Minimum aperture for globulars and galaxies

Tony Flanders wrote:

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.)

If the 4 inch refractor is used at the proper power and under reasonably-dark
skies (ZLM 6.0 or fainter), many people (other than Steve O'Meara) can see at
least some structure or detail in a number of galaxies with averted vision.
M31 begins to show some of its spiral structure in a low-power wide-field
eyepiece, along with at least one of its dark lanes, although like most
galaxies, it isn't all that bright. M51 will show a sort of "ring" structure
hinting at its spiral nature, while M101 will also show some patchyness in its
outer haze at low to moderate power. I can see a large diffuse patch in one
end of M81, although it doesn't show the spiral arms in a 4 inch. M82 and
M104 will show some dark lane-like structure, while a number of edge-on
spirals like NGC 4565 will show their needle-like form fairly well. NGC 253
looks mottled, as does M33 (one arm starts to become visible), NGC 2903, M66
and a few others. As for an 8 inch, this aperture increases the number of
galaxies which show at least some kind of detail by quite a bit, although with
many of the fainter ones, any structure that is visible is mainly mottling,
overall shape, star-like nucleii, or, in the case of near edge-on spirals, a
dark lane. Observing galaxies is always a challenge, and learning how to
tease out some of the detail that is visible is part of the fun. Clear skies
to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote in message ...

If the 4 inch refractor is used at the proper power and under reasonably-dark
skies (ZLM 6.0 or fainter), many people (other than Steve O'Meara) can see at
least some structure or detail in a number of galaxies with averted vision.

You're absolutely right -- it's all a matter of how much detail you want.
Even a 60mm scope is big enough to classify most of the Messier galaxies
as spiral versus elliptical with a reasonably high degree of confidence,
and M31 is obviously a spiral to the naked eye. Not that you can see
the spiral arms, but no elliptical galaxy could possibly have that
shape and that kind of central condensation.

Josh Roth here at S&T told me that he recently glimpsed M33's spiral
arms using an O-III filter in his 60mm refractor, and 60mm is certainly
enough to see the emission area NGC 604 in that galaxy. I've heard of
people seeing M31's dust lanes in hand-held binoculars, although I'm
sure that feat is far beyond me.

- Tony Flanders

(Tony Flanders) writes:

Even a 60mm scope is big enough to classify most of the Messier galaxies
as spiral versus elliptical with a reasonably high degree of confidence,
and M31 is obviously a spiral to the naked eye. Not that you can see
the spiral arms, but no elliptical galaxy could possibly have that
shape and that kind of central condensation.

You are exaggerating _grossly_ on both acounts, IMO. First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.
Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

pej
--
Per Erik Jorde

Per Erik Jorde posted:

Even a 60mm scope is big enough to classify most of the Messier galaxies
as spiral versus elliptical with a reasonably high degree of confidence,
and M31 is obviously a spiral to the naked eye. Not that you can see
the spiral arms, but no elliptical galaxy could possibly have that
shape and that kind of central condensation.


You are exaggerating _grossly_ on both acounts, IMO. First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.
Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

Well, he might be exagerating a little, but a good 60mm refractor when used at
a *low* enough power under reasonably dark skies will show M31 and its much
fainter extended outer haze. Its core region is usually the only thing which
shows up well at the moderate powers that many of the common "department
store" refractors use, but drop that power to 20x and the faint vaguely patchy
outer extensions become quite easy to see, along with the marked dropoff of
light intensity along the northwest side marking the location of the first
major dark lane. This kind of brightness profile (small brighter nuclear
region and extended faint outer haze) is enough to show that M31 is a spiral
galaxy, although it does not show what kind it is nor does it show much of the
arm structure. M110 on the other hand does not show the kind of smaller and
markedly brighter central core which would hint at it being a spiral. Its
smooth edges, slowly increasing brightness level with decreasing distance from
the center, and the lack of a smaller fairly well-defined brighter (but
non-stellar) core indicate that it is probably an elliptical galaxy, an
impression which is confirmed with larger apertures.
It is quite easy to use the overall brightness profile of galaxies which
show little other structure to get at least an idea of whether they are
spirals or ellipticals. In addition, with an 8 or 10 inch telescope, I can
usually tell what kind of galaxy I am looking at for a *large* number of
galaxies as faint as 13th magnitude (or sometimes fainter). With that
aperture, I can usually see at least some indications of structure (mottling
or irregular edges, small bright nucleii, dark lanes) that can point to the
object being a spiral without necessarily seeing the spiral arms. Of course,
with a ten inch, there are a number of galaxies which will visibly show the
spiral structure, so for those at least, there is little doubt of the
classification. Even without those, the brightness profile is sometimes
enough on its own to tell whether a galaxy is an elliptical or a spiral.
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Per Erik Jorde wrote in message ...

First, a small
scope (most scopes, really) typically shows just the central hub of
spiral galaxies, and these are not much differen from ellipticals.

Well, central hub means different things to different people, but
it's true that when a galaxy is at the edge of visibility, you rarely
see the full extent as listed in the catalogs. But the fact remains
that whenever I see a new galaxy -- and most of those are pretty faint
for whatever scope I'm using -- I try to classify it as a spiral,
ellipse, or irregular. I'm certainly wrong sometimes, but I'm right
much more often than not.

I've even seen a fair number of galaxies right at the edge of
averted vision, where I wasn't 100% sure if I was seeing them
at all, but I was 100% sure that if I was, it must be an edge-
on spiral because no elliptical ever has such a thin aspect ratio.

It would be fun to try this particular experiment with the Messier
galaxies and a 60mm scope, but alas, I probably have the classification
of most Messier galaxies buried somewhere in my mind, so it wouldn't
be a fair trial. Maybe if somebody else found them for me, and didn't
tell me which one I was looking at ...

Second, one only has to glance over at neighboring M110 to see an
elliptical galaxy that looks very similar to the naked eye view of M31.

I'm sceptical, but I'll give it a whirl next time I'm under dark skies.

- Tony Flanders