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...
Prerequisites
To see these Messier objects in a light-polluted site you’ll need a moonless night of outstanding transparency. You’ll also need to shield yourself and your telescope from as much of the local lighting as possible. Try to position yourself so that trees or other objects block nearby lights. If you can’t do that, consider creating a portable screen to block the most obnoxious lights. Use a light-shield or shroud to keep stray light out of your telescope’s light path. Finally, do your best to let your eyes dark adjust. I often keep my eyes closed for a few minutes before putting my eye to the eyepiece (and only open my non-eyepiece eye to get my eye to the eyepiece).
The Urban Challenge List
These are the Messier objects that I find most challenging to see while observing under the brightly lit skies of northern Virginia. These objects are listed roughly in the order from hardest to easiest, but the ordering is very subjective and each object’s visibility will be highly dependent on the particular conditions. The comments reflect my own experiences observing these objects with a 10 inch (250mm) Dobsonian telescope. Note that in light-polluted skies none of these objects are visible in my 90mm refractor or my 4.25 inch Newtonian.
M74
Known as “The Phantom,” M74 really lives up to its name. I have found this galaxy to be hands-down the most challenging object in the Messier list. In fact, I have been able to see M74 only once from my washed-out skies, on a night of exceptionally good transparency. Even with a detailed chart showing me exactly where to look, it took averted vision, some gentle sweeping, careful jiggling of the telescope, and a black sheet covering my head before I was able to pull out M74 as an exceedingly faint smudge of light.
M33
Although it is relatively bright overall, M33 is roughly twice the size of the full moon—which gives it a very low surface brightness. From urban areas, your only hope is to glimpse the central core, which is unfortunately very weak and only very slightly brighter than the rest of M33. Try to see M33 in the late fall right after a sharp cold snap: M33 will be positioned roughly overhead and the cold-snap will enhance the transparency by pulling most of the moisture out of the atmosphere.
M101
This face-on spiral galaxy is much like M33. In theory it has lower surface brightness than M33, but I find it somewhat easier to see than M33. I think the difference is that M101 has a stronger, relatively brighter core than M33.
M100
I think M100 is the most challenging Messier object in the Virgo Cluster. I have had better success seeing it with medium-low magnification (about 75x) than using the low magnifications (40x to 50x) that tend to work best for most other faint galaxies.
M98
This galaxy is the second hardest Messier object in the Virgo Cluster. It’s very faint and hard to locate. It takes slow, careful sweeping using averted vision to find it. Like M100, I have better luck using medium-low magnifications.
M109
This is a rather faint galaxy, and glare from nearby 2nd magnitude Phecda (γ UMa) makes it all that much harder to see. The low magnification views that favor this object come with wider fields-of-view, making it hard to keep Phecda and its glare out of the eyepiece. But even without Phecda's interference, M109 is a pretty tough object in washed-out skies.
M110
In washed-out skies M31 and M32 are easy to see but light pollution makes it quite challenging to see M110, the third member of the trio. Wait for the Andromeda galaxy to be positioned nearly overhead and then look for a very soft, faint hazy spot to the northwest of M31's core. I find M110 is best seen with low magnification (30x to 40x).
M97
To glimpse this large planetary use a relatively low magnification (50x or less) and averted vision. Look for a very subtle, round disk that hovers ghost-like at the edge of visibility. M97 perhaps shouldn’t be on this list because with a narrowband filter it is an easy target: when I use an Orion UltraBlock filter, M97 pops right out and is immediately, obviously visible.
M1
I find M1 surprisingly hard to see in washed-out skies, regardless of the magnification I try. It is not as hard to see as M97, but unlike M97, M1 does not respond well to narrowband filters. I find it pretty much as hard to see M1 with a narrowband filter as it is without.
Final Comments
I want to highlight one Messier object that is not on this list: M76. By the numbers, M76 is the faintest Messier object and it has the lowest average surface brightness. I often read that M76 is considered the hardest of the Messier objects. That has not been my experience. Even in light polluted skies, I find M76 rather easy to pull out. I can see it with direct vision even at moderate magnifications (70x to 150x) and without any filters—although M76 also responds well to narrowband filters. I suspect this is because the central bar of M76 has rather uneven brightness, with some parts of it having relatively high surface brightness.