Bill Meyers wrote in message ...

Hello, all,
What in your opinion or experience is the minimum aperture necessary
for satisfying views of globular clusters and galaxies?

Well, this question is really quite hopeless as stated,
but I'll do my best. For simplicity, I'll consider just
four telescopes, all of which I've used fairly frequently,
and which form a vaguely equally-spaced sequence:

* 4-inch refractor
* 8-inch reflector or catadioptric
* 12.5-inch reflector
* 17-inch reflector

For viewing globular clusters, I'm never really satisfied
unless I've resolved some stars. I can't really say just
how many is enough; in fact, the view of a barely-resolved
globular cluster has a charm of its own, with a dozen
stars twinkling in and out of the halo as I apply averted
vision. But on the whole, the more stars, the better. Of
course, no amateur scope can resolve a globular cluster
fully, like a Hubble photo, where you can count thousands
of stars within a few arcseconds of the center.

Next, I would divide globular clusters into four classes.
The first class consists of 47 Tucanae and Omega Centauri,
which are truly glorious in a 4-inch refractor under dark
skies. The second class is what I would call the great
globulars. Just which ones make the grade is a matter of
taste, but anybody would include M5, M13, M22, NGC 6397,
NGC 6752, and probably another dozen Messier and far-
southern non-Messier globulars. These are pretty well
resolved in a 4-inch refractor under dark skies, and
they're really quite glorious in an 8-inch scope under
dark skies. The third class is the major globulars,
including most of the other Messier globulars and quite
a few NGC globulars. An 8-inch scope may show a few
stars in those, but it really takes a 12.5-inch scope
under dark skies to do justice to them. And finally,
there's all the other globular clusters in our galaxy,
a motley assortment. A few of them are barely detectable
even in a 17-inch, due mostly to heavy obscuration by
galactic dust.

On the whole, globular clusters survive light pollution
pretty well, but they do require extra aperture to get
views that are roughly equivalent to the dark-sky views.
Under typical suburban conditions, I would move everything
over one class. There, an 8-inch scope is just beginning
to resolve the great globulars, and a 12.5-inch scope is
required to do full justice to them.

The story is quite different when it comes to galaxies.
With a handful of exceptions, like M82, galaxies show
very poorly in the presence of light pollution. Yes,
you can get a decent view of M51's spiral arms in a
big scope from one of the darker suburbs, but I still
wouldn't call most galaxy views satisfactory unless
the skies are reasonably dark.

In discussing galaxies, I'll have to explictly exclude
the two Magellanic clouds and our own Milky Way. All
of those show some detail even to the unaided eye,
and quite a lot of detail through tiny instruments.

Steve O'Meara can obviously see a lot of galaxy structure
in a 4-inch refractor, but for normal mortals, I think
that 8 inches is the minimum. And even then, only a
handful of galaxies show decently -- M31, M33, M51,
M101, M83, M82, M66, maybe M81 under pristine skies.
(I'm sure I've forgotten some.) Things improve quite
a lot in a 12.5-inch scope. All of the galaxies listed
above really begin to open up and show intricate detail,
which an 8-inch scope just hints at. And a bunch more
galaxies start to show tantalizing detail. But it
takes a 17-inch scope to start to unlock the wider
realm of galaxies -- and that's just the beginning.
I've never seen a view of any galaxy in any scope, no
matter how big, where I didn't wish for just a little
more aperture -- or better yet, a lot more.

- Tony Flanders