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Re: (IAAC) limiting magnitude
>Fortunately, the magnitudes derived by such methods are not *too* bad for
>visual use as long as they are in the right system
Thanks for extra details on galaxy magnitudes, Jeff. I also echo Randy's
suggestion to read Roger Clark's book... It provides good background reading on
deep-sky observing, although I wouldn't take everything Dr. Clark says as
gospel. I recommend Luginbuhl & Skiff's _Observing Handbook and Catalog of Deep
Sky Objects_ even more strongly! This has the broadest, most detailed coverage
of both background topics and particular objects I've ever seen. If Brian Skiff
doesn't have time to comment on this thread right now, you can still read what
he has to say in this book. :) The tough part is finding a copy now... It
appears to be out of print.
--
By the way, your experience is quite different from mine, Jeff. I generally
find magnitudes commonly quoted in, e.g., NGC, IC, UGC, MCG, and Zwicky
catalogs are pretty hard to interpret for my observing! I can sometimes guess
from the general magnitude RANGE whether an object will be visible with a given
scope and sky: for instance, a quoted mag of 15 (visual) PROBABLY indicates a
galaxy is a challenge in the 20", while a quoted mag. of 11 MAY imply that it's
easy. Of course, this assumes I will know the source for the magnitude: again,
this depends on knowledge of the quoting catalog, and even then the observer
can be mislead. And then there's the very considerable effect of the observer's
perception, and the particular sky conditions on any one night and at any one
site. Plus with emission features, you have to take into account the utility
(or not) of various filters, etc. In the end, as I said before, magnitudes are
pretty fuzzy guideposts for amateurs...
To take one example, I observed the pretty little Abell galaxy cluster A262 in
Andromeda last Fall. Among the cluster's many members are the two galaxies NGC
710 and and NGC 717. Now the quoted mags for these are 13.7 and 13.9 (visual),
respectively, according to Jeff Bondono's 'dObjects' software. (This is a great
secondary source for tons of deep-sky data by the way! But just to allay any
question, I double-checked these numbers in NASA's Extragalactic Database on
the Web: RC3 quotes n710 at Mv_t0 13.2, n717 at Mv_t0 13.3. Meanwhile, the SAX
SDC astronomical catalog browser showed NGC2000 mags of 14.0 for both galaxies!
Drop a line if anyone's interested in URLs...)
Anyway, based on these numbers, an observer might have expected the two objects
to appear somewhat similar... In fact, my logs didn't bear this out: I logged
n710 as "one of the brightest members of 262", difficult to distinguish from a
star but still an easy direct vision object. n717 on the other hand was "only
visible to averted vision" and "largely featureless". Quite distinct! If anyone
is interested in the whole log of A262, it's at:
http://www.tiac.net/users/lewkaren/netastrocatalog/msg00384.html
An even more telling example was an observation of the mighty Abell 1367 in
Leo's haunches a year ago: IC 2955 is quoted at V=14.0, while nearby NGC 3862
has V=12.7. But my logs actually show IC 2955 as appearing the BRIGHTER of the
two that night, in that scope... My full log is at:
http://www.tiac.net/users/lewkaren/netastrocatalog/msg00174.html
As always, form your own impressions every one! But keep coming back to our
little observers' campfire here to share what you find out. :)
Clear skies,
Lew
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