Sky Quality Meter (SQM) readings

erick (Eric) · #1 13-02-2011, 11:04 AM

Suburban Bacchus Marsh, Victoria

3:30am 13 Feb 2011

SQM-L - 20.55 magnitudes per square arcsecond

koputai (Jason) · #2 13-02-2011, 11:15 AM

Hi Eric,

What sort of meter were you using?

Cheers,
Jason.

Robh (Rob) · #3 13-02-2011, 11:37 AM

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 · #4 13-02-2011, 11:40 AM

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 (Eric) · #5 13-02-2011, 11:41 AM

Quote:

Originally Posted by koputai

Hi Eric,

What sort of meter were you using?

Cheers,
Jason.

Unihedron SQM-L

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

JohnG (John) · #6 13-02-2011, 11:56 AM

My backyard: 21.4 - 21.6.

Snake Valley I normally get around 21.65.

Cheers

mithrandir (Andrew) · #7 13-02-2011, 12:14 PM

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 (Kev) · #8 13-02-2011, 12:39 PM

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

Cheers Kev.

Robh (Rob) · #9 13-02-2011, 12:54 PM

Quote:

Originally Posted by mithrandir

Willow Tree (just off the Kamilaroi Hwy) - 20.5 from the Milky Way. 21.5 elsewhere.

Andrew

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 (Eric) · #10 13-02-2011, 05:16 PM

Quote:

Originally Posted by Kevnool

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

Cheers Kev.

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 (Kev) · #11 13-02-2011, 06:03 PM

Quote:

Originally Posted by 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.

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

.

Cheers Kev.

jamespierce (James) · #12 14-02-2011, 09:15 PM

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 (Rob) · #13 15-02-2011, 02:41 PM

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 (Darren) · #14 03-03-2011, 12:46 AM

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

erick (Eric) · #15 03-03-2011, 07:45 AM

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 (Darren) · #16 03-03-2011, 08:07 AM

Quote:

Originally Posted by 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!

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 (Andrew) · #17 03-03-2011, 09:02 AM

Quote:

Originally Posted by 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.

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 (Darren) · #18 03-03-2011, 09:10 AM

Quote:

Originally Posted by 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

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 (Eric) · #19 03-03-2011, 10:03 AM

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 (Andrew) · #20 03-03-2011, 06:27 PM

Quote:

Originally Posted by erick

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

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.