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

>	[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
>	http://spur.barksdale.af.mil
Well Tom,
I had the same idea some years ago. I wished to evaluate transparency on
defocused stars (about one arcminute in diameter in a 8-inch SCT and with a
system of occulting discs, I could have tried to determine the surface of
the mirror needed to detect the defocused star. I haven't pushed this idea
further for the moment as I am now using a bigger dobsonian and this
protocol is harder to achieve on it. In fact, I remember that I had another
project with this equipment: I wanted to evaluate visual magnitude on
galaxies (ie defocusing stars of known brightness to achieve the galaxy's
size and find which lokks the same)...
But as far as measuring transparency is concerned (and the difference
between LM estimates and extended objects visibility), the problem remains
with our protocol above: the star (which looks stellar before beeing
defocused) can be less affected by haze than extended objects...
clear skies, Yann.
Yann Pothier	tel: 01 43 41 43 29
11 impasse Canart, 75012 PARIS, FRANCE
Email: ypothier@abi.snv.jussieu.fr
Site : http://pegase.unice.fr/~skylink/publi/cielextreme