|
What’s troubling you Bunky? Bad ol’ light pollution got you down? You want to observe, but with no Moon or planets in the sky you think it’s a waste of time? Easier just to plunk yourself down in front of the boob tube and tune in the latest installment of “Who Wants to be a Pin-headed Weasel?”
Light pollution is unfortunately just a fact of life for the modern amateur astronomer. Thanks to the efforts of committed amateurs who’ve banded together as the International Dark Sky Association (IDA) to fight this evil of Modern Times, the corner has been turned on light pollution in many communities. For most of us, though, it will be a long time before the skies of our towns and cities return to the comparably pristine condition they were in in the 1960s (when the light pollution problem got started with the unchecked expansion of the suburbs). What do you do until things change for the better? Veg out? Surf the Internet? You can certainly do Lunar and planetary observing. For some people this becomes a passion, and they just never worry about chasing down galaxies. The Moon and planets (with the exception of Pluto) are not affected by light pollution at all. But this is not the answer for all amateurs. For many of us the call of wild interstellar and intergalactic space is just too strong. We hunger for the deep sky.
But how do you satisfy this hunger when you live in the city and work a 60 hour week? There’s always the local club—most have a dark site of some kind available for member use. But this is usually not an every-night thing. Or you can seek out a remote site on your own (though secure dark sites close to home where you can observe well and in safety are hard to come by). Or there’s the back yard!
The humble backyard is where most of us got out observing starts, anyway. It’s certainly convenient and hopefully secure. But what can you possibly see of the deep sky from the sodium-orange skies of the average subdivision back-garden? One heck of a lot. And not just easy stuff like double and multiple stars. I mean nebulae, galaxies and star clusters by the score, the whole deep sky menagerie. There’s no denying that all of these beautiful night birds look better from dark sites. But many, many wonders are visible from the average suburb or city.
By making use of your home site, you can also stay in “practice.” This is a very important benefit if you are in the habit of expending considerable time and resources attending star parties. If you wind up observing only once or twice a month, your deep sky locating and observing skills will atrophy for sure. Get out under the dark skies of the Texas Star Party and you’ll find yourself at a loss—for a while, anyway. Besides, observing is fun. And pushing your scope and yourself to the limits can turn up some real beauties, even in the suburban—or urban—skies, making your nightly trek to the backyard an end in itself.
Where to start? The first thing we need to do is dispel a myth. A harmful myth. It goes like this: “A big scope, a large aperture scope, is useless under city skies. The big lens or mirror collects too much light, too much light pollution. You’ll see much more in a small scope. Get a 90mm refractor instead of a 12 inch dob!” Hogwash. The truth is that a large scope beats a smaller one under City Lights just like it does everywhere else. Yes, the gain may be less in light pollution than in dark skies magnitude-wise. But it may actually be more dramatic from light-polluted environs. For example, from my heavily light polluted back yard (Dorothy and I live in the historic district near the downtown of our city of 250,000), M13 looks like a blob in one of my 8 inch telescopes. Albeit a BRIGHT blob, but really just a blob, nevertheless. In contrast, in my 12.5” dob the cluster takes on at least a hint of its spectacular appearance from a dark site. It is indeed a huge ball of Suns.
There is one truth to the old saw about big scopes and light pollution. In your large short focal length scope, the sky background will be pretty bright. Objects will still look much better than they do in a small scope, though, and you can suppress the bright sky background by increasing magnification.
Do you need any special accessories to help you tackle the bright sky? Certainly a light pollution reduction (LPR) filter can be helpful. As long as you understand that one is not a replacement for dark skies. The biggest drawback to LPR filters is the fact that they are only good on nebulae. Sadly, the “good light” from the stars lies in the same passband as the “bad light” from Earthly streetlights. Some folks will tell you that the mildest filters, the Orion Skyglow or the Lumicon Deep Sky, can help a little on galaxies. But I’ve never seen this. My favorite filter? Easy, the Lumicon OIII. This sucker can make M97, the Owl Nebula, go from invisible to showing his eyes from my urban backyard!
Quite a few amateurs are disappointed by the performance of their new LPR filters. They just don’t seem to do much of anything at all. Why? LPR filters work by reflecting unwanted wavelengths of light. They are made by depositing chemical layers on a piece of optically flat glass. Each layer reflects a certain wavelength of “bad” light. This works well until you start shining light into these filters from the eye end! Ambient light coming in from the eye end of your eyepiece will wreak havoc with your filtered view. You must do something to shield the eyepiece from ambient light sources (by “ambient light” I mean your neighbor’s porch light or other nearby lights).
In truth, when many urban observers moan about their “light pollution” what they are complaining about is two things: skyglow and ambient light. You can’t do anything about skyglow. But ambient light from nearby fixtures is at least as harmful, and you can do something about it. Ambient light prevents you from attaining any degree of dark adaptation at all—you can’t really take advantage of what photons there are. It also reflects off your eyepiece, weasels its way into the optical train of your scope, and, in general, makes things far worse than they should be for your degree of light pollution.
But, yes, you can do something about ambient light. The simplest thing to do is to make light shields for you and your scope. If you use a dew cap on your refractor or your SCT, you already have an effective light shield for Mr. Telescope. If you use a Newtonian, especially a dob, though, the secondary mirror and focuser may be quite close to the end of the tube, and really vulnerable to stray light. Make an extension for your tube using any material that’s handy—plastic or even cardboard. I made a nice light shield for the upper cage of my truss tube dob out of Kydex plastic, which is mounted to the UCA with Velcro.
And for you? How about a hood? A piece of dark cloth (ripstop Nylon is nice) draped over your head can work wonders. You’ll look ridiculous, but blocking stray light from your eyes and eyepiece really will help. To help preserve your dark adaptation when you’re not under the hood, use an eyepatch (which you can get at the drugstore). Place it over your observing eye when you have to go inside to answer that inevitable phone call.
But the real solution to the ambient light problem is to put your scope in an observatory. Not only does this mean that your scope is always setup and ready to go—and will therefore get used often--it means that you are completely blocked from nearby lighting and are able to attain at least some dark adaptation. Not ready to build? There are some nice halfway measures you can implement for just a few dollars. Large moveable light shields can be placed around your scope to shield both you and it from nearby lights. One way of doing this is to make a frame from PVC pipes and place plastic tarp material over this frame. Or you can build a wooden frame of some kind and cover it with cotton or canvas like a theatrical “flat” used in stage sets. You can move these shields around with you if you need to move the scope to escape trees. A 4 by 8 foot movable light shield or two can eliminate most of your ambient light woes.
Ok, this sounds good, but you can’t see anything if you can’t find it! When the skies are sodium-orange with only a few stars peeping out how can you hope to find objects located far from bright stars? It’s easy. Ditch that Telrad. The zero power Telrad is a fine invention, but because it doesn’t provide any more light than what your naked eye can gather, it isn’t a big help in light polluted areas. On the other hand, a good 50mm finder can pull many guide stars out of that bright sky. Digital setting circles can also obviously be a boon for the urban observer.
You’re in the right place but still can’t see anything? There are some tricks to enhance your chances of penetrating the glare. A well-known and efficacious technique is averted vision. Look away from rather than directly at a dim object in order to use the dim light receptors of your eyes. Jiggling the scope can also help, as the eye is more sensitive to dim moving objects. But the best thing you can do is increase magnification. This comes as a surprise to many novices and even to some seasoned amateurs—after all, everybody knows that wide fields and low power are best for deep sky work, right?
Wrong. Increasing magnification can make dimmer objects pop out of the skyglow, as it tends to spread-out the background light of the sky, increasing contrast between it and your target. If you can’t seem to catch an object, bump up the magnification. You should also make it a habit to examine each field minutely. Take your time and really look. You can’t expect dim objects to jump out at you when you’re observing under City Lights. For example, M51 and NGC 5195, which are a spectacle in a 6 inch f/8 from country skies, are a pair of dim BLOBS from the city. But 51 is doable—I’ve caught it with a 4 inch f/11 reflector from a site barely a mile away from a major shopping mall.
And just what can you expect from an urban observing program? From my downtown site I have done the entire Messier list with my 4” f/11. Yes, even M74 and M33. It did take some patience, naturally. Waiting for “good” nights and for my targets to be high in the sky was part of the secret. Observing nearly every clear night was another big advantage. I found that my observing skills improved rapidly and dramatically when I started putting in a lot of telescope time. How did things look? I enjoyed every object, though, truthfully, star clusters, both open and globular, were the real prizes. Brighter open clusters especially are hardly affected by bright skies at all.
But the most important thing was that I was having fun! Sure, many galaxies didn’t look like much. But it was interesting to see what details I could tease out of them despite the light pollution. Visual astronomy is at least partly imagination all the time and from anywhere. M63 didn’t look as good from the backyard as it did from my club’s dark site—but it doesn’t look as good from said dark site as it does in a photo, either. I found that I could appreciate it for what it was: a giant, mind-bendingly distant city of billions of Suns. Even in the city, there was beauty in it, beauty that haunted my dreams long after my faithful little scope was packed away.
Rod Mollise:
For further information on urban observing, or just to shoot the breeze on scopes, observing and astronomy, contact the author at the following email address: RMOLLISE@aol.com. Rod also maintains a web site at the following URL: http://members.aol.com/RMOLLISE/index.html.
[ Back to Articles ]
|
