Hi Alex & All,
Quote:
Originally Posted by mental4astro
There is one thing I don't get about SQM readings - do they take into account the quality of the transparency of the sky too, or just "how dark" it is?
Because an SQM reading can mean bugger all if transparency is stuffed.
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There are several components that make up an excellent sky and they include minimum of artificial light pollution, minimum of natural light pollution (Zodiacal light, gegenschein, sky-glow etc) altitude and lastly solid particles in the atmosphere that scatter light (dust, soot, water-droplets volcanic aerosols etc). To rank them in order of importance, (1) Artificial light pollution (2) Solid particles, (3) Natural light pollution and (4) Altitude. (Though 3 & 4 are probably a close-run thing).
The Earth's atmosphere (assuming a complete absence of solid particles) is uniform over the surface of the globe in
composition and therefore equally transparent. The only variable here is thickness -- increasing altitude means you have to look through less of it. It is largely transparent except over long distances.
The amount of solid particles is going to vary a lot from location to location and indeed from day to day, month to month and year to year depending upon the weather, climate and how many "Plinian"-type volcanic eruptions have occurred in recent times. These types of eruptions inject large quantities of very fine silicate dust into the stratosphere. At the present time, the stratosphere is pretty clean and the last truly big event was Mt Pinatubo in the mid-90s. At least living in the bush means you only have to live with "natural" solids.
The Unihedron SQM & SQM-L aren't the absolute end of the story, but I believe are a very strong and importantly
objective indicator of the quality of a site. They measure the brightness of the background sky in magnitudes per arc-second Sq. They can't take into account the amount of star-light blocked-out by solids in the atmosphere (upper and lower) but they do, for all practical purposes take into account all the other factors that will make the sky brighter or from scattering incoming light.
One Cosmos, you mentioned readings in excess of 22.0. Normally, a reading much over 22.0 shouldn't occur. How long have you had the unit? I returned mine recently (after 11 years) for re-calibration and replacement of the I.R filter because I was sometimes getting readings in the 22.1 - 22.25 range. I compared it with a new unit that was giving a different reading. Over time this filter can become frosted due to interaction with water (ie dew, fog etc etc). It was retested after replacement of the filter and is now accurate again. In winter on moonless nights around midnight returns reading in the 21.6 to 21.8 range (usually depending on the amount of humidity) and in summer 21.8 to 22.05. The cost of the service was just the cost of return postage to Unihedron in Canada.
I live in a site that is very, very low in artificial light pollution, 70km south of Orange, 30km north of Cowra and 260km from Sydney (as the photon flies) at 321m elevation. To find a "perfect" site (leaving aside elevation) I'd think you'd need to be at least 500km from any major city, about 150km from any major rural centres (think Orange, Wagga, Tamworth, Dubbo etc etc) and 70km from any good-sized towns with populations over 5000.
Plenty of places to choose from that meets these criteria, but actually living there on a day-to-day basis is a chore as you have to travel for at least an hour for "anything" -- there are little to no facilities. A compromise must be made on that front.
Best,
L.