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Daniel-K

Dobfest II - Off the grid camping in brecon

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I don't believe that either. SQM manufacturer reckons 22 is highest possible. La Palma observatory recorded 21.9 in V-band, and that's for the sky background (i.e. starlight subtracted). SQ meter records all light, including starlight, within its detection angle. If you've got the Milky Way overhead you get a reading brighter than would be obtained for sky background alone. Highest I've ever recorded was 21.7 (in Northumberland).

I've spoken to one professional astronomer who expressed scepticism about the accuracy of SQ meters, though generally they seem to be consistently calibrated. My guess is that the main source of error is from improper use, also a tendency to accept highest possible readings rather than take an average and discard outliers. Other possible sources of error include obstruction on lens, obstruction in detection area, meter not at correct operating temperature etc.

Like I say, the SQ reading is our own version of the "fishermen's tale". Take everything with a big dose of salt (i.e. scientific scepticism).

Edited by acey

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Peel fell?..mate of mine said it was truly fantastic there...

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We was thinking of that way but TBH,if we're going that far why not go the whole hog and head Oban way which needs a lot of planning so Scotland's on hold till next year

Me and Kieran went last year, and although there is wigtown to the north about a mile away the site is one of the best I've been to. I dont have an SQM but can confirm that its very dark and you cant recognise anyone till your within 6ft of each other even then its difficult. Cant wait for this years event

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Peel fell?..mate of mine said it was truly fantastic there...

I use more sheltered sites - it can get very windy. As long as there's a decent view to the south, just about anywhere is fine once you're far enough from Newcastle so that its light dome is not too obtrusive.

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Have SQM, give me a lift and I'll measure your skies.... When I compared 2units, we got agreement with a few tenths, I always measure several directions and make many measurements and get a feel for the average. Milky Way makes a few tenths effect. If you use one in a sealed room it refuses to give you a result. I would feel that 22is the max, but I am happy to he proven wrong.

PeterW

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Certainly possible to get a reading higher than 22, e.g. in middle of a forest at night, but not if it's pointing at a cloudless and unobstructed sky. If agreement between units is only to within a few tenths then a reading of say 21.5 could potentially mean anything between 21 and 22 (or perhaps 21.2 and 21.8). My understanding is that they're better than that - would certainly hope so.

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Few tenths.... Think upto 0.2. I am sure we have enough available for a comparison... Sure beat my backyard 18.5!

Cheers

PeterW

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my local dark site which is in snowdonia,30 mins drive. a few weeks ago there was a fella there taking sqm readings for the national trust(so i imagine he nos how to use the meter) there trying to get dark sky status. any way he was averaging hi 20,s and elan is certanly darker than there

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When I take my readings (I always do four or five at a time, all aimed at zenith) I find there's generally stability to one decimal place, and if different meters agree to within about 0.2 then it suggests that rounding to the nearest 0.2 is probably reasonable. Still doesn't explain how people sometimes get readings of 22 or higher, but the power of wishful thinking can do remarkable things.

The IDA (International Dark Sky Association) has three dark-sky bands, starting with Bronze at SQ20.0, which I reckon is a pretty low bar. If the sky is that bright I don't bother to observe. Milky Way becomes visible around 20.3. They currently set Gold as 21.75 or above. But as far as I'm aware, there isn't a precise protocol about how that figure is determined (e.g. whether it's an average over a certain number of nights, or just the best that anyone manages to measure). Rather more important, to my mind, are the rules on lighting that go with dark-sky status, which at least help to preserve whatever conditions already exist. Though that's only meaningful if there's a sufficient buffer zone to prevent increasing encroachment of light domes. I understand that the IDA is moving toward a more all-sky approach that would include light domes as part of the consideration, rather than just zenith sky brightness.

There are of course plenty of anecdotes about places that are "really dark", based on people's personal perceptions, but a lot depends on how well dark-adapted the individual is, and how much experience they have of dark site conditions. Personally, the thing that always strikes me about my dark site (once I'm fully dark adapted)  is how bright it looks.

My suspicion is that the Bortle Scale (among other things) has encouraged an exaggerated sense of what dark-sky conditions are like. A professional astronomer told me that on cloudy nights at La Palma observatory the sky is visibly orange (on clear nights it reaches 21.9 in V-band), so it wouldn't rate too well on Bortle (which requires clouds overhead to be invisible for Class 3 or better). I find it frankly impossible to believe there is any place in the world where the zodiacal light looks coloured (a criterion for Bortle Class 1). My dark site comes out as 5 ("suburban") or possibly 4 (rural/suburban) - and that's in a remote part of Northumberland, where I've recorded SQ readings up to 21.7.

Edited by acey

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When I take my readings (I always do four or five at a time, all aimed at zenith) I find there's generally stability to one decimal place, and if different meters agree to within about 0.2 then it suggests that rounding to the nearest 0.2 is probably reasonable. Still doesn't explain how people sometimes get readings of 22 or higher, but the power of wishful thinking can do remarkable things.

i will find the thread that was on here for those readings, im not arguing with you, i dont own a sqm reader but i do find it interesting. and i see it as a gauge rather than scientifically proven

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Interesting point about there being a lot of light around at a truly dark site. I guess that is down to your eyes getting as dark adapted as it is possible to get. So they can make use of the little light that the Milky Way affords us.

I spent most of my stag do in a cave deep under the Brecon Beacons. That was dark....

Paul

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Quote from Unihedron website:

"a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80."

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

At a dark site, most of the light comes from light pollution (light domes on the horizon), or from natural airglow (which is like auroral light, but fainter and across the whole sky). You only need to look at foreground objects (e.g. trees against the sky) to see how far from black the sky is. Total darkness is found in sealed rooms, caves etc, or from looking at the background sky through a telescope at very high power with your head covered. After half an hour of that the sky is really dazzling.

Edited by acey

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The SQM must be warmed up by taking a few readings first- and then an average of the subsequent readings taken. Taking readings this way has given a best of 21.7 mag here, repeatedly-I did have one night of 21.8 mag avg, but as this only happened once I have discounted it. If the SQM reads 22+ I think the sensor isn't warmed or atmospheric issues are causing the high number.

Its funny, light low cloud can cause a much lower SQM, but after a certain point ( more cloud etc) the meter can read darker. I try to use NELM + SQM to determine the amount of light extinguishment from cloud, dust whatever. If I remember my best NELM was 6.5 @ 21.7 and the views were very good.

Once a nice dark site is found, transparency is the limiting factor and that is out of our control.

Edited by jetstream

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there seems to be too many variables and inconsistent reading with these  

You guys have a bunch of good sites that you know are dark- the SQM readings in a transparent sky should stay the same there, my sites do. Some are 21.4 avg, a few 21.6 and two are 21.7 and the readings don't go darker unless its quite cloudy.

The SQM has saved me a lot of gasoline....I used to travel much farther to a site that the LP map said was darker, until I bought the SQM- the sky darkness was the same at both sites so I save 20 miles or so. The SQM-L has been a very consistent reliable performer here.

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You guys have a bunch of good sites that you know are dark- the SQM readings in a transparent sky should stay the same there, my sites do. Some are 21.4 avg, a few 21.6 and two are 21.7 and the readings don't go darker unless its quite cloudy.

The SQM has saved me a lot of gasoline....I used to travel much farther to a site that the LP map said was darker, until I bought the SQM- the sky darkness was the same at both sites so I save 20 miles or so. The SQM-L has been a very consistent reliable performer here.

That's a good point Gerry, I have only recently and quite tenuously began to use mine, but I need to apply it for contrast between varied locations. Some good tips on getting the best out of this devise.

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I've used a sight tube at that site and saw mag 7th stars at the zenith. That's dark. I'm not convinced by meter readings, too many variables IMO

But hey ho it makes for a fun discussion :D

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I find the SQ meter extremely useful, but like any instrument it needs to be used and interpreted correctly. I find it especially interesting to record variations at a site over the course of the night, e.g. the progress of twilight. It's the standard device in light pollution studies and will remain so until anyone can come up with something better. It has its limitations, but in many cases these are not as great as the limitations of the user.

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I have to add that it usually takes a difference of .2 mag for me to think that the difference is accurate, but a couple of sites are 21.7 vs 21.6- and Iv'e taken enough readings to believe it. Seeing M33 naked eye at one of them makes me want to believe....it might be more accurate than I think.

The SQM-L and gasoline have improved my views tremendously.

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I have to add that it usually takes a difference of .2 mag for me to think that the difference is accurate, but a couple of sites are 21.7 vs 21.6- and Iv'e taken enough readings to believe it. Seeing M33 naked eye at one of them makes me want to believe....it might be more accurate than I think.

The SQM-L and gasoline have improved my views tremendously.

 Out of interest, what sort of naked-eye limiting magnitude were you achieving when you saw M33 naked-eye? (I.e. actual limit, rather than suggested SQ equivalent value).

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I use a couple of stars around Gamma Cass that time of year and they are just over 6.4 NELM, sometimes in the winter extremely transparent skies come through, the dark blue according to cleardarksky. This site is the lake ice at our cabin and provides exceptional views.

I remember you posting in my thread when I reported it that it takes at least 6.7 NELM to see, but I have never seen stars that faint- at least that I'm aware of.

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ive seen NELM star of 6.8 from the two main dark sites i use and  one gives far better details in DSO's than the other but yet the NELM is the same? those SQM readers have far to many variables to be accurate. the best way is too look through your scope instead of worrying about a stupid LED read out.. you know its a good site when you look at an object and see detail that makes your jaw drop to the floor. BTW can we keep on topic  after all it is a report and not a discussion.  

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North America and California give me a pretty good transparency idea

PeterW

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