For urban astronomers, galaxies are consistently disappointing. Galaxies that are stunning from a dark-site can be stubbornly invisible in even mildly washed-out skies. A very slight loss of contrast is enough to hide spiral arms, dark lanes and any other structural details. What makes galaxies even more frustrating is that none of the usual tricks for enhancing contrast work on them. Because they are extended objects with low surface brightness, increasing magnification just makes their already faint and diffuse light even more diffuse and fainter. Because galaxies radiate primarily stellar light, narrowband filters which can be very effective on emission and planetary nebulae don’t help at all with galaxies. So what’s an urban astronomer to do? Just give up on galaxies? Well, luckily there are a few galaxies that can be reliably seen under washed-out skies...
(Photo of M31 by Richard Arendt.)
The Messier catalog lists some of the finest objects in the northern skies. Observing all of the Messier objects is one of the classic rites of passage for amateur astronomers. There are also a variety of challenges related to the Messier Catalog, the most famous of these being the Messier Marathon: a race to see all of the Messier objects in a single night. There are also binocular Messier challenges and photographic Messier challenges. My own addition to this is an Urban Messier Challenge: a list of Messier objects that are particularly challenging for urban and suburban observers dealing with skies washed-out by light pollution. In dark skies you can see all of the Messier objects with a 3 inch (75mm) aperture, but with heavily light-polluted skies you’ll have a hard time seeing the Messier objects on my urban challenge list even with significantly larger apertures. Here is the challenge list...
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