Post by Chicago Astronomer - Astro Joe on May 25, 2011 7:47:29 GMT -6
Urban Astronomy by a British Astronomer
Here is a nice write up on successful urban astronomy under light polluted skies......
Observing from the City
By Alan MacRobert
Light pollution is the bane of amateur astronomy -- but you can see a lot even through the worst of it.
For amateur astronomers these are the best of times and the worst of times. Never have such large and sophisticated telescopes and such powerful accessories been so readily available at moderate prices. Never has so much celestial information been available at the flip of a page or click of a mouse. But never have so many people lived under such awful skies. Many Sky & Telescope readers can follow the motion of Pluto in 3000 A.D. on a screen, but cannot step outdoors and find Polaris through the light pollution.
This paradox will grow ever more extreme as equipment improves and dark skies retreat. The future of amateur astronomy is, perhaps, a microcosm of the rest of the world's future: better technology in a poorer environment.
Which of these two trends will outrun the other and become dominant is anyone's guess, in amateur astronomy as in the larger world. One result, however, is already becoming clear. The environment is public -- but equipment ownership is private. The stars belong to everyone, but access to them is becoming privatized. Many once-common celestial sights already require expensive instruments or the money and time to travel to distant, unspoiled locations.
No matter what the future holds, however, some observers will never let anything stop them.
These are people who set up telescopes in city lots and observe with blankets draped over their heads to block streetlights, while keeping an ear out for muggers. These are people who spend a year examining bleary star images through an apartment window and come away with a sheaf of variable-star light curves. These are people who time the instants when stars are occulted by skyscraper walls and determine the rate of precession of the Earth's axis.
"Normal" observers who have (or travel to) decent skies tend to regard such enthusiasts as crankish inhabitants of an unimportant amateur-astronomy backwater. They are wrong. As the world grows more densely populated, urbanized, and brightly lit, city observers are the vanguard exploring trails to our future.
Rooms With A View
Years ago I discovered the unexpected possibilities of city observing after moving into downtown New Haven, Connecticut. I assumed that skywatching would cease to be part of my life. But our garret apartment had a plastic bubble skylight over the kitchen sink, and one night, just for laughs, I tried looking through it with 7×50 binoculars. Amazingly, I could make out some stars.
Observing anything under these conditions seemed so remarkable that I did some experimenting. I arranged a way to stand on a stool with my head in the bubble surrounded by light shields. Using a variable-star chart for Mira published in Sky & Telescope, I discovered that my limiting magnitude with the binoculars was as faint as magnitude 8.6. I even spotted Mira near its minimum light.
Star images were distorted by the plastic bubble, it was true, but they were there. Surely lots could be done with a limiting magnitude this faint! In the following months I explored the binocular sky more carefully than I ever had before. I would pick a small area of sky and research everything about it -- the distances and spectral types of stars, interesting objects to try for -- and draw little maps. I set up a writing board in the bubble and arranged to rest comfortably there. It was my own little world, with my feet on the stool and my head in the stars. I followed the monthly pulsations of T Monocerotis, the unpredictable quiverings of Y Tauri, and the nightly creep of asteroids. Binocular double stars could be surveyed at leisure, and I spent long periods mapping everything I could see in Orion's Sword. I identified scores of features on the Moon.
Unexpected benefits began to appear. Restrictions on your observing impose discipline; rather than aimless sightseeing, I had to do desk work with maps and catalogs beforehand to develop good projects. This turned out to be the key to rewarding astronomy. The inside surface of the bubble proved to be a rock-solid "mount" for the binoculars when they were pressed against it face-on. The beautifully steady views made up for the plastic's poor optical quality and the gray film of pollution coating its outside. And it was a new experience to be in shirt-sleeve warmth examining Orion, while an icy winter wind screamed by inches away.
The lesson was clear. Desk work, steady optics, and comfort make for fine sky-watching in the worst environment.
If so much can be done in the city with binoculars, a telescope offers much wider possibilities. The Moon and planets show every bit as clearly to the urban astronomer as the rural one. True, a city is full of heat sources that can cause atmospheric turbulence and degrade the seeing. But often the city haze actually seems to steady the view.
Jim Phillips of Charleston, South Carolina, was one of many city observers who answered a request in Sky & Telescope for their stories. Phillips dedicates his observing time to the Moon and planets. "Quite honestly," he writes, "I am amazed at how many nights of good seeing I get from my 'middle of the city' location." He uses a custom-built 8" f/13 refractor in a roll-off-roof observatory behind his house. "I realized long ago that I was likely always to live in or near a city, and, after great thought, concluded that rather than a portable telescope I could take beyond the lights, I would prefer an observatory in a light-polluted area. To me the advantages of having an observatory with charts, books, and catalogs handy, and the ability to begin observing within minutes, far outweigh the light pollution and partially obstructed sky.
"I have split doubles at or near the theoretical limit of my telescope. Detail on Jupiter is excellent, as is detail on the lunar surface." Phillips ended up taking over the revived Lunar Dome Survey of the Association of Lunar and Planetary Observers.
It doesn't take such a powerful telescope to overcome viewing problems. Robert W. Bethune of Grand Rapids, Michigan, used a 3 1/2" Questar for nearly two years "from the heart of Seoul, Korea, a city of many millions with both smog and light pollution. The compound where I lived was heavily lit, with several streetlights nearby plus bright lights on the walkways and in windows." Nevertheless, he writes, "the Moon was of constant interest. This past summer offered excellent opportunities to study the Jovian and Saturnian systems. There are a number of variable stars, double stars, and even nebulae that can be observed with practice through heavy pollution. My greatest success was timing the bright-limb occultation of an 8th-magnitude star during the recent Pleiades passage."
Bethune offers several tips: "One can be aggressive about certain lights. I dealt with offending sidewalk lights by simply opening them up and loosening the bulbs, remembering of course to restore them afterwards.
"It helps to retire late and rise early. Even in the biggest of cities, things slow down in the dead of night. Planning ahead also helps. One becomes skilled at digging into sky calendars and planispheres for events that will take place within one's limited hunk of sky. The most important lesson is to accept the limits of the situation, after careful experiment and investigation have revealed what they are."
Pointing a telescope out a window is supposed to be utterly taboo for anyone who aspires to the title of amateur astronomer. Temperature differences between outdoors and indoors are supposed to destroy the steadiness of the seeing. This is certainly true at times -- but not always, as Michael Boschat of Halifax, Nova Scotia, discovered. "For planetary work I stick my 3" refractor out the front window, and at times the images are so steady I can use 200 power. I did so for Mars last summer and saw markings even though the planet was low in the sky over a neighbor's roof."
Then there's always the Sun. Sol Steinberg is a retiree living in a garden apartment complex near Camden, New Jersey. "My windows face east, into the 'garden' surrounded by 11 buildings." Using an inexpensive 3" reflector on a tabletop mount, he has photographed the Moon and followed the satellites of Jupiter and phases of Venus. "The Sun has become my regular morning subject," he writes. "The 55-mm eyepiece projects a bright image on my bedroom ceiling, but nothing equals direct viewing with a filter." He began following the comings and goings of sunspots. "A new group appeared yesterday, and this morning two of the spots have clearly become arcs -- a new and exciting phenomenon to me."
And the Deep Sky Too
Nebulae and galaxies, with their low surface brightnesses, are hit hardest by light pollution. Therefore many urbanites give up on them before really trying. One who refused to be defeated was Jenny Worsnopp near New York's Lower East Side. "Manhattan is the worst any amateur astronomer has to live with," she wrote. "But my great love is deep-sky objects. What to do?" The answer was to go out and give it her best shot.
From her apartment's roof Worsnopp had an open view of the sky as well as such luminous spectacles as the Empire State, Chrysler, and Citicorp buildings. The last throws a brilliant fan of light toward the zenith as some architect's twisted notion of art. Nevertheless Worsnopp logged 46 of the 110 Messier objects from her roof using a 6" f/8 reflector. "The open clusters and bright globulars are visible even in the early nighttime," she wrote. Many buildings turn off their decorative lights at midnight, "and after that, the sky is ours. Sort of.
"I recently got a pleasant surprise. I went to the roof at 12:30 a.m. and found the galaxies M104, M66, and M81 -- and, amazingly, M97," a large, dim planetary nebula that can be difficult under any circumstance. "I used an OIII nebula filter and averted vision."
Such feats do require practice and skill, not to mention lengthy star-hopping from a naked-eye starting point that may be tens of degrees away. "City observing," Worsnopp comments, "is good training for those exotic objects that we all want to see from better sites. City observers don't look in their finders and see the Messiers glowing, waiting to be centered; we have to find the spot exactly, so it makes us good map readers. I guess my feeling is that if I can see it here, you, no matter where you are, can see it too."
Worsnopp's tally is surpassed by that of David H. Frydman of London, England. "I have observed from cities for 30 years," he writes. Using a refractor "with a 5" f/5 Jaegers objective, I have observed 350 of 600 deep-sky objects that I know are possible (excluding double stars), most of them many times." He offers a number of pointers:
"There is a window with a radius of 25° from the zenith where faint objects are well seen even in London pollution. Every effort should be made to observe within this window, or at as high an elevation as possible.
"Exclude as much local light as you can. Observe after 11 p.m., choose the most shielded site, and, if necessary, put an open box over your head and telescope.
"The best conditions are after rain and in high winds, as clear country air is blown over the city. During gales I have seen the Veil Nebula, the Owl Nebula M97 including its dark 'eyes,' and enormous detail in M33. Normally the first two are quite invisible in the 5" refractor."
Highly detailed charts, Frydman stresses, are essential. "You see one or even two magnitudes fainter if you know exactly where an object is and keep waiting until it comes into view."
The visibility of a deep-sky object in light pollution depends much less on its total magnitude than its surface brightness. So when looking through catalogs for promising targets, seek those that combine brightness with small size.
Many tiny planetary nebulae have quite high surface brightnesses. Alister Ling published a list of 60 "planetary nebulae visible from the city" in the now-defunct Deep Sky magazine (Summer 1986 issue). Many of them are practically starlike and require very high power to resolve -- not to mention excellent charts to identify them in the first place. Ling gives several tips for distinguishing them from stars. One method turns bad seeing to an advantage: a tiny planetary twinkles less than a star, for the same reason a planet twinkles less (both have appreciable disks). Another tip-off is an unstarlike greenish or bluish tint.
The best "city planetaries" in Ling's list that are larger than 15 arcseconds across and brighter than magnitude 10.0 are not widely known: NGC 1535 in Eridanus, NGC 3242 in Hydra, and NGC 6826 in Cygnus. Clearly this is a big open field to explore.
A more recent article on observing high-surface-brightness planetary nebulae in the city was published by Donald R. Ferguson in the April 1995 Sky & Telescope, page 96. Ferguson includes a list of 18 of these objects that he collected with a 3.5" telescope from within "the vast urban sprawl of Houston."
Nebula Filters
City and suburban observers gained a new claim to the deep sky when nebula filters were developed in the late 1970s. These function on a straightforward principle. Emission nebulae give off light at narrow wavelengths that differ from those of sodium- and mercury-vapor streetlights. By using a multilayer interference filter, the spectrum of visible light can be cut finely enough to separate these wavelengths. The result is a much darker sky, somewhat dimmer stars and galaxies, and only slightly dimmer planetary and emission nebulae. This enhanced contrast can, in many circumstances, more than make up for the relatively small amount of light lost from the nebula, and so it stands out more clearly.
These filters do not bring country skies to the city, but they do help. One technique for detecting nebulae, especially tiny planetaries, is "blinking" with the filter. Hold it at the eye and move it rapidly in and out of the line of sight; a nebula will blink relative to the surrounding stars. Alternatively, blinking can be done by tilting the filter back and forth while looking through it, since it loses its effectiveness when at an angle.
Several nebula (or "light pollution") filter designs are available. They use somewhat different strategies for different types of objects and conditions. A detailed review of them is in the July 1995 Sky & Telescope.
The CCD
The biggest promise that technology holds out -- for those who can afford it in both money and time -- is the CCD camera. By 2000, CCD (charge-coupled device) cameras had taken over and vastly expanded high-end amateur astronomy, and their prices are declining every year. A CCD camera has two enormous strengths. First, the CCD chip is many times more sensitive to light than either your eye or photographic film. Second, it feeds a digitally recorded image from the telescope directly into your computer, where the image can be enhanced, analyzed, measured, and manipulated.
The most important manipulation is the ability to subtract away an extremely light-polluted background, as if by magic, with hardly any loss of data. An 8" telescope can now record 15th- or even 16th-magnitude stars in the worst city light pollution or moonlight. This is several times fainter than the same telescope can show stars to the eye under black, mountaintop conditions!
Drawbacks to CCDs include the very small field of view, the difficulty of aiming this field where you want, and problems of focusing. The equipment may be temperamental; the telescope mounting must be as rigid and controllable as for long-exposure astrophotography. And, of course, you're looking at a computer screen, not stars. It has been said that CCD astronomy is about working with equipment and computers, not skygazing.
The most important advance that CCDs represent is the science that can be done with the recorded images. For much of the 19th century, amateurs were almost on a par with professional astronomers in terms of the useful science they could do. Then amateurs fell very, very far behind -- but now CCD cameras in dedicated hands are making up some of this lost ground. Amateurs are discovering asteroids in great numbers, performing professional-quality variable-star studies, detecting the 19th-magnitude optical afterglows of gamma-ray bursts near the limits of the observable universe, taking spectra of stars and galaxies, imaging the planets more finely than was once thought possible, and much more.
No machine, however, will ever replace the simplicity and delight of examining the stars directly, as a part of living nature.
Duck and Cover
"Light pollution" is the glow in the sky itself. It should not be confused with local lights that shine directly into the observer's eyes.
Local lights are more aggravating but easier to defeat. Many observers have cooperative neighbors who turn off outdoor lights on request. A good way to break the ice on this issue is to offer views through your telescope.
If you can't observe in the shade of trees or walls, you might rig a tarpaulin to shield your site. Max Wyssbrod lives in Lucerne, Switzerland, which he calls "the brightest country in Europe." His "cloth observatory" consists of four aluminum poles 10 feet long that fit into tubes cemented into the ground in a 10-foot square. The four walls are black cloth; guy ropes add stability. The whole rig, along with an 8" Schmidt-Cassegrain telescope, takes 15 minutes to set up.
Another strategy is to shield only your eye and the back end of the telescope. An old-fashioned photographer's black cloth or equivalent, or a cape that can be thrown up over your head, does the trick. Any telescope in bright local lights should also have a long dewcap or side shield to keep the light out of the tube. Eyepieces should have rubber eyecups.
"I use a black hood, and blinders I made from cardboard fitted to each side of my face," writes Charles Haun of Morristown, Tennessee. "This works quite well."
Hiding under cloth and wearing blinders may seem an ignominious way to experience the glories of the cosmos. But such is the garb that amateur astronomers shall increasingly wear as they march bravely into the future.
www.astronomy-education.com/index.php?page=191
I agree with all this gentleman has to say about city astronomy and practice it at every Chicago Astronomer star party observation session.