SQM - Sky Quality Meter

erick (Eric) · #1 14-01-2011, 10:07 AM

I have an SQM-L on its way to me.

I would be interested in comments/advice on using it based on your experience. I cannot locate many reviews, just one interesting thread on Cloudy Nights. And the long SQM Data thread I haven't read.

Should I just read all these (long?) threads?

gary · #2 14-01-2011, 01:23 PM

Hi Eric,

Happy New Year.

Some of the guys here routinely take them up to Coona and out to Ilford.
As you can appreciate, you point them up, they measure the amount of
light and they give a number. When we have compared one site from another
or the same site with two different meters or the same site measured in
a few places on the sky, one of the things missing is any sense of the
error bars.

The guys have the original versions of the unit and the readings they provide
some argue are flawed.

As many of us from the Southern Hemisphere will testify to when visiting the Northern
Hemisphere, one of the first things we say when we look at dark northern
skies is, "Where are all your stars?".

Down here at the southern latitudes you and I enjoy, the Milky Way passes
overhead and the very bright core of the Milky Way passes overhead at the
zenith. By comparison, at latitudes such as the south of Texas, Scorpius
and Sagittarius come up over the horizon in the south and only reach an
elevation of about 30 degrees. When you look north, there are large conspicuous
areas of black and it is little wonder birds manage to navigate by Polaris at it
sits in a comparative sea of nothing.

So chances are when one is in the Northern Hemisphere, when you point the
device up, there is a larger possibility of getting a reading from a dark patch
of sky. However here, all those damn stars, bright nebulae, globulars and the
Gegenschein and the like seem to get in the way and wreck the reading, so if
one does not take care, you might get a number from your favourite dark sky
site that a guy in Hong Kong can better from his apartment block balcony.

So one has to be careful when taking a measurement to compare apples to
apples as from what I understand, they don't differentiate between light pollution
and starlight.

The newer model with a dash L (-L) suffix apparently has a narrower
detection cone, which may be better suited for these latitudes.

erick (Eric) · #3 14-01-2011, 01:40 PM

Thanks Gary

Yes, I have read that pointing it at the Milky Way at the Zenith will give a brighter reading. I also read that you should try and not capture a bright star in your measurement. These all make sense but I'll have to experiment.

I'm also interested in comments I have read, that a brighter reading than expected may be high wispy cloud that is not immediately obvious, or maybe a high vapour content in the air.

My reading suggests that the SQM-L was developed because the SQM does capture a greater angular part of the sky and horizon light domes or nearby objects (buildings) higher than 30 deg, upset the supposedly zenith reading. I am fascinated that both of these are such a common problem for their main sales market!

mithrandir (Andrew) · #4 14-01-2011, 02:03 PM

Eric,

I took my SQM-L up to a place several Km outside Willow Tree. There were no street or house lights, a bit of traffic on the Kamilaroi Highway, but none directed to where I was observing.

On a perfectly clear night with Crux just west of the meridian the reading from that piece of sky was about 21.6

To the northwest towards Quirindi was about 22.5.

Depending on the amount of moisture in the air - it does not have to be cloudy to increase the skyglow, just humid - I can get a bit beyond 20 at the local pony club in the northwest of Sydney.

Andrew

gary · #5 14-01-2011, 02:15 PM

Hi Eric,

One of the things that amuses me now and then is when you read a report and the
author says, "it was so dark that you could not see your hand in front of your face."

To me, that probably indicates that it was a comparatively poor dark sky
observing site.

As countless Australian observers know from visiting very remote desert
and semi-desert regions, where the nearest town is, perhaps, hundreds of
km away and has a population of maybe 16 and where there is absolutely no
chance of light pollution, the dark areas of the sky are never completely black.

I've seen the same thing when visiting remote desert regions in places such
as Africa, including in places where you come across indigenous tribes people
wandering the deserts naked and still carrying nothing more technologically
advanced than a spear, and yet when you are under the skies at night, the
dark areas of the sky are still not totally dark.

There are many reasons for this, including phenomena such as Zodiacal Light
(sunlight scattering from the zodiacal dust cloud in space) and a phenomena few
enthusiasts take into consideration which is airglow, caused by high energy
particles hitting the tenuous upper atmosphere and causing ionization and the
release of photons in the process.

So often a better metric of an observing sky site is where some of these more
subtle phenomena and others such as the Gegenschein are discernible.

Computer graphics these days is a multi-billion dollar industry with demand
coming from Hollywood, games designers, flight simulator designers
and so on. Being able to very accurately model the light from the night sky
is an area of active research for some in the graphics business and academia.

One of the better papers on the topic I have come across appeared in the
SIGGRAPH proceedings by Jensen et. al. entitled "A Physically-Based
Night Sky Model". The authors come from Stanford, MIT and the one of the
big homes of computer graphics, the University of Utah.

The paper makes for excellent reading and is available online here -
http://graphics.ucsd.edu/~henrik/pap...y/nightsky.pdf

It helps gives some insight into the myth that a good dark sky site is, well, totally dark.

erick (Eric) · #6 14-01-2011, 02:30 PM

I am driven to this by my recent 10 nights at a site over 100km north of Broken Hill. There was no artificial light on the full 360 deg of horizon. A couple of people were on site with just a handful of weak white lights, none in direct view. I had several hours of dark adaption. Any cloud that came over - on a couple of nights - was pure black to my sight.

But, as dark as it was, it seemed "bright" to me - I could fairly easily make my way around the site - and I was not getting the contrast I had hoped for in my views of galaxies. I also could not pick out the "Dark Doodad" in Musca in my binoculars as clearly as I have done at Snake Valley. When I asked the property manager, he felt it was darker in the winter months - so I am planning to visit again mid year.

I felt I could see a faint shadow being cast by my hand held to the sky (no Moon nor Jupiter, nor Venus). But, for the first time ever, I noted that, when I rubbed my hand across the plastic table top, I could see sheets of static discharge at the edge of my hand - quite a sight! But perhaps that was the ultra dry air conditions (rel humidity below 20%, temps in the high 20s overnight!)

So, I need an objective measure of "how dark".

(BTW, a fuller report on my "bush trip" is to come.)

erick (Eric) · #7 14-01-2011, 02:31 PM

Quote:

Originally Posted by mithrandir

Eric,

....

Depending on the amount of moisture in the air - it does not have to be cloudy to increase the skyglow, just humid - I can get a bit beyond 20 at the local pony club in the northwest of Sydney.

Andrew

Andrew, so the reading drops with increased humidity? Records the sky as brighter?

mithrandir (Andrew) · #8 14-01-2011, 02:52 PM

Quote:

Originally Posted by erick

Andrew, so the reading drops with increased humidity? Records the sky as brighter?

Yes, more humidity -> brighter sky.

erick (Eric) · #9 17-01-2011, 01:08 PM

Quote:

Originally Posted by gary

The paper makes for excellent reading and is available online here -
http://graphics.ucsd.edu/~henrik/papers/nightsky/nightsky.pdf

It helps gives some insight into the myth that a good dark sky site is, well, totally dark.

Thanks Gary, that was interesting reading.

Paul Haese · #10 20-01-2011, 10:49 AM

Used one of the SQM versions on a few occassions. Reading depend on where in the sky you point the SQM and what time of night and part of the year. The units give a fairly broad reading only really. I reckon you made the right choice with the more accurate SQM-L.