Re: (IAAC) limiting magnitude

Clouseau@webtv.net wrote:
(Why DID you pick that username anyway, Harold? :>)
> I am trying to find some informtion that may not exist. Is there a
> publication that with a known limiting magnitude will tell you if an
> object you are trying to observe will be bright enough to see?

Your frustration is the same one we all face, Harold: for as you're already 
finding, there are magnitudes and then there are magnitudes! :)

Put simply, there IS a simple relationship between the faintest naked-eye star 
you personally will see on a given night, and the faintest star you'll see with 
a given telescope and eyepiece on the same night... And with time, you'll find 
out what that relationship is for your eyes and equipment.

But to get from there to knowing "what you can see" is a whole other question: 
fortunately or not, this is where you start to get into the *ART* (or the "dark 
science") of observing... :> First of all, the *stellar* limiting magnitude for 
your eye (or eye+eyepiece+telescope system) really doesn't determine your 
ability to perceive a particular *extended object* or feature: With stars, you 
are trying to see a single point source. For the extended object, a diffuse 
source against a diffuse background.

What's worse, the "magnitudes" frequently quoted for extended objects (i.e., 
galaxies, nebulae, comets, etc.) are EXTREMELY difficult to interpret! In some 
cases, these magnitudes are the TOTAL magnitude of all visible light from the 
object, out to some unstated boundary or "isophote" determined by an astronomer 
long ago. In other cases, the mag given with a galaxy may not be it's VISUAL 
brightness: instead, it may be a "red", "blue", "photographic", "photovisual", 
or about 10 other different magnitudes. It's all a question of how obscure the 
object is, what catalogs it appears in, etc.

And as if all THAT wouldn't thoroughly confuse us, then advanced observers 
start talking about "optimal detection magnifications" for an object! (This is 
basically the magnification at which the object shows the MOST contrast with 
the surrounding sky... Which of course depends on the sky you view under - AND 
on how light is distributed around the object's area!)

Also, don't forget star clusters - which are actually both stellar and extended 
objects at the same time! Not to mention "stellar" galactic nuclei, or 
"unresolved" globular cluster cores, etc., etc., etc.


In other words, Harold, you're question is not a trivial one to answer! But 
there *IS* one thing you can always rely on to help you know when an object 
might be within your reach... As you might have guessed, I'm talking about our 
little mailing list! ;>

For just imagine you're a fairly inexperienced observer (and experience *is* 
important to how much you'll see). Let's say you observe from a somewhat dark 
site, but still not far from town (I've heard such sites called "exurban"). If 
you're using an 8" telescope at 70x, how likely are you to see the Crab Nebula 
in Taurus? And if you see it, what will it LOOK LIKE? Just a blob? Or a mottled 
haze? *Or* will it look like the striated mass of filaments overlying a diffuse 
multilobed cloud that you see in photos?!?

Well, rather than noting that it has "magnitude 8.4", just try looking up 
"Messier" and "1" in the Web archive! If you do, you'll find Adam Albino's 
beautiful description with his 8" scope. Reading that, you will at least know 
what you COULD see, if you really took the time to try...

"A large soft elongated glow at 39X that is an easy Messier in the 8".
Not viewable in my 50mm. At 69X a hint of an "S" shape becomes apparent,
and at 125X and above, very easy. A small 13.5 mag star(s) is visible
with an averted view near the center. The actual pulsar, of course, was
not visible (16 mag?) At 125 and 250X some texture (unevenness) is
noticeable which I take as filaments that are better seen in photos. In
the 80mm, the "s" shape was seen at 125x although not clearly defined.
No stars or "texture" seen at this aperture. I wonder at what aperture
the central pulsar, and filaments would really be seen."


Then, of course - and this is important! - you can post what YOU saw with your 
8" under your skies. And another observer who is a beginner just like you were 
will know that they ARE likely to see that "S" shape, after all... And they'll 
thank you very much for your time. :)

OK, advertising mode off! And if I haven't answered your question well enough, 
Harold, don't hesitate to say so! There are some VERY expert observers on this 
list, and I'm sure one of them will be able to help.

Clear skies!