How to *objectively* measure sky quality ?

[skysurfer]

I mean particularly in transparency, not seeing.

* NELM: is very personal. Under the same sky one person sees until +6.2 and the other to +5.5.
* SQM: is brighness in magnitudes per square arc second. Unreliable. A "perfect" dark sky (which never happens) has a luminance of zero i.e. an SQM of -infinity as magnitude is logarithm of luminance. Real examples like a fully overcast sky of very hazy sky at a very dark location render a high SQM of more than 22 while no, resp. fewer stars are visible.

The best alternative is : Take a photo of a starfield around the zenith (always the same lens (or zoom factor in case of a compact camers), aperture, ISO setting, white balance and exposure time. Compare the photo with a star chart (Stellarium, Skysafari or a paper atlas) to get the actual limiting magnitude.

Does somebody have ideas on this ?

[glend (Glen)]

In my opinion all that matters is you pick a system that suits you to gauge relative quality. I like my SQM and use it at every dark site I go to and the backyard and in terms of realative darkness it is a good measure but it cannot measure 'seeing'- that is still subjective for most peope as eye sight, age, etc affect it as well as atmo, heat, angle of view through the atmo, etc.

[mithrandir (Andrew)]

SQM isn't subjective which I prefer, but depending on which part of the sky I point my SQM-L at I can get vastly different readings. There is a formula to convert SQM to NELM:

NELM=7.93-5*log(10^(4.316-(SQM/5))+1)

and one for NELM to SQM:

SQM = 21.58 - 5 log(10^(1.586-NELM/5)-1)

[algwat (Alan)]

So, how does this more technical method compare to this simpler one that any body could use visually , or with binoculars?

http://www.iceinspace.com.au/forum/s...d.php?t=121660

I was thinking of a zenith cam approach, or if you have an allsky measure the zenith area, or a few degrees around zen. It could also be used as a twinkle-o-meter to measure air turbulence? These are all concepts I am slooowly working into my DMK ALLsky meteor camera.

Interested to read if anyone is looking to use the current available java based languages to program your own solutions to these astronomical problems?

kind regards, Alan Watson