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Old 11-12-2015, 09:46 AM
glend (Glen)
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glend is offline
 
Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,121
Kevin, sorry I responded late last night and didn't have time to find the link. So here is the link to the Light Pollution Map:

http://www.lightpollutionmap.info/#z...yers=B0TFFFFTT

When you open it up, close the central white box which is just informational, and you will see a default map of Europe. Don't be put off by that as it's the starting point. Just click and drag the map overlay over to Australia, then you can zoom in on your area of the Central Coast of NSW. The map provides pretty good street level detail I find, and you can easily see the effect of shopping centres, and other major light sources. There are buttons so that you can go back to previous years data, and it is sad to see how light pollution spreads. I have not worked out a way to save my position and still have to drag it back each time I check it. My local light pollution level is just in the light blue range, but it is getting worse (or closer to Green) each year of the map history. Your area near the top of Brisbane Water is probably close of Orange, or Yellow at best, depending on the exact location of your street, but you can zoom in on it.

Now as to the Sky Quality Metre (SQM) , the details are here:

http://www.unihedron.com/projects/darksky/

The SQM works by calculating the magnitude per square arc sec.

It is a very useful tool for comparing night sky locations, and Allan and I use it to rate the various dark site locations we visit. Typically your looking for SMQ readings well above 21.0 to qualify as a dark site. Bretti, for example, regularly returns readings above 21.7, with Lake Chaffey near Tamworth slightly behind that figure (due to Tamworth Sky Glow on the north-western horizon). Seasonal factors can affect the SQM reading, for example, having the expanse of the Milky Way directly overhead can produce a lower reading due to the relative 'light pollution' from the Milky Way.

Experience with sky assessment can give you a perspective on linking the Bortle Scale with the SQM number but they are separate and the Bortle Scale is somewhat subjective. The advantage of the SQM, in my opinion, is that it is producing a reading based what it is gathering from the sky and thus is objective in nature.

As to my comment on star twinkling, this is a quick look sort of baseline. Atmospherics (winds aloft, differing density movements, etc) produce the observed affect. Ulrich's recent trip around Australia demonstrated this well, as his posts back generally reported a very stable atmosphere - based on his visual assessment of the twinkle. Full details are here:

https://en.wikipedia.org/wiki/Twinkling

Hope all of this helps in some way. The links are worth reading.

BTW, the highest SQM reading that I have seen from my backyard observatory is 20.92, which is pretty good and certainly better than any of the major cities. However, there is a big difference between 20.92 and Bretti's 21.77 and that's why some of us continue to make that trip as often as we can.



Last edited by glend; 11-12-2015 at 09:58 AM.
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