Measuring transparency (was Re: (IAAC) Obj: Jonkherre 900)
I have to agree with Yann that Limiting Magnitude may not be the most
accurate measurement of visibility for extended objects. Of course,
don't forget that many deep-sky objects are not "extended" in the usual
sense of that term! For instance, the visibility of globular and open
clusters which are well-resolved will relate closely to LM. So will the
visibility of faint companions in multiple stars, as well of course as
the minimum magnitude range for negative variable-star observations.
For what we're talking about here - galaxies, unresolved clusters, and the
various kinds of nebulae - I do agree that some OTHER measure really is
needed in addition to LM, though! Measuring angular sizes of certain well-
known extended objects (M31, M33, Scutum Star Cloud, etc.) is one method.
Unfortunately, though, you can't always count on having one of these be
sufficiently close to the Zenith at the time of your observation! I like
Yann's idea of using a "galaxy count" within certain standard clusters
or groups of galaxies: but there again, observers without large scopes
may not have any such group near Zenith and within range of the scope.
And of course, folks will have huge variations when they use the common
"1 to 10" rating scales - a "10" for someone used to City observing might
only be a "4" for someone spoiled by the high desert...
So in the absence of any better standard for extended objects, and since
I'm a meteor observer who measures it often anyway, I just use LM myself.