…continuedObserving from the City
. . . 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-inch 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-inch 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-inch 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.