Sky Quality Meter

[BPO]

Now that's just bragging!

Hmmm. Well, now that you mention it, I suppose it does. But that wasn't my intention!

My actual point was that even from a very dark site you are stretching it to get much above 22 with an SQM. Even the manufacturer says such readings are almost certainly erroneous.

My aim for the month is to get out in the Peak District on a decent night with one of our SQM-L's and see if I can uncover some proper dark skies. I'm told they're out there...

There's nothing quite like a truly dark sky, and, as you said, they're out there.

One thing I'm about to try is see how much of an effect - if any - the recent Christchurch earthquake has had on sky darkness to my northeast. Light from the city is essentially undetectable in normal circumstance (it's a small place and quite distant, with several mountain ranges in between us and it), but perhaps the SQM-LE will detect a difference now that much of their electricity has been disrupted.

[hunterknox]

Hmmm. Well, now that you mention it, I suppose it does. But that wasn't my intention!

Didn't think you were bragging, really. I'm just jealous .

[BPO]

Didn't think you were bragging, really. I'm just jealous .

If you or any other SGL members ever make it down this way you're most welcome to visit my observatory.

Here are a couple of panoramic views (approx. 180° each) of the site:

http://www.observatory.org.nz/temp/pan1_W-NE.jpg

http://www.observatory.org.nz/temp/pan2_NE-S.jpg

[dogfish]

I've just bought a holiday home in Galloway. Anyone want to lend me their SQM?

Martin

[ollypenrice]

I get a few visiting my place. They seem interesting but I have never looked at what the scale means. They usually show 21.6 and one guest goes to the Namibia astro place every year and gets the same readng there, though in the middle of nowhere the horizons are a little better than mine. I'm impressed by how consistent the meters are, though., which lends henm credibility.

Olly

[acey]

I think the SQM-L is an excellent device and I'm very glad I bought one. In Northumbria I generally get something in the range 21.3-21.5 for clear-sky readings. If it's completely cloudy then it's darker! My figures notionally correspond to limiting magnitude 6.3-6.4 though I can never see stars beyond 6 - I expect younger eyes would. According to the conversion calculator (which uses various assumptions) a reading of 22 would be limiting mag 6.6. There are people who claim naked-eye limiting magnitude 7 for exceptional sites which would be about 23 on the meter.

Conversion Calculator - NELM (V) to MPSAS ( systems

[SteveL]

Sorry for the large image, but this is the data collected from the SQM USB and some other sensors while at kelling in September 2010.

[acey]

Interesting! What exactly is Clear Sky Factor and how is it measured?

[SteveL]

Its the sum of the difference of adjacent readings over a period, in a rough attempt to spot clouds. A clear sky will change smoothly over time, a sky with clouds will change rapidly. Its not a substitute for a cloud detector, its just a way to try to give a rough approximation of one. I adjusted my code ever so slightly so it ignored values when it changed from totally saturdated to a proper value, otherwise I always got a CSF spike at dusk and dawn.

Clear Sky Detection experiment

[BPO]

Steve, that's very interesting! Did you code that application yourself, or is it available "off the shelf" from somewhere?

[BPO]

I think the SQM-L is an excellent device and I'm very glad I bought one. In Northumbria I generally get something in the range 21.3-21.5 for clear-sky readings. If it's completely cloudy then it's darker! My figures notionally correspond to limiting magnitude 6.3-6.4 though I can never see stars beyond 6 - I expect younger eyes would. According to the conversion calculator (which uses various assumptions) a reading of 22 would be limiting mag 6.6. There are people who claim naked-eye limiting magnitude 7 for exceptional sites which would be about 23 on the meter.

Much depends upon where you're pointing it. For instance, the MW is often crossing the zenith when the SQM-LE at my site is operating, therefore it will always be detecting some amount of light. And that's not taking into account natural skyglow.

When I've discussed this with another local astronomer, he is of the opinion that the "target" should be the SCP, as there are few stars there greater than mag 5-6. The result would be a much higher reading, probably in the 22.25-22.5 range, perhaps greater, but it would be less representative of the sky as a whole.

Of course, if you take readings from a dark site on the new moon and with 8/8 cloud cover, then you'll get a very high reading, which may account for the questionable upper-22s to 23 readings we sometimes see and hear of and read about, but then you may as well bung the meter in a sock and bury it under a huge mound of coal dust, because the reading won't be any less useful.

If you're obtaining readings of, say, 21.5 to 22.25 under realistic circumstances and in usable conditions, it's fairly safe to assume you are at a truly dark site.

[SteveL]

Steve, that's very interesting! Did you code that application yourself, or is it available "off the shelf" from somewhere?

All coded myself (in Perl), borrowing some code from Unihedron for the SQM stuff.

[BPO]

All coded myself (in Perl), borrowing some code from Unihedron for the SQM stuff.

Nice!

[661-pete]

'Bumping' this thread since I've just bought myself one - the Unihedron with lens but not USB - also from Teleskop Express (very good service )

Basically, I'm suffering badly from lost motivation, almost on the point of giving up astronomy altogether. I wanted to test my French location to see whether there's any point in continuing. The street lights in the village were 'improved' a few years ago - to my detriment - and floodlighting was set up on the nearby church - although luckily I am allowed to switch off the latter when I have the need by arrangement with the local Mairie - I borrow a key from them.

To tell the truth, wondering whether to even keep the house in France or sell (although house prices there are plummetting - would be difficult to sell )

Sorry, enough waffle and whingeing.

Well, we got over there last week and I specifically managed to test the sky with the floodlights on - on the first evening. I hadn't had a chance to get the floodlights key from the Mairie which was closed anyway. So I had to test with the floodlights - which point straight at the zenith - on.

In the best place in our garden, shielded by the house from most of the streetlights, I got 20.6 at the zenith.

I wish I'd been able to get a reading without the floodlights, but alas! the rest of the week was clouded-over and most days pouring rain.

Question: should the 20.6 encourage me enough to keep going? I'll never match Olly's almost-perfect location, but it looks better than I'd feared.

Anyway, I think the SQM was a worthwhile buy. But then - would I have said that if I'd read - say - 18?

[acey]

I reckon anything over 20 is potentially good if direct light can be avoided so that the eye can adapt, but 21 is where it begins to get truly dark. My UK dark site is generally around 21.5. My back garden is 18, but I can still see clusters and the cores of some galaxies (though I rarely bother trying - far better to drive).

Something I find striking is how bright a twilit sky of 20 looks when you're at a dark site with well adapted eyes. The same sky brightness seen with non-adapted eyes at a light-polluted urban site would appear black - but only because of the lack of adaptation. In this apparently "black" sky you'd see no milky way, and stars down to about mag 4. At the dark site you'd see those same stars against a blue background, and might make out the milky way.

The SQM is excellent as a way of putting a precise figure to what you should be able to see with your own eyes. If the Milky Way is clearly visible then you've got a genuinely dark sky and will get get good views of galaxies. If the Milky Way can't be seen then you may still be able to get reasonable views of some galaxies.

Converting SQM readings to limiting magnitudes is problematic for many reasons. The online calculator at the SQM site gives 20.6 as equivalent to limiting mag 5.9, which sounds very good, but whether you actually see stars to that limit is another matter. At my site with a reading of 21.5 I only see down to about 6, rather than the predicted 6.5. Partly that's down to my eyes, but probably also the limitations of the model used to make the prediction. In any case, a reading of 20.6 is a lot better than most people can enjoy, even if it falls some way short of ideal conditions.

http://unihedron.com/projects/darksky/NELM2BCalc.html

I wouldn't worry too much about local floodlighting as long as you can shield your site from it. Skyglow is an accumulation from lots of light sources over a wide area rather than whatever happens to be right next to you. If you turn off the floodlights then it shouldn't really make a difference to the sky reading. The difference will be in your ability to dark-adapt, if you've managed to eliminate light at your site. Of course, if the floodlights can be switched off then they should be, since they're a waste of energy.

[hunterknox]

Pete: depends what you're comparing to really, and why you bought the house in the first place.

If you have a look at the MyDarkSky SQM map (link in my signature) then 20.6 would be considered pretty good from these shores, especially with the floodlight on. Population-wise I'd guess only a couple of percent of people would live anywhere like that dark on these shores, and most of the forum would probably kill to be in your shoes.

That said there are darker, accessible places in the UK, however these would possibly be a worse bet from weather-wise. It also depends what the whole sky picture is like. It's 20.7 at zenith round my way, but the nearest town's only 5 miles away and that's a significant source of sky glow up to about 45 degrees from the horizon.

If you bought the house purely for astronomy then there are probably better options both here and in France. If you like the house in general and astronomy is only one of many benefits then you'll need to think harder about it, but then you probably figured all this out already.

By the way, we'd love it if you'd consider adding a few blobs to the MyDarkSky map. You could give us our first reading from French soil:). We don't want to know where you live but somewhere in the general direction would be lovely.

While I'm here: I've just noticed that the maps are starting to fill up outside our homelands in the North West, so a big thanks to all the new MyDarkSkyers and a special shout out to the London super-enthusiasts who persevere regardless of their abysmal light pollution, and to Trull's relentless Aberdeen crew:hello2:.

[661-pete]

I can certainly see the difference with my eyes, between floodlights on and floodlights off. There's a visible cone of light pointing straight at the zenith. So I'm hoping for a bit of an improvement once I can get a reading with them off!

Hunterknox - thanks for the heads-up. In fact I'd already had a look at your website: I take it that you are either the same chap, or work together with, 'ArcturusMDS' over on UKAI. But I didn't realise that you accept input from outside the UK. I've registered on your site now, and I'll certainly plonk down a 'pin' for my recent reading once my account's active. But I may be able to amend it for a better reading later on....

I could also put in quite a lot of readings for my home area in Sussex which seems a bit sparse of 'pins' at the moment - always glad to help. But I think it would be best to wait until the end of the 'twilight season', which for my UK latitude lasts until 19 July . At my French place, of course, I'm much luckier: I get at least four hours of astronomical dark even in mid-summer (I'm the same latitude as OllyPenrice, although several hundred Km further west).

"Why did we choose that place in France?" Well I could fill a book with my thoughts about that one, the plusses and minuses and all the doubts and second thoughts we're now going through in our minds. I wanted a decently dark place but not too far out from civilisation, not at my age! I was originally gunning for a place in the celebrated 'triangle noir' (an area of the Causses du Quercy National Park roughly bounded by the cities of Cahors, Figeac and Gramat, which is supposed to be one of the best parts of France for LP). But we didn't find a suitable house in that area, so we settled for some distance west of Cahors instead, between Cahors and Agen. Also we wanted quiet, picturesque but not mountainous, countryside with plenty of good cycling, and we've got that.

Anyway I'll be following your site with interest!

[Beulah]

It's a very good site. I'd love to borrow an SQM from somewhere and take some readings of my locality. I'll be emailling the local astronomy group to see where I can borrow one.

[hunterknox]

Thanks for the support Pete and good luck with your house. I like the sound of the 'triangle noir' but I can also see the allure of a bit of flattish cycling!

You're right about the astronomical twilight thing. It doesn't help dark sky surveying but, that said, our friend in the North Steve Owens put up an interesting blog post recently pointing out that astronomical twilight is an academic consideration for him on the south side of Glasgow (you have to scroll down the screen about half way for the graph). Sadly a lot of folks are in the same position. I'm planing on running the same experiment down our way.

By the way, ArcturusMDS is our tech guy and we have another mastermind behind the scenes. , I'm just a pretty face of MyDarkSky. We're all based at Macclesfield Astro Soc thought that particular website is independent of the society.

[hunterknox]

Beulah - we also take naked eye limiting magnitude (NELM) readings . SQMs are a great thing to have for a society and dead useful on a road trip because you don't have to wait to be dark adapted to use them, but sadly they're not as cheap as we'd like. Mine is borrowed but I might buy one before I go house hunting again.

We were thinking of merging our NELM and SQM maps at one point, and also thinking about getting some satellite overlays. I suspect that satellite and ground data will be complementary rather than consistant - satellites give a good general picture but the ground readings allow for far greater resolution.

[SteveL]

I am trying to work out the best way of mounting my USB SQM permenantly at home (not having a Obsy yet mean it will be on the roof above my office somewhere), just need to ensure its waterproof and easy to get to so I can clean the window on the top.

[George]

SQM of 23.6 makes that a Mag 7.2 sky... really?

Are there any house building plots available in that area

[ArcturusMDS]

By the way, ArcturusMDS is our tech guy and we have another mastermind behind the scenes. , I'm just a pretty face of MyDarkSky. We're all based at Macclesfield Astro Soc thought that particular website is independent of the society.

Not sure about that bit to be honest

also thinking about getting some satellite overlays. I suspect that satellite and ground data will be complementary rather than consistant - satellites give a good general picture but the ground readings allow for far greater resolution.

Also I am working on overlaying some light pollution data. Still testing things at the moment but you can view them using the following links just to get an idea.

MyDarkSky - SQM Map

MyDarkSky - SQM Map

[mcrossley]

Forestry Commission Scotland says of Galloway Forest: "The Forest Park’s scores range from 21 to 23.6" Hmm...

23.6 does seem very high though I see MyDarkSky has a reading of 22.72 in the forest is is pretty good by any standard. Last time we were up there I can vouch that it was 'pretty' dark, NELM was way beyond 6 (though not actually determined), the gegenschein was a very obvious bright patch - mistaken for a cloud at first!

[acey]

The sky brightness at La Palma observatory is 21.9 V-mag/sq".

La Palma Night-Sky Brightness

The manufacturer of the Sky Quality Meter states: "a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80".

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

An independent study found that the SQM readings are typically 0.1 to 0.2 mag/sq" higher ("darker") than professional Johnson V-mag measurements; so an SQM at La Palma might read 22.1.

http://unihedron.com/projects/darksky/sqmreport_v1p4.pdf

It is of course perfectly possible to get higher readings when the sky is made darker by cloud. The meter measures brightness, not transparency.