Hi Paddy,
Not a dumb question at all mate. We all know what a light-meter is. Well, the SQM is a genuine "dark-meter" that has very good scientific credentials.
The Unihedron SQM and SQM-L are here:
http://www.unihedron.com/projects/darksky/
There were two reasons I bought one (I've got the "-L" model).
(1) I wanted to obtain verifiable scientific proof for the purposes of resolving an argument at Sydney Observatory, regarding the difference in sky-darkness between a 4-5 day moon and no moon in the sky is non-existent or at worst negligible. Sydney Obs was so impressed, they bought one too. I won the argument.
(2) It provides an accurate, objective and repeatable verification of conditions under which observations were made.
Without going into all the ins and outs, I have witnessed countless arguments over the years, usually conducted over the internet about how a person has claimed to have observed Object "X" or "Y" and people have then cast doubt over that observation and attempted to discredit it.
To call a spade a spade,
some people will tell lies about whether they were able to observe/detect a particular object in a particular sized telescope. I try very, very hard to avoid becoming entangled in such arguments (because they are usually unproductive and frequently end in tears), but have witnessed dozens. They rarely end happily ...
When an observation is questioned, in order to support the observation, people will usually talk about how the sky was that night and it was
soooooo dark etc etc and then go on to say they could see 8th magnitude stars naked-eye, and the gegenschein was casting strong shadows on the ground and could see the zodiacal band with direct vision etc etc ... and on it goes. It usually ends in name calling of a magnitude that would leave a thread on "climate change" looking pretty tame.
If properly used
and if the operator does not lie about the reading obtained, the SQM can be used to accurately and objectively quantify and objectively compare (with other sites) how dark the sky was with no fudging. Again to emphasise, it cannot preclude people fudging the reading -- and let's face it, if people are willing to fudge an observation, they probably won't balk at fudging a figure.
There are, without doubt, similarly a lot of people out there, for reasons best known to themselves who will fudge observations but usually supply no objective proof of how good the sky was. I hear a lot of people boast about how good such-and-such a site is and how faint the stars are you can see, and that it is like that all the time and that is why I can see things so much fainter than you can -- but won't use something like this (with a witness) to back up their claims.
The SQM (or SQM-L -- depending on the model used) gives a readout of the sky's surface-brightness magnitude measured in in magnitudes per arc-second. The higher the reading, the darker it is -- pure and simple. A simple equation is used (or performed on a web page with an applet) to do the conversion to the ZLM (zenithal limiting magnitude) -- which for all practical purposes means the same thing as NELM.
The SQM is now used at many professional observatories around the world because it is really that accurate (about 0.05 mags) and that good. The best true dark skies in the world that are at altitude (like Mauna Kea, Chile etc etc) give a reading a little better than 22 -- maybe 22.2. I believe (I've never tried it) Siding Spring Mountain is just about that or perhaps a little shy of 22. The ASNSW Ilford site on top nights is about 21.8-9 -- as is my spot at Mudgee.
Bargo (where I regularly observe) on an average night is about 21.2 and on really good nights gets to about 21.4-5. Home at Engadine on a moonless night I normally get about 20.1-2.
For what it's worth, a reading takes about 6-8 seconds to make. I usually take 4 in a set over about 30 seconds every hour or so. I discard the first reading, then average the following three. In my experience the first one is usually a little out from the other three in a set of four and usually reads about 0.05-0.1 mag/arcsec^2 higher than the others -- that's why I dump it.
I own one because I hope its use will contribute some "sanity" and a little objectivity in recording visual observations -- that are, in the end, highly subjective.
Best,
Les D
