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What's a Bortle between friends?

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Derbyshire Dave


  Although it rained most of yesterday, I got caught out, when I went out with the dog at 10PM it was really clear. I knew it probably wouldn't last, and it didn't!!! But rushed in and got the 8x42 birding binoculars for a look-see.

  I was amazed that I could see the Beehive clearly. We live in a village, and about six months ago they changed the sodium lights for led downlighters. They have obviously made a big difference, because I'd looked for the Beehive before and not found it easily. It really stood out last night.


  So, this got me to wondered about my Bortle value, and looked on the light pollution map .... Bortle 5, with SQM 20.42 mag./arc sec2.


  But I'm not sure how often that is updated. I haven't a meter to measure it, and vaguely recall there are rough ways of estimating it, like how many stars you can see with the naked eye (or maybe binoculars) in the Orion Nebula.


  Just wondered what experience might have been gained by ems members, and whether they have any tips..





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I think i'm Bortle 6 here, if i remember correctly, which i think is more optimistic than it really is.


I think the Beehive stands out because its in a fairly 'starless' area of the sky.

I will have a look at it myself with my binocs and see how well it stands out.

With binocs i always look in the vicinity of the sky between Regulus in Leo and Pollux in Gemini to locate M44 as Cancer from here is a pretty 'blank' constellation.


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First Light with a Sky Quality Meter

The meter concerned is a Unihedron SQM-L, which is the narrower FoV version adopted by most sky surveys.

It has been used on my drive at home, with and without a shade protecting it from a nearby LED street lamp; at the Wymeswold dark site, and indoors.

The darkest it has ever recorded was 23.05 Mag/arcsec2 in my bedroom at night.  This and all other readings have been plausible.

Outdoors, the meter is fitted to a tripod and levelled.  Before all recorded readings are taken, three measurements are taken to ‘warm up’ the meter, as recommended by Unihedron.  A succession of readings are then taken, the highest and lowest are discarded and the remainder averaged.


My usage so far has been to manage expectations e.g. to establish how much variability there is between successive readings. 

Principal findings so far have been:

·          On my front drive (suburban fringe of Loughborough) with a clear sky and influenced by a nearby streetlight: 15.93 Mag/arcsec2

·          On my front drive, clear sky, shaded from streetlight: 19.23 Mag/arcsec2

·          Wymeswold, in front of the pavillion, with dappled cloud: 19.28 Mag/arcsec2

·         On my front drive, no streetlight, dappled cloud: 18.44 Mag/arcsec2

·         On my front drive, no streetlight, clear sky: 19.55 Mag/arcsec2

·         Switching off the streetlight gained me 2.5 Magnitudes

·         Shading the streetlight was only 0.3 Mag worse than turning it off, so the shading seems to work.

·         Patchy cloud cost me 0.84 Mags and caused so much variability that the meter only really produces meaningful results under clear skies.

·         It is rare for two successive readings to be identical.  The mean difference between successive readings has been 0.03 Mag/arcsec2.  So the last decimal place is important.

·         A scatter plot of successive readings shows no evidence of a systematic increase or decrease in values over time.


I'll be bringing it to Dark Site meets.

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