Tony Flanders' Astronomy Site
Messier Guide: Introduction
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Over the past few years, I have attempted four complete surveys of the Messier objects, using a 70mm refractor and a 178mm (7-inch) Dob from both urban and suburban sites. I succeeded in viewing all of the objects with both scopes from the suburban sites, although several of them were very difficult in the little refractor. From the urban sites, two of the objects eluded me in the 178mm Dob (M68 and M98), and a couple dozen proved invisible in the smaller scope. The urban observations were summarized in the April 2002 issue of Sky and Telescope. I have also viewed many of the Messier objects in 7x35 binoculars in both settings.

Equipment

The observations were done with three instruments: two telescopes and a pair of binoculars, which I will abbreviate throughout as follows:

178mm: Starmaster 7" F/5.4 Oak Classic Dob
70mm: Televue Ranger 70mm F/6.9 refractor
7x35: Nikon 7x35 binoculars

I did most of the observations with a single eyepiece: the Vixen Lanthanum 8-24mm zoom, yielding 20X-60X on the 70mm refractor and 40X-120X on the 178mm Dob. I also used a 30mm Celestron Ultima for wide-field work on the refractor, and a 35mm Rini in a 2-inch barrel on the Dob. In some cases, I might have gotten somewhat better views using higher powers than those provided by the zoom, but I chose to restrict myself for the sake of simplicity.

I also used a Lumicon UHC narrow-band filter on some of the objects. In most cases, the filter was useless or counter-productive, but it was quite helpful for a few objects, notably M8 and M97. The Orion Ultra Block filter is much the same. I have not used broad-band filters much, but the general consensus is that these filters are only marginally useful for visual observing.

Aperture Versus Light Pollution

There is essentially no truth to the claim that large apertures are useless, or even counter-productive, under heavy light pollution. On the contrary, satisfactory deep-sky observing definitely requires more aperture under heavy light pollution than under dark skies. This subject is explored in more detail in the web page Aperture Versus Light Pollution, with comparative views of several Messier objects to illustrate the general principles.

Finding Objects and Seeing Objects

I star-hopped to all of my targets using a Telrad on the Dob and a red-dot finder on the refractor. I sometimes used the two scopes independently, and sometimes mounted the refractor on the Dob so that I only had to find each object once. I mostly used custom star-charts printed by Sky Map Pro, but I sometimes used Sky Atlas 2000.0 or Uranometria. In general, Sky Atlas 2000.0 does not show enough stars to locate the more difficult Messier objects under heavy light pollution, and even Uranometria is only marginally adequate.

Finderscopes are somewhat harder to master than Telrads or red-dot finders, but once the necessary skill has been acquired, they are very useful under heavy light pollution, where relatively few stars are visible to the naked eye. Both of my scopes have unusually wide maximum fields of view, allowing them to serve as their own finderscopes, but I strongly recommend a finderscope for any city dweller whose scope's maximum FOV is less than 2 degrees.

Some kind of mechanical finding aid, such as setting circles, digital setting circles, or electronic Go To capability can certainly be useful in any conditions, and particularly so under heavy light pollution. I do not get much joy from such equipment; I prefer spending my time looking at charts and looking at the sky to interacting with machinery, but I have no trouble understanding people who feel differently.

Regardless of whether you use mechanical aids or star-hop, it is essential in the case of the fainter objects to know *precisely* where your scope is pointed, and where to start looking for your target. It is not sufficient to aim it vaguely in the right direction with a Telrad and hope that the object will show up in the eyepiece. That technique works fine for objects that are immediately obvious as soon as they enter the field of view, but to locate a faint galaxy against a bright background, you need to know precisely what quadrant of the eyepiece to start scanning with averted vision. For faint objects, I generally prefer star-hopping from a bright star to using the point- and-shoot method with a Telrad. The essence of star-hopping is that you always know exactly where you are every step of the way. I find that easier than matching up what I see through the eyepiece with my charts after my Telrad has gotten me within half a degree of the target.

Likewise with setting circles and Go To drive. Unless these are accurate to a few arcminutes, so that you can be *sure* that the target is centered in a high-power field each and every time, you will still need to consult your charts and match up star fields to know where in your eyepiece to start looking for your target.

Averted vision is essential for finding faint targets; it becomes second nature to any deep-sky observer. In evolutionary terms, peripheral vision is meant for detecting moving prey and defense against moving predators, and indeed, many faint targets show up best if you sweep the scope slowly over their location, but are much harder to see when they stay in the same place. Conscious control of breathing can be very useful for seeing faint objects, as it is for most difficult physical tasks. In particular, there is a natural tendency to hold one's breath when concentrating hard, and this is almost always counterproductive.

Magnification

Magnification is usually best discussed in conjunction with aperture; 100X is a high power in my 70mm scope but not in my 178mm scope. This can be expressed in terms of X per inch or X per mm of aperture, but I prefer to use "exit pupil", which is the aperture divided by the magnification. Thus, 100X in my 70mm scope yields an exit pupil of 70mm/100 = 0.7mm. Smaller exit pupils mean higher magnifications.

For deep-sky observing, any exit pupil over 4mm counts as low power. 2mm or 3mm is medium power, and 1.5mm and smaller is high power.

It used to be thought that low magnifications were essential for viewing objects with low surface brightness; the theory was that higher magnifications would spread the light out over a larger area and make the objects harder to see. This sounds good in theory, but in practice, higher magnifications are often helpful for seeing difficult objects, especially under heavy light pollution. Part of the reason is that although higher magnifications do reduce the apparent surface brightness of the target, they also reduce the background skyglow, so that the ratio between the two remains unchanged. Urban skyglow seen at a 1mm exit pupil is much like a rural sky seen at a 5mm exit pupil.

Stars are point sources of light, and are not spread out by higher magnifications -- not much, anyway -- so higher magnifications almost always bring out fainter stars. For viewing star clusters, a safe rule of thumb is to use the highest power that frames the object well, typically yielding a field of view at least twice the cluster's diameter.

For viewing nebulous objects, it is very hard to generalize. Without a doubt, people under-magnify these more often than they over-magnify, but there are exceptions to every rule. Usually, I find that higher powers work better the brighter the sky glow, but sometimes the reverse is true. The rule of thumb is to start with the lowest possible magnification, at which many of the fainter objects will be completely invisible. Then work up until the object becomes visible, then continue raising the magnification until the object starts to deteriorate. Sometimes some features of an objects will look best at medium power while others will look best at high power. If I had to pick a single magnification range most likely to work well with faint fuzzies, it would be 1.5mm - 2mm.

Observing Sites

The urban observations were done at various sites near my home in Cambridge, Massachusetts, about 5 miles from the center of Boston, a metropolis of several million people. The suburban observations were done at various sites in Lincoln, less than 15 miles from downtown Boston. Both areas are probably somewhat darker than an average urban or suburban site, respectively. In Cambridge, I estimate the limiting magnitude at around 4.6 on a good night. M44 and the Double Cluster are visible naked-eye with difficulty, and M31 is visible with great difficulty. In Lincoln, I can usually see stars to around mag 5.2, and the summer Milky Way is quite obvious, although somewhat lack-luster.

All of my observing sites, both urban and suburban, are completely free of light sources above my head (e.g. streetlights), and in places where all of the ambient lights could be blocked by an arm or a cupped hand.

The terms urban and suburban should not be taken too seriously; what matters is not whether the residents consider themselves to be urban, suburban or rural, but the level of light pollution, and each case must be evaluated on its own merits.

It is always a good idea to observe deep-sky objects when they are near their highest, and on nights of good transparency, but both of those are immensely much more important in the presence of heavy light pollution. A small amount of haze, which would be only a nuisance at a dark site, can be a killer in the city; it gets you both coming and going, dimming the light from your target while simultaneously brightening the skies tremendously. Likewise, deep-sky objects might suffer half a magnitude of extinction when twenty degrees off the horizon at a dark site, but in the city, that half-magnitude of extinction is compounded by a skyglow that is several times brighter than at the zenith, allowing only the very brightest objects to shine through.

My observations of the southerly Messier objects must be calibrated according to my latitude of 42N. Objects that gave me great trouble, like M20 or M68, would be much easier to see under the same degree of light pollution from a site 5 or 10 degrees farther south. Conversely, many of the objects in the southern summer Milky Way must be impossible to see from the cities of Northern Europe, 5 or 10 degrees farther north.

Ratings

I have rated each of the Messier objects with four number-letter combinations, like this:

Obj S178S70U178U70
M351A2B1A3B

S178, S70, U178, and U70, are my ratings for each object in under suburban and urban skies, using my 178mm and 70mm telescopes. The number indicates the difficulty of seeing the object, as follows:

  • 1 - very easy, obvious even to a beginner
  • 2 - easy, immediately obvious to an experienced observer
  • 3 - moderate, may take a little looking
  • 4 - hard, probably requires averted vision
  • 5 - very hard, borderline observation, intermittently visible
and the letter indicates how interesting or beautiful the object is:
  • A - spectacular
  • B - beautiful or unusual
  • C - unspectacular but interesting
  • D - detectable but nearly featureless

The ratings are fairly arbitrary, both as to difficulty and even more so as to beauty and interest; every time I re-rate them, they ratings end up somewhat different.