Sky Quality Meter (SQM) readings

[ArcturusMDS (Darren)]

Originally Posted by erick

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

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

[ArcturusMDS (Darren)]

Originally Posted by erick

Suburban Bacchus Marsh, Victoria

3:30am 13 Feb 2011

SQM-L - 20.55 magnitudes per square arcsecond

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 (Eric)]

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 (Darren)]

Originally Posted by 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.

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 (Eric)]

Originally Posted by ArcturusMDS

You've got 22.02 to beat!

Ok, I'll try.

[ArcturusMDS (Darren)]

Originally Posted by erick

Ok, I'll try.

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

[erick (Eric)]

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 (James)]

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

[ausastronomer (John Bambury)]

Originally Posted by ArcturusMDS

Has anybody got 22.72 beat yet. Best reading we've had so far in the UK.

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 (Eric)]

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

[ArcturusMDS (Darren)]

Originally Posted by 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

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

Originally Posted by erick

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

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

[jamespierce (James)]

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

[ausastronomer (John Bambury)]

Originally Posted by 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

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 (Eric)]

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 (Eric)]

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

[MrB (Simon)]

Originally Posted by ausastronomer

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.

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 (Eric)]

Originally Posted by 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.

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 (John Bambury)]

Originally Posted by erick

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?[/FONT]

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