Sky Quality Meter (SQM) readings

[erick]

Suburban Bacchus Marsh, Victoria

3:30am 13 Feb 2011

SQM-L - 20.55 magnitudes per square arcsecond

[koputai]

Hi Eric,

What sort of meter were you using?

Cheers,
Jason.

[Robh]

Hi Eric,

I can give you a few SQM-L readings under moonless nights.
Penrith (UWS Campus), date ?, 10pm, 19.2 mags/arcsec^2
Glenbrook (suburb lower Blue Mts), several occasions, around 10pm, 20.2 mags/arcsec^2
Linden (mid Blue Mts), 31/12/10, 11pm, 21.2 mags/arcsec^2

Regards, Rob

[ngcles]

Hi Eric,

Wish I had that sort of reading outside my back door! Even my "nearly dark" site in the Southern Highlands southwest of Sydney on the best nights gets to about 21.4 -- more commonly aboout 21.25. The reading in outer suburban Engadine on really good nights is about 20.2.

Lucky boy!


Best,

Les D

[erick]

Unihedron SQM-L :)

http://unihedron.com/projects/sqm-l/

[JohnG]

My backyard: 21.4 - 21.6. :thumbsup:

Snake Valley I normally get around 21.65.

Cheers

[mithrandir]

With an SQM-L and little cloud:
Home (NW Sydney suburbs) - 18.5 to 19.2 depending on which way I point it. Parramatta direction is worst.
Dural Pony Club (~5Km north of home) - around 20.5 to the north. ~19.5 in any other direction.
Willow Tree (just off the Kamilaroi Hwy) - 20.5 from the Milky Way. 21.5 elsewhere.

Andrew

[Kevnool]

What sort of readings did you get from Fowlers Gap Eric ?
They would of had to been excellent.

Cheers Kev.

[Robh]

Yes. With the weave of the Milky Way high in the sky, you can get some skewed readings for an otherwise dark site.

Regards, Rob

[erick]

Well....................none actually. I bought it after the visit to Fowlers Gap. I expected the sky to be so dark with so much contrast - but it really wasn't?? I really didn't understand. Nearest human habitation of any significance was Broken Hill 110km away. Just a few little solar lights and a couple of 240 Volt outside lights on at the Homestead, and none close to where I was set up. So I want to take readings there - next time I get back. I'm debating whether I can get back in winter. Manager felt that the sky was much darker in winter.

[Kevnool]

Nah Eric its the stars that light up the sky out there (Starshine).
That zodiacal light is a pain.

The best way to see darkness is to hold your hand up in the air then look how dark your hand is ....Thats dark.

It will get cold out there in winter brrrr it is steady tho :cold: .

Cheers Kev.

[jamespierce]

SQM-L

Inner city Melbourne best I've seen - 18.0

Just north of Ballarat (our family dark site) - 21.0 (I've seen it darker, but being fairly close to town it's varies quite a bit due to haze and local conditions)

[Robh]

On the significance of the measure magnitudes per square arcsecond:

The unit is most often used to describe the surface brightness of celestial objects like galaxies and nebulae. An overall or integrated visual magnitude is usual given for such objects. The Sculptor Galaxy NGC 253 has an overall visual magnitude of 7.1, being compared to a star of visual magnitude 7.1. However, for NGC 253 this light is spread over a large area and each square arcsecond of the surface of NGC 253 is a lot less bright then the light of the mag 7.1 star spread over a square arcsecond. In fact the average light given off by each square arcsecond of NGC 253 is equivalent to a magnitude 21.4 star. This is known as its surface brightness. Thus NGC 253 has a surface brightness of 21.4 mags per sq arcsecond. Alternatively, surface brightness can be given in mags per sq arcminute. For NGC 253, this is 12.5 (just take off 8.9).

Now, the skyglow can be measured as the surface brightness of the sky i.e. how bright each square arcsecond of the sky appears.
This is what a Sky Quality Meter measures.
Roughly speaking, the surface brightness of the sky is ...
17 centre of a major city
18 high density suburb around city centre
19 outer suburbs with lower density population
20 low density fringe suburbs, uninhabited areas adjacent
21 rural area with a town in the distance
22 isolated location. Darkest sky possible.

In my mountain suburb of Glenbrook, the skyglow is 20.2.
A half Moon (first or last quarter) increases the skyglow to 19.7.
The Full Moon brightens the sky to 18.

According to Tony Flanders, an object is detectable if its surface brightness is up to 3 magnitudes fainter than the skyglow.
However, the ability to detect an object depends on how sharp its edges are. If it fades away gradually towards the edges, it is harder to detect.
See ...
http://mysite.verizon.net/vze55p46/id18.html

Why can you see an object that is actually fainter than the skyglow?
The object's surface brightness and the surface brightness of the sky have an additive effect. However, there is a level at which a faint object's surface glow does not add enough to the skyglow to be distinguishable from it.
Even at the darkest possible site, there is a limit of detectability for any Earth-based telescope.

Regards, Rob

[ArcturusMDS]

I'm involved in a project that may well be interesting to people here with Sky Quality Meters. it's called MyDarkSky and it is completely free and none profit making.

We currently have a handful of dedicated users armed with Unihedron Sky Quality Meters and we're are surveying sky quality in as many places as we can get to. Our aim is to find the darkest sites for observation but also highlight light pollution hotspots.

If anybody here owns an SQM and would be interested in submitting some data please register and join in.

If you don't own an SQM but would like to be involved anyway we also have a method of assessing the Naked Eye Limiting Magnitude and users can add data that way.

Currently the maps are a little sparse but new Data is being added all the time. Most of our data is in the UK but I'd love to see readings added globally.

It's a free website created by amateur astronomers for amateur astronomers and the more users we have the better it will be for everybody.

Please take a look and let me know what you think. MyDarkSky (http://www.mydarksky.com)

[erick]

Sounds good Darren, I'm sure you'll have interested skywatchers here with meters to hand.

Only problem is that we'll fill Australia with black dots! ;)

[ArcturusMDS]

Brilliant. We could always add more levels of black if 22 isn't enough for you.
Has anybody got 22.72 beat yet. Best reading we've had so far in the UK.

[mithrandir]

Darren, we'd have trouble beating that for most of the year. There is too much light from the Milky Way and any dust scatter spreads that around. For example, the sky around Crux only gets down to about 21.

Andrew

[ArcturusMDS]

Well you can't complain much if it's the Milky Way causing the bad readings can you ;)

I've never been anywhere that dark myself so I can't comment. I've got to drive for a couple of hours to get a decent view of the Milky Way. My 13 year old son still hasn't seen it properly.

[erick]

Darren, I'm getting a Server error (below) when I click "register" on the site?

Problem at your end or mine?

Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

[mithrandir]

Eric, it worked for me at around 8:00 AEDT.

Also it is Globe At Night time again. http://www.globeatnight.org/index.html

February 21 - March 6 figures are to be submitted this week.

There is another round March 24 - April 6.

[ArcturusMDS]

Dammit Sorry. I'd been updating a few things and mucked something up. All is fine again now.

[ArcturusMDS]

Register on the site and add it. If you would prefer I could add it for you but half the fun is doing it yourself. It's pretty straight forward to do.

[erick]

I will Darren. Am off to Snake Valley Star Camp today and expect to have some good dark readings from there to enter - sometime next week.

[ArcturusMDS]

Great. We've our first reading from New Zealand now too so you've got some local competition. You've got 22.02 to beat!

[erick]

:sadeyes: Ok, I'll try.

[ArcturusMDS]

Erick have you registered yet? you should add the 20.55 you took at Suburban Bacchus Marsh!

[erick]

Yes, just completed. I had to remember how to get my SQM serial number. I'll add data after this camp. Mid 21's last night but a bit too much cloud around.

[jamespierce]

21.5 at the ASV Heathcote Site last night, though there were so many lights on the field I wouldn't be surprised if we were creating our own light pollution :P

[ausastronomer]

A reading in excess of 21.9 indicates the batteries are going flat in the SQM. If someone gets a reading of 22.72 they need fresh batteries and the reading is totally inaccurate.

Cheers,
John B

[erick]

Interim report from Snake Valley Astro Camp - 21.45 at 2:50am third night.

[ArcturusMDS]

Can't work out if you're being serious or not.



Can't wait to see these readings enetered onto the system. They're getting better eachtime. With a bit of luck you might beat the 22.02 from NZ. Although John B will want your reading disqualified if you do :thumbsup:

[jamespierce]

My understanding also was that 22 was the effective limit of the scale.

[ausastronomer]

Hi Darren,

I couldn't be more serious. The theoretical maximum reading is 22.0. With Sky Glow and the light output from the stars themselves this brings the practical maximum to about 21.9!

How I learned about this? One evening at Coonabarabran in October, 2007 I was observing under "stunning" observing conditions with some friends. My observing partners were Andrew Murrell (another 3RF Volunteer), Gary Kopff from Wildcard Innovations, Dave Kriege from Obsession Telescopes and Monte Wilson from AS of NSW. We were discussing how good the observing conditions were. M33 was visible naked eye, notwithstanding it was only about 20 degrees above the horizon. Dave Kriege has observed all over the world from some very dark skies, including, the Atacama Destert in Chile, Lake Titicaca in Bolivia, and Mauna Kea in Hawaii. Dave commented that these were the best observing conditions he had experienced anywhere in the world apart from one solitary night at high elevation on Mauna Kea. There were also 5 other US visitors with us including Scott Tannehill and Don Wyman.

We decided to put the sky conditions to the test with the SQM. Andrew Murrell pulled out his Unihedron SQM and got a reading of 23.45. I commented, "Andrew that can't be right, 22.0 is the maximum". I then used my own identical meter aimed at the same area of sky and got a reading of 21.88. Gary Kopff, who knows a little bit about electronic devices, suggested to Andrew that he should put fresh batteries in his unit, as they may be depleted. Andrew put a fresh battery in his unit and got a reading of 21.85, aimed at the same patch of sky. It was then very obvious that the partially depleted batteries had given an incorrect "false" high reading. On a number of subsequent occasions I have received readings over 22. I have immediately put in a new battery and got an accurate reading of between 21.4 and 21.7.

I believe you should eliminate any reading over 21.9 from your database because IMO it's not an accurate reading.

However, if you don't believe me you had better record an entry of 23.45 on
12/10/2007 for Coonababran NSW taken by Andrew Murrell.

Cheers,
John B

[erick]

I note that Unihedron's database of readings has 32 readings over 21.9. The darkest reading is 23.27.

Given John's tests, I wonder how low batteries affect readings across the scale? Could all readings be elevated by battery condition?

[Rob_K]

Sounds like John has outlined the pitfalls well. Surely it would be easy to do a quick calibration reading before (or during) use. Just a reading in total darkness (dark room at home, or other completely shrouded space). As Eric said, the worry is that if depleted batteries give false high readings, then how depleted do the batteries have to be before they have some (even minor) influence on the readings?

Cheers -

[erick]

Unfortunately Rob, trying to read in a dark room doesn't work. If it sees no light, it reports "underexposed".

[MrB]

I know a bit about electronics too (not comparing myself to Gary) and this is enough to scare me right off this device.
That is just flat-out bad design. Should never happen.
It may just be a firmware bug they are not aware of? Has anyone contacted them?

The only reference I can find in Unihedron's literature about the battery is if there is no reading at all.

[erick]

I see the instruction sheet says:- "Any kind of 9V battery is usable. The SQM-L contains a voltage regulator to power the sensor, microcontroller and other components." which suggests some level of battery depletion would be allowed for?

[BPO]

Hi guys.

The highest ever reading by my SQM-LE was 22.02 in December 2009. (It's been in storage since then). This was on a moonless night at my very dark permanent observatory site, where there is essentially no light pollution.

The SQM-LE is powered from a permanent supply: it has no battery.

However, anything much above 22 would be fairly suspect, and, as has already been mentioned in this thread, even the manufacturer states that readings significantly above 22 are likely to be inaccurate.

[ausastronomer]

Hi Erick,

Andrew and I both have the early version standard SQM. I guess I have had the unit coming up towards 5 years. I don't know what circuitry these older devices have to allow for battery depletion. Possibly none. However, there are a lot of these older units floating around so I would guess most of these > 22.0 readings might eminate from these units.

What I have done is repeated the excersise enough times to know that depleted batteries cause a fasle high reading with at least mine and Andrew's units.

It's also worth mentioning that all SQM units have a tolerance allowance of 10% (+ or - .1 reading).

Cheers,
John B

[ausastronomer]

Hi,

Where is your observatory located? I am not asking for the street address and number BTW, close enough is good :)

Cheers,
John B

[BPO]

Hi John.

It's called Benmore Peak observatory, in the South Island of New Zealand. The site's almost 2,000m ASL, and there's nothing much of anything to speak of for quite some distance in any direction.

[mithrandir]

FYI, Darren says the readings reported on MyDarkSky must be in UTC, so you might want to fix the time on that reading.

Eric, I haven't seem Snake Valley readings appear yet.

Andrew

[ausastronomer]

Well, having a wife who is a New Zealander (we all make mistakes) and having done 30+ trips to NZ in the past 10 years I know how dark it can get in some parts of the South Island of NZ, particularly towards the West Coast. I have never been to Benmore but from memory it is about 100km east of Wanaka? I know how dark it gets South West of Te Anau in Fiordland National Park around Doubtful Sound and Dusky Sound; and further North on the West Coast of the South Island around Westland National Park (Franz Joseph Glacier and Fox Glacier). Allowing for the fact the units can have a 10% (.1 mag) error I would think your reading is accurate as it would be a genuine 21.9 to 21.95 (allowing for unit tolerances). I would think this is about as high a genuine reading as anyone is going to get. This area is certainly as dark and provides as good a sky conditions as anywhere on the planet I have been to. Most importantly, transparency is always very good (when the clouds part) as airborne particulate is usually low. Some places in OZ are just as dark, but dust particulate often knocks the sky conditions back a notch. Unfortunately I don't own any "toy" telescopes to take with me when i go to NZ, so I am stuck with my 16 x 60 binos. I think your reading is right. Some of the high readings from places I know aren't nearly as dark as your area, I take with a grain of salt.

Cheers,
John B

[BPO]

Hi John.

My site is 80km ENE of Wanaka, in the Mackenzie Basin high country. The climate is quite arid by NZ standards, far more so than that of the west coast. Precipitation in the area is around 200-300 mm per year. It is drier at my site, which is 1,500m above the surrounding terrain.

http://www.observatory.org.nz/about.html

http://www.observatory.org.nz/temp/pan1.jpg (4.16 MB)

http://www.observatory.org.nz/temp/pan2.jpg (1.80 MB)

http://www.observatory.org.nz/temp/Pan_max.jpg (5.22 MB)

[erick]

Anthony said I could share his email:-

"The SQM has a method for not showing a display if the battery is too low, but there is a possibility that a fluctuating battery voltage, or dying
battery will present different voltages during the measurement cycle.
Generally this has not been a problem. Another way to check the battery
voltage is to get the unit temperature by pressing once, then again and
holding the pushbutton. If the temperature shown is bad, then the battery is probably on the verge of its life.

Also, we have found that not all batteries have the same terminal dimensions, and this could cause a loose or poor connection. This can be
corrected by lightly squeezing the flower shaped connector on the SQM
battery lead with pliers then re-attaching the battery.

Also, Alkaline batteries do not do well in very cold conditions.

The unit does not have a fixed high value because of the nature of the
sensor. In some cases a dark room might yield 24, or just time out.

Oddly, there are places where cloud cover results in darker skies because of the great distance from light pollution. Such places are the desert, the ocean, and the hilltop at Cherry Springs PA. In these cases, not even the light from the sky reaches the meter, and an apparently abnormal reading is obtained.

I hope that helps to answer your questions.

Best regards,

Anthony Tekatch
Unihedron"

[ArcturusMDS]

Erick,

I also emailed Anthony and got the same response.

We're trying to do some testing with a dozen or so devices we have. So far we not had a problem, maybe we've not got a battery bad enough just yet. we'll keep testing in various conditions and let you know what we find. we do also regularly calibrate the devices against each other and have yet to find any device inaccurate. we'll keep testing for now.

[ausastronomer]

Guys,

Please allow my single digit IQ brain to analyse this logically. I indicated earlier that 22.0 was the theoretical maximum and I felt 21.9 was the "practical" maximum because of skyglow and starlight. I did forget to adjust this for the fact there are bugger all stars visible in the Northern hemisphere to create starlite glow :) I did neglect to allow for the tolerance error of the unit which is stated at + or -10%, or .1 mag. Lets also assume some units could be a "touch" out of calibration, or have a slightly greater tolerance error, so lets allow a tolerance of .2 mag. This takes the practical maximum to 22.1.

Attached is a PDF file extracted from the Unihedron database on their website of all readings > 22.1. There are 8 of them. What I find truly amazing is that the highest 5 readings come from 4 different dark sites in CONUS. ONE OF THE MOST LIGHT POLLUTED CONTINENTS ON THE PLANET. It would be a tie between North eastern CONUS and Western Europe as to which area is the most light polluted area on the planet. Here is a link to the world light pollution map

http://www.lightpollution.it/download/mondo_ridotto0p25.gif

The top 2 readings come from Potter County Pennsylvania which just so happens to be smack bang right in the middle of that really big patch of light pollution in the middle of North East CONUS. I have no doubt the skies there are reasonable (they hold a Star Party there), but darker than anywhere else in the world where people have taken readings from over the past 5 or more years? It's also worth noting that these eight readings >22.1 are all taken in 2005, 2006 and 2007 which in all likelihood would indicate they were taken with one of the older units.

I note the highest reading from Australia is 22.07 from outside Mullawa in WA, which is about 100km West of Geraldton and 500km North of Perth. I have no doubt it's right. There are 3 men and a dog per 1,000 square km out there. But not close to the equal of several readings in CONUS? I note the highest reading from New Zealand is 22.02 from Benmore Peak Observatory. Again I have no doubt it's correct as indicated in one of my earlier posts. But again not close to the equal of several readings in CONUS? I suggest some of you revisit that light pollution map.

You guys believe and think what you wish, that's your choice. But, if you don't believe me, please email Anthony and ask him to include that 23.45 reading of Andrew Murrell's taken at about 11:45pm on 12/10/2007 at Coonababran NSW

Cheers,
John B

[ArcturusMDS]

John,

I didn't say I don't believe you. The fact that i'm looking into the issue and testing the units we have suggests I'm open to the fact you could be right.

[ausastronomer]

Hi Darren,

I think looking at the Unihedron Database in all likelihood the problem could be isolated to the older type units. You may not find any issues with the newer units. I think it significant those 8 readings I mentioned would have all been taken with the older units, which is what Andrew and I have. I have repeated the high 22's readings on at least 4 different occasions (not counting Andrew's 23.45) from skies I know aren't 22+ skies, only to find the reading drops by at almost 1 mag when I put a fresh battery in.

Cheers,
John B

[ArcturusMDS]

Currently in our Database I think we have 3 reading over 22. The Benmore Park 22.01 reading and a reading of 23.1 in a very dark region of Scotland but with full cloud. The interesting one is the reading of 22.72 in Galloway Forest Park which is designated a dark sky park. The reading is quite a bit higher than other readings taken in the same area. The reading was taken with an SQM-L. My contact in Galloway Forest Park is investigating.

Those light pollution maps do make things look terrible here and in many places the light pollution is terrible. But it's not all bad, there are regions with little or no light pollution at all. Just not as big as your regions.

[BPO]

Perhaps I should add that the 22.02 reading was not the "best" or "worst" from my site...it was basically the only proper one I took, aside from a few tests, before putting the meter back in storage, where it's remained ever since.

[timokarhula]

Hi all,

It was me who submitted the SQM-L 22.07 reading from outside Mullewa, W.A. I visited a remote farm together with Finnish amateur astronomers in December 2009 to do some really dark DeepSky-observations. They had also a SQM which showed 22.09 as best. In a series of three readings I got 22.01, 22.02 and 22.07. That was when I pointed to a relatively "blank" region of the sky, not the Milky Way or the zodiacal light. From here we observed the Light bridge between LMC and the Milky way. See my thread

http://www.iceinspace.com.au/forum/showthread.php?t=68088

From my regular observing site here in Sweden, the SQM-L has shown 21.48 at most and my naked eye limiting magnitude was 7.4. My Australian site is way darker than that!

/Timo Karhula

[erick]

"Pony Club" - Mangrove Mountain. 11:30pm last night - 21.1

[ArcturusMDS]

Erick, Add Pony Club to the mydarksky website if you can.

Are you guys still enjoying long dark nights at the moment? Were still stuck in Astronomical Twilight here. Although I did get my first glimps of the milkyway this summer a few nights ago.

[erick]

Done Darren, I was going to get to it soon.

[ngcles]

Hi All,

Unusually good night out at my "nearly dark but reasonably convenient" site in the southern highlands (nr Bargo). There is a significant difference between Saturday and Sunday nights (this was a Sunday) I think.

Tonight at 1am I recorded 21.38, 21.38, 21.40 (I take three readings and average -- so 21.39) with my SQM-L pointed at zenith (nr Grus, Microscopium) and 5 degC. This equates to a ZLM of 6.33 (rounded).

This is the best reading I've had there in a couple of years. On Sundays there there is considerably fewer lights on in Sydney (like sports grounds, car-parks etc) and I think this makes a substantial difference. I saw 4 Vesta very easily -- direct vision at mag 5.7 and 40 deg elevation in Capricornus earlier at about 11pm. Could also see with direct vision nearby 27 Capricorni at mag 6.26 when closer to Zenith with no difficulty. The light-dome from Sydney in the northeast was somewhat lower than normal and less bright and the Wollongong light-dome was hardly worth speaking of.


Best,

Les D

[ArcturusMDS]

Thanks Erick. It's good to see new Data entered. It's very quiet over here at the moment. Hopefully is should pick up again in 3 or 4 weeks.

[ArcturusMDS]

Any chance you could register on the website and add your results?

I'm guessing the dark nights are getting shorter down under now as the nights are getting longer here. A few more reading from your guys would be nice before the window closes.

We've been stuck with Astronomical Twilight for a couple of months but things are moving again now.

[erick]

Tuesday 22 May, 2012, around midnight, about 1 deg C

Yerrinbool NSW - 20.85

[koputai]

Hey Eric, have you taken a reading at Ingleside yet?

Cheers,
Jason.

[cfranks]

If it stops raining sometime soon, I'll try to get my SQM-LE working and get a reading for Tungkillo, SA.

Charles

[BPO]

Mind the Milky Way.

[erick]

Yes, I did point it away from Sagittarius ;)

Ingleside, Jason? OK, next week sometime when the clouds go away - if I can avoid the waxing Moon. Don't get your hopes up :prey: It's probably in the 19s or 18s?
I only get the telescope out here to show people the Moon, Jupiter, Saturn and maybe some bright stars and doubles. I can glimpse the bright galaxies, but most people cannot.

[koputai]

Yeah, I don't expect it will be great, but it's what we have to live with around here. It will be interesting to see.

Cheers,
Jason.

[erick]

Well, that was surprising. First surprise was that the clouds cleared!

Second is that at 8pm, reading is 20.05! It'll rise a bit through the night as the lights reduce a bit in the city.

[koputai]

That's the good thing about the Northern Beaches Eric, it usually pretty ordinary early on, but as the lights go out it gets pretty good considering the proximity of the city. South is always bad, but North and East are pretty good by 11pm to midnight.

You don't have to go far for real improvement either, on Pittwater or around at Church Point, after 10 or 11pm it's very nice indeed.

Cheers,
Jason.

[cfranks]

I finally got some readings last night but really don't know what to make of them. At least compared to the numbers that have been listed in this thread. My readings started with MPSAS 18.6 and climbed to 20.6 some hours later. My problem is that it was 100% thick cloud, so thick there was no glow to be seen from the moon. SkippySky showed the whole of SA to be clear so probably the SQM (Unihedron SQM-LE) was looking at that! Is there a calibration routine I should have run? I read through the Help file but may have missed something.

Charles

[mithrandir]

Charles, the darkest you could possibly get (other down a mine with the lights off) would be a new moon night, far from town, under total cloud. The limit for SQMs is supposed to be about 22. I get high 21s at Willow Tree pointed NE away from the Milky Way.

I know you said you have an SQM-LE, but getting more than 22 with an SQM-L means you should try again with a new battery.

Andrew

[cfranks]

Thanks for puuting me on the correct path Andrew, I was not sure what the readings meant and thought the higher number meant a more cloudless sky. It is currently at 18.4, cloudless but wet with a half moon so that is about all I can expect. I'm not sure if mine has a battery, I use an external power source.

Regards
Charles

[mithrandir]

Charles,

As it uses external power yours doesn't have the low battery issue. The limit of about 22 still stands. I can get high 18s on moonless nights from my back yard in NW Sydney.

Andrew

[BPO]

Hi. At a truly dark site during new moon you'll approach 22*, so long as you aim the SQM at a "dark" part of the sky, but under overcast skies you'll easily exceed 22.

On Unihedron's site there are readings in the 23+ range from places such as Cherry Springs. Those were taken under heavy cloud cover. Had it been clear the readings would have been much reduced, probably around upper 20 to 21ish.

*Reading from my dark mountain site have slightly exceeded 22, but I remain unconvinced, although it's been claimed that SQMs in deepest darkest Namibia and the Aussie outback have also handily beat the 22 limit. Anthony Trekatch at Unihedron told me that he suspects low 22 readings (eg 22.0 to 22.05) from such sites may be accurate.

[ausastronomer]

That sounds 100% accurate to me and consistent with my experience. Any readings over very low 22s would indicate to me the batteries are depleted. I have seen mine and Andrew Murrell's unit read 1.5 lower when we put fresh batteries in and it was obvious the intitial readings were inflated.

I have achieved readings of just under 22.0 from very dark skies and would expect readings up to 22.1 or thereabouts, from the darkest places on the planet. The 23 + readings from the USA, which is the most light polluted continent on the planet, are clearly in error IMO.

Cheers,
John B

Cheers,
John B

[MrB]

I'm assuming these overcast readings are from extremely dark sites where no light pollution exists to illuminate the underside of the clouds?

[BPO]

Yes, clouds above a dark site cause very high readings.

Away from lights in the mountains, deserts, mid ocean, etc, during new moon and under clear skies will result in readings no more than the very low 22 mark, according to Anthony Tekatch and experience.

Bright stars and the MW passing through the FoV of the SQM-L(E/U) models will cause lower readings, even at truly dark sites during new moon, so best to aim at something like the SCP for consistent numbers.

[mithrandir]

Up at the farm I got a difference of around .75 between the MW around Crux and the sky to the north west. The best I've seen up there was high 21s (need to get rid of traffic on the New England Hwy), and in the morning it was high 18s in the west between astronomical twilight and nautical twilight. 18.5 is about as good as it gets at home.

SCP would be a poor choice here - that's the direction of maximum skyglow.

[BPO]

Yes, even relatively faint natural light sources certainly make a difference, so the aim is to find an area in the sky as devoid of light as possible and always try to take readings from that point in order to collect consistent and meaningful figures.

Obstructions are another hazard. Anthony suspected that trees may have been the culprit in the case of the very high readings obtained at the heavily wooded Cherry Springs Park.

[MrB]

A couple of readings, directed away from MW, no moon, no cloud with brand new SQM-L and a fresh battery:

Home(Rockingham WA) : 20.10
Dark Sky(Nannup WA): 21.65

I got one single reading of 21.74 at Nannup, but consistent readings of 21.64 and 21.65.

[Paul Haese]

Did a couple of readings from Clayton and at home.

Home came up at 19.5 (which surprised me).
Clayton came up at 21.6 on two occasions and 21.5 on another. On a cloudy night at Clayton I have to use a torch as walking down the drive can lead to walking into trees.

Readings taken with an SQM (L)

[mithrandir]

I just got 18.88 and 18.96 (SQM-L) at home with no moon, even with the verandah light on but out of the direct line of sight. Guess it's time to check the battery.

[MrB]

Had a few nights of good transparency this week and got consistent readings of 20.75 from Singleton WA. Previous best from the same location was 20.6

[erick]

Haifa, Israel - 17.1

Yikes! Can see only a handful of stars naked eye!

[Paul Haese]

Last new moon it was 21.86, then 21.96, then 22.03. Dark as. Hope your missing that Erick.:)

[erick]

Yes I am, but wait until I get down to the Negev desert :thumbsup:

[jamespierce]

Little Desert Nature Lodge (near Nhill in Victoria) - 21.82 (once Venus set)

[PeterHA]

On Tuesday night we had 20.6 with SQM (not the L version), around 10:30 PM at the Johns Hill Reserve, Ridge Rd. Kallista in Melbourne east. Venus was still up.
Only two scopes there a 12" Dobson and a 4" Apo, was a good night.

[erick]

30 minutes from the outskirts of Haifa, but still close to various villages - 19.8 It's the "darkest" spot on the light pollution map close to Haifa.

[saviofong]

My experience in Tibet, never reach 22th mag, at most 21.5 or so. Even at 5100m in the middle of nowhere, the closest real cities is 1000Km away, no light or air pollution, sky is crystal clear, at daytime you can see snow mountain >600Km away, or set of moon age 28th (Sun 30 degree above) by naked eye. I found the air is so thin there, the star light is brighter as not as attenuated as on sealevel, so interfere the SQM reading. Yes, at downtown of capital city Lhasa, I got ready approaching 20th Mag.