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Re: (IAAC) limiting magnitude



At 07:55 PM 3/2/98 -0500, Lew wrote:

>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. 

Frequently, especially in the case of galaxies, the integrated magnitude is
that of the object out to a brightness of 21 mag/sq. arc seconds, or
thereabouts. This is something of a convention in galaxy catalogs, but the
boundary does have a tendency to slip back and forth between about
18m/arcsec2 and 23m/arcsec2 depending on the zero points used and the, er,
not to put to fine a point on it, sloppiness of the work.

Fortunately, the magnitudes derived by such methods are not *too* bad for
visual use as long as they are in the right system, but the *sizes* that
are derived are generally complete junk for visual observers.

Finally, if we are lucky, Brian Skiff will crawl out of the woodwork and
correct whatever mistake it is that I inevitably made in the above....

>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.

That's exactly right. There are ways to convert from all of these magnitude
systems to something more useful such as the V or Cousins V systems, which
will more or less correspond to the eye's response to light. However,
photometry in another system or integrated spectral type must have been
done in addition to the cataloged brightness.

However, as comet observers have known for years, the size of the
instrument used for visual observations can make a big difference in the
estimated magnitudes of objects. When a comet is observed by a big
instrument, the magnitude estimate will tend systematically to err on the
side of too faint - that is, the observer will estimate a fainter magnitude
than reality indicates. This is counterintuitive, but it is the reason that
comet researchers recommend using the smallest instrument that will clearly
show the comet when making an estimate.

This translates to our deep sky experiences as well, of course. Even the
useful quoted magnitudes can look way off, depending on what instrument we
use. And it also throws a wrench in a project that I always thought might
be worthwhile for amateurs to engage, that is, making visual magnitude
estimates of DSO's using comet-observers' disciplines. Unfortunately, over
about 10 years of trying to encourage this, I have encountered little
interest. Still, if the project were mounted, the systematic errors can be
corrected, just as they are in comet work.

>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!)

Yep, this can be a confusing and variable-ridden question. Nils Olof Carlin
has devised a 'rule of thumb' that seems to hold up well: optimum
magnification is that which magnifies the object to one apparent degree OR
that which magnifies to the point that the field stop cannot be seen,
whichever comes first. This rule of thumb has many exceptions, of course,
but it has served me fairly well when observing from light-polluted Ohio
and not-so-light-polluted Arizona. Carlin's rule of thumb was derived from
a re-evaluation of Blackwell's work, and does not contain the errors that
bias Clark's book.

>Also, don't forget star clusters - which are actually both stellar and
extended 
>objects at the same time! 

Lew, I have a couple of Herschel 400 star clusters in/near Orion that I am
sure I am staring right at, but that I cannot pick out of the starfield.
Any special technique?

>In other words, Harold, you're question is not a trivial one to answer! 

That is quite an understatement, Lew! :-)

>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.

I really doubt that I can help too much, but this seems a convenient place
to plug my deep photometry web page. It contains charts of faint starfields
for which reliable photometry has been done. You can use the charts to
figure out a limiting stellar magnitude for you and your site on a given
night, at least. As to how to convert that to something useful for deep-sky
observing, I will have to defer to other folks to figure that out. Here's
the URL:

http://shutter.vet.ohio-state.edu/astronomy/mags/



--
Jeff Medkeff          | Acting Assistant Coordinator
Rockland Observatory  | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section

On the web at http://shutter.vet.ohio-state.edu/


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