not use a light pollution filter under lightly or moderatly light polluted skies. In the suburbs, there is usually one direction in which you can image that is not completely overcome by sky glow. A light pollution filter (I use the IDAS LP when I do use them) will require much longer exposures. It’s not quite as bad as narrow band, but there is a tradeoff between just shooting more subs with a little light pollution, or having to take much longer subs with a light pollution filter. Where this line is depends on how bright your sky is, and what object your trying to image. You should get one anyway, and you should experiment with it. My best results have been by using a LP filter in conjunction with an RGB filter set on a CCD camera. This eliminates the color shift apparent in most one shot color cameras (DSLR for example) when using these filters. When I do use a LP filter, I always use it on a monochrome CCD camera, not a DSLR or one shot color CCD. Color cameras have much lower sensitivity to begin with, and then putting a light pollution filter in front of it just makes it even worse. If your patient and can shoot lots of subs with your OSC though, don’t let me stop you.
7. Practice. Astrophotography is a technical art, and it requires skill. Skill comes only with practice, not reading books and blogs, or buying expensive equipment. You must practice your art as often as you can. Experiment for yourself, see what works, what does not, and refine your own technique. “I don’t want to do that, I just want to set up and be able to take great pictures”… let me be blunt with you… you should probably find another hobby. 😉 I’m not sure in the strictest sense a hobby is something your do for pure amusement, but rather the challenges and the accomplishments it brings. I learned to guide during full moons for example, and it took a while to get the hang of it. Maybe I’m just a slow learner, but I was a not a slow learner on the one or two nights I had good dark skies and clear weather at a state park.
I practice excessively at home so that when I go to my dark sky site, I’m ready to take advantage of the superior conditions there. I’m amazed at star parties when I see people who have driven miles from their home cities to take equipment out of the box for the first time… They are usually frustrated shortly after dark and end up going to bed early. A good star party is one where I never sleep before dawn. But that’s just me.