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.
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
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 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.
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
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
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.
I have rated each of the Messier objects with four
number-letter combinations, like this:
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:
and the letter indicates how interesting or beautiful the object is:
- 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
- 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.