Sky Quality Meter (SQM) readings

[koputai (Jason)]

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

Cheers,
Jason.

[cfranks (Charles)]

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 (Eric)]

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 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 (Jason)]

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 (Eric)]

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 (Jason)]

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 (Charles)]

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 (Andrew)]

Originally Posted by 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, 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 (Charles)]

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 (Andrew)]

Originally Posted by 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.

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]

Originally Posted by mithrandir

[T]he 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.

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 (John Bambury)]

Originally Posted by 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.

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 (Simon)]

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 (Andrew)]

Originally Posted by BPO

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.

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]

Originally Posted by 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.

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 (Simon)]

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 (Andrew)]

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.