Some observers and vendors, especially vendors of small
telescopes, claim that large apertures are useless or
even counter-productive under heavy light pollution.
In my opinion, and the opinion of most experienced
observers, there is essentially no truth to this claim.
If anything, satisfactory deep-sky observing requires
more aperture under heavy light pollution
than under dark skies.
However, large apertures are not a substitute for dark
skies, any more than dark skies are a substitute for
large apertures. Small apertures and bright skies both
hurt deep-sky objects, but they hurt them in fundamentally
different ways. The reason lies in the distinction
between an object's integrated brightness -- the sum of all
the light that comes from it -- and its surface brightness,
or brightness per unit surface area.
The galaxy M33 is one of the brightest of all the Messier
objects as measured by integrated brightness; however, that
light is spread out over an area considerably larger than
the full Moon, making it one of the faintest of all the
Messier objects as measured by surface brightness. The
tiny planetary nebula M76 is about fifty times fainter
than M33 as measured by integrated brightness, but its area
is only about one thousandth that of M33, giving it a
surface brightness twenty times higher. This subject
is explored in more detail in the section
Surface Brightness.
Small aperture places a lower limit on the integrated
brightness of the targets that can be observed, while
skyglow places a lower limit on the surface brightness.
Thus, M33 is faintly visible to the unaided eye in dark
skies, and very easy in small binoculars, but it is extremely
hard to see from the city in any telescope, no matter how big.
M76, by contrast, is a bit of a challenge in my 70mm scope
even under the darkest skies, but it is quite easy and
attractive in my 178mm scope even from the city. In fact,
it shows nearly as well in that scope from the city as
it does under dark skies; its surface brightness is high
enough to overpower the urban skyglow.
Stars and star clusters are the objects where aperture and
sky brightness are most nearly interchangeable. My 178mm
Dob under urban skies sees very nearly the same stars as
my 70mm refractor under fully dark skies; therefore, a
star cluster looks much the same through the bigger scope
under urban skies as through the smaller scope under dark
skies. But even that isn't completely true, because
getting an optimal view in the bigger scope requires
higher magnification, which is likely to frame the cluster
poorly, and make it stand out less well from the background.
Moreover, many clusters, especially globular clusters,
are only partially resolved even in the biggest telescope,
and the unresolved stars merge to form a nebulous background
which is hurt severely by light pollution.
Bright planetary nebulae like M76 and M57 are the objects
where aperture matters most and sky brightness least.
These objects are like their namesakes the planets;
they easily outshine even the brightest skies, but
require high magnification which can be provided only
by large apertures. But even bright planetary nebulae
often have faint halos that can be obliterated by
light pollution.
For galaxies, dark skies are of paramount importance.
With a handful of exceptions, all of the Messier galaxies
show far better in my 70mm refractor under dark skies than
in my 178mm Dob under urban skies, or often even under
suburban skies. Our own Milky Way galaxy is a good
indicator. If the skyglow is bright enough to swamp
the Milky Way, making it invisible to the naked eye,
then it will also swamp the outer disks of most galaxies
as seen through a telescope, and you will be reduced to
viewing the nearly featureless cores. If the Milky Way
is visible but unexciting to the naked eye, so will most
other galaxies be through a telescope. If Milky Way
glistens with detail, so will most other galaxies.
The links below show pictures of various Messier objects
through both my 70mm telescope and my 178mm telescope
under urban and suburban skies. For the prototypic
galaxy M31, even the fairly modest difference in sky glow
between the two sites is far more important the aperture.
For the star cluster M38, aperture is far more important.
The situation is more complex with respect to M42, the
Orion Nebula. This objects has aspects with high surface
brightness which benefit most from aperture and also
aspects with low surface brightness which are affected
more by sky glow.
