RE: Measuring transparency (was Re: (IAAC) Obj: Jonkherre 900)

> From:	Lew Gramer [SMTP:dedalus@latrade.com]
> I have to agree with Yann that Limiting Magnitude may not be the most
> accurate measurement of visibility for extended objects. 
> 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.
> So in the absence of any better standard for extended objects
	[Tom Krajci Capt] Here's a thought, that I haven't had time to
evaluate with observations:  Evaluate the visibility of faint, defocused
stars as a way to measure transparency.  This would have to be standardized,
or have correction factors taken into account such as:  scope aperture, star
magnitude, amount of defocus. . .but there are always stars available for
defocusing, so this method may by quite handy, and if correction factors are
applied correctly, then any size scope can evaluate transparency this way.

	Have others done this before?  Am I taking a bad approach here?

	Capt Tom Krajci
	B-52 Intelligence Officer