sqm readings

[faulksy]

last night my self dan and barry went to my local darksite. very good place well into the mountains with no visible light domes. i have been around quite a lot of wales over the years and this is the best place i no of localy. 40 min drive. there was a fella there last night who worked for the national trust and he was taking sqm readings, he recorded 21.6 from were we were observing which is great, but i dont believe the readings. there is lots visible naked eye and spiral arms are visible easy in the lies of m51 e.t.c. but the sky isnt black, more like a dark grey. i have been down to the elan valley area 3 times and the sky is black like velvet, steve swampthing told me about this site and he can vouch for it, so im wondering now was hhe taking the meter readings wrong, was he doing it right and the skys are that good, if thats the case then elan valley must read 22 or more. whats peoples thoughts on these meter readings

[jetstream]

Lots of factors at play Mike,including your eyes-maybe you dark adapt well,I do too.When first out of the vehicle the sky will look jet black,but then turn dark grey as the pupils open for some people.Also sky transparency is a factor,a pretty big one.The SQM's are accurate,but don't indicate the level of extinguishment from suspended dust,aerosols etc in the atmosphere.Even my 21mm Ethos exit pupil of 4.3mm is a bit much for me,lightens the sky.I would say you have good eyes for the dark.Swampthings site may be more transparent than yours...?I SQM and use NELM to determine the atmospheric light absorbtion,etc.http://unihedron.com/projects/darksky/NELM2BCalc.html

[faulksy]

thanks for that gerry, says mag 6. i understand that when you come out of a car your eyes need to adapt so the sky looks black. maybe it is good there. if your getting readings similar and your skys are dark grey, then maybe it was right. cheers gerry

[Wyvern]

Hi Faulksy,

By sheer chance, we were out last night/this morning (19th-20th) getting more SQM readings in the Elan region for the IDS application.

We visited and recorded at 12 locations, and zenith readings ranged from 21.52 to 21.85 - equating to NELM of 6.39 to 6.56

Best Elan reading so far has been 22.77 - equating to NELM of 6.94

Thought might be of interest to you,

Regards,

Les

[faulksy]

Hi Faulksy,

By sheer chance, we were out last night/this morning (19th-20th) getting more SQM readings in the Elan region for the IDS application.

We visited and recorded at 12 locations, and zenith readings ranged from 21.52 to 21.85 - equating to NELM of 6.39 to 6.56

Best Elan reading so far has been 22.77 - equating to NELM of 6.94

Thought might be of interest to you,

Regards,

Les

very much so. thank you les and brilliant findings. i love it down there

[Daniel-K]

whats the highest reading that these can go up to

[laser_jock99]

SQM readings vary considerably from night to night (due to sky conditions) in my experience and also depend on where in the sky you are pointing. For instance if the Milky Way is bright and directly overhead you'll get a lower reading than if you were pointing more in the direction of the Galactic Poles.

Another possible method is to take measurements on a cloudy night as this is quite literally more of a reflection of local light pollution levels (reflected off the underside of the cloud base).

With the Milky Way overhead my darkest reading was 21.85 but with total cloud cover and no moon a deeply dark 22.7 (to replicate this indoors I had to put the SQM inside a drawer, inside my very dimly lit obsy....)

In total darkness you might get a reading of 23?

[jetstream]

Laser,I have noticed the reflection effect from low clouds too-off any bright light source(town,city) which lowers the SQM reading.Here was my sky last night,I'm by the big red "lake" in cloud pic-Lake of the Woods/Rainy Lake

[ollypenrice]

I'm sceptical. My site has an SQM best of 21.9 but I really don't believe that the Elan valley will beat it. We are at 3000 feet, the nearest major light sources are far further away than is possible in the UK, the sky is drier... So I think the SQM is useful but you shouldn't become a slave to it. The only person I know personally who has seen the 22s was an Australian with a site in the outback. One of our regular guests goes to Tivioli in Nabiia each year and records the same zenith readings as we get here and that is with the same metre.

When the Milky Way is high here, and it is very, very dark, the sky is not jet black and once adapted you can walk about quite easily. The Milky Way, in a nutshell, is bright and lights things up. Steve O'Meara describes the effects of Milky Way light reflected up from the ground at his Hawaii site. It obliges him to close his unused eye, which he doesn't normally do.

Olly

[Wyvern]

I totally agree that changing sky conditions will effect your SQM readings from night to night, even just a couple of hours can make a difference as we found out during our last excursion around the Elan. A site we measured first of all displayed a 21.22 reading - NELM  of 6.24, and a second reading some 2 1/2 hrs later as we passed back through on the way home then gave a 21.88 reading, NELM of 6.57. Guess that's why you simply average all the readings out over time to get a "truer" figure .

For interest, I copy the International DarkSkies guidelines for obtaining readings with a SQM device.

Quote - 

"This is perhaps one of the easiest ways to achieve quantities measurements of the darkness of your location’s dark skies. First, you will need a Unihedron Sky Quality Meter. This is available for $119.99 USA dollars plus shipping and handling from Unihedron’s website.

It is simple to use and effective at measuring sky brightness at zenith. Simply hold the device above your head, pointing the photometer at zenith, and click the button. The screen will then display the magnitude of the sky brightness at that point; the higher the number, the darker the sky. Take three to five measurements per location and average the results. Never take a measurement directly underneath a light source or anything that might block the clear sky. Always take measurements with clear open sky, cloudless, to prevent any sky glow measurements that will deter the accuracy of the device. Also, always take measurements on clear, moonless (New Moon) nights. Measurements taken during Full Moon and while the Moon is visible will not reflect the natural darkness of your location."

As to the "high" readings we obtain here, ....... I just point the SQM at zenith, press the button and read the reading.!!

Regards,

Les

[acey]

The manufacturers Unihedron say at their website that they don't think a clear sky reading any higher than 22 is possible if the device is used correctly. They have a collection of readings submitted by users, some of which are darker than 22, but that probably indicates errors which could arise from various causes other than a meter defect, e.g. not holding the meter at arm's length, in which case it might be partly shielded from the sky by the user's body.

The meter is notionally calibrated in V-magnitudes per square arcsecond, but its sensitivity is not exactly the same as the Johnson V-band, so it cannot be taken as exactly equaling the V-band which is used by observatories, where the reported figures are typically close to 22 mag per square arcsecond. Another point to bear in mind is that the Sky Quality Meter averages light from a fairly large area, i.e. it captures some starlight as well as sky brightness, and if the Milky Way is overhead then this can be significant, leading to brighter readings.

Subjective judgements about the blackness or otherwise of the sky are heavily dependent on the state of dark adaptation and other factors. To give an illustration, I'm always struck by the brightness of the night sky (at a dark site) when I see it next to a black edge such as a roof - the part of sky adjacent to the black obstruction looks substantially lighter than the rest of the sky. This is a phenomenon called the Mach effect, and is a property of the human visual system. When my eyes are fully dark adapted and I've been looking through the eyepiece for a long time, I can find the sky dazzling when I look up at it. This is with a sky that is typically in the range 21.3-21.7 SQ.

Limiting magnitude estimates made on the basis of SQ measurements are prone to wide error, since everyone's eyesight is different. Best thing is to find your own limit for a particular reading, calculate the difference between your mag limit and the predicted one, and make that adjustment to all predictions. For example, if SQ 21.5 is supposed to equal limiting mag 6.5 but you can only see to 6.0, then subtract 0.5 mag from the predicted value for any other SQ reading.

Total blackness has a value of infinity - I've never tried my meter out in a blackened room to check but the red display would create some illumination anyway. In practice anything higher than about 25 mag per square arcsec is effectively total darkness as far as human vision is concerned. We can never get a sky as dark as that on Earth because the atmosphere itself emits "airglow" (effectively a very faint auroral light), and this is what sets the limit at around 22. During periods of maximum solar activity, when airglow is at its brightest, the V-band figure at pristine sites can be closer to 21.5.

The sky is always brighter towards the horizon, even in the complete absence of light pollution (because you're looking through more glowing air), so readings are always considered to refer to zenith, which is the darkest part. From my own experience I would say that light domes near the horizon are not a problem unless you happen to want to look at objects situated near (or in) them.

At a pristine site with a perfectly clear sky, readings will vary in the course of a night because of fluctuations in airglow. Fluctuating readings at UK dark sites could be due to a number of other factors too: from experience I would cite the likely main ones as varying light pollution (distant lights coming on or off), varying transparency (thin, unseen cloud or haze affecting readings), and the movement of the stars (since the device is capturing some of the overhead starlight). The device is also sensitive to temperature (including its own internal temperature) which is why I always take several readings at a time, and usually only note it to 1 decimal place, unless I'm getting readings that remain consistent to 2. A further cause of changing readings is of course twilight - readings can only be expected to remain roughly constant during the time of full astronomical darkness.

I observe in Northumberland (not far from Kielder) and take readings at intervals over the course of every observing session. The darkest readings I get are a little over 21.7. I'm in my 50s, and never see stars fainter than mag 6.0.

[ollypenrice]

That's a characteristically informative post, Acey, with several points of which I was unaware. I've often pondered the effect you identify as the Mach effect - the fact that foreground objects can appear far darker than the background sky.

As well as the MW it would also be possible, inadvertently, to stumble across the Gegenshein, I guess.

Olly

[Moonshane]

great posts and really interesting stuff. it does remind me somwhat of a what's best - Televue or Pentax thread though

personally, I just wish I could escape the interference of neighbours' bathroom and kitchen lights once in a while with good weather anyway.

[Mr Spock]

I'd settle for a dark grey sky instead of an orange one

[Mike73]

I totally agree that changing sky conditions will effect your SQM readings from night to night, even just a couple of hours can make a difference as we found out during our last excursion around the Elan. A site we measured first of all displayed a 21.22 reading - NELM  of 6.24, and a second reading some 2 1/2 hrs later as we passed back through on the way home then gave a 21.88 reading, NELM of 6.57. Guess that's why you simply average all the readings out over time to get a "truer" figure .

For interest, I copy the International DarkSkies guidelines for obtaining readings with a SQM device.

Quote - 

"This is perhaps one of the easiest ways to achieve quantities measurements of the darkness of your location’s dark skies. First, you will need a Unihedron Sky Quality Meter. This is available for $119.99 USA dollars plus shipping and handling from Unihedron’s website.

It is simple to use and effective at measuring sky brightness at zenith. Simply hold the device above your head, pointing the photometer at zenith, and click the button. The screen will then display the magnitude of the sky brightness at that point; the higher the number, the darker the sky. Take three to five measurements per location and average the results. Never take a measurement directly underneath a light source or anything that might block the clear sky. Always take measurements with clear open sky, cloudless, to prevent any sky glow measurements that will deter the accuracy of the device. Also, always take measurements on clear, moonless (New Moon) nights. Measurements taken during Full Moon and while the Moon is visible will not reflect the natural darkness of your location."

As to the "high" readings we obtain here, ....... I just point the SQM at zenith, press the button and read the reading.!!

Regards,

Les

I agree with Olly that I'm a little sceptical of the 22.77 Elan reading, its a long way off your own readings from the other night. I've found that my readings vary by up to 0.3 SQ depending on transparency and the brightness of the MW.

I also take a few readings and disregard the first couple which are always alot higher than when the meter has settled. For me getting a 'accurate' reading is more important than getting a high one.

Oh and making sure the meter is pointing at the sky rather than your feet also helps....But who would do a stupid thing like that eh?! 

[Beulah]

Acey, that is a fantastic post, thank you.

I recently acquired an SQM to use for my job and tend to take a few readings and use an average. For example, I took around four readings in my garden and came up with an average of 21.20 or around 6.2 LM (I hold the SQM aloft like He-Man does...  ).

When the moon's light started to show on the horizon the readings dropped so I knew it was working correctly.

Wyvern, I applaud you and your wife's efforts in championing dark skies in Mid Wales, keep up the great work.  It's no mean feat gallivanting around the hills late at night with job commitments, etc....

[faulksy]

Acey, that is a fantastic post, thank you.

I recently acquired an SQM to use for my job and tend to take a few readings and use an average. For example, I took around four readings in my garden and came up with an average of 21.20 or around 6.2 LM (I hold the SQM aloft like He-Man does...  ). When the moon's light started to show on the horizon the readings dropped so I knew it was working correctly.

Wyvern, I applaud you and your wife's efforts in championing dark skies in Mid Wales, keep up the great work.  It's no mean feat gallivanting around the hills late at night with job commitments, etc....

that is brilliant from your garden, you dont need to go anywere else

[estwing]

Auspom has a story about the outback that'll want you to throw your scope in the bin!!

[Beulah]

Come on, Auspom, spill.

MIke,

I was very surprised. I was guessing around 19.00 due to a horrific light dome in the east. Local light pollution can sometimes be very nasty - I got lucky that night.

But sadly aperture fever has progressed into Darker Sky Acquisition Syndrome... And I don't know how to cure it easily.....

[estwing]

DSAS...deadly!

[laser_jock99]

I agree with Olly that I'm a little sceptical of the 22.77 Elan reading, its a long way off your own readings from the other night. I've found that my readings vary by up to 0.3 SQ depending on transparency and the brightness of the MW.

I also take a few readings and disregard the first couple which are always alot higher than when the meter has settled. For me getting a 'accurate' reading is more important than getting a high one.

Oh and making sure the meter is pointing at the sky rather than your feet also helps....But who would do a stupid thing like that eh?! 

My 22.77 reading was under a cloudy moonless sky - with no light lights illuminating the underside of said clouds! When the sky is clear I 'only' get 21.85 max, to get 22+ readings you need to be somewhere very dark - like in an unlit room at night!

As I said cloud underside readings are a good measure of local light pollution. Light pollution travels a long way, to escape light pollution you need to be 100+ miles away from big cities- I can pick up LP from cities 80 miles to the south of me on high contrails.

A high contrail drifting through Sagitarius- the camera picked it up though it was invisible to the eye. I calculate that at say 20,000 feet it could be easily be illuminated by Swansea & Cardiff.

Air quality/transparency is key in SQM readings I think. It doesn't take much moisture to degrade my readings for sure. Hence the night-to-nigh variability I see.

[Beulah]

Gasp, amazing!! I wish I could give you more 'likes', laser_jock!

The first image shows how much the contrail affect the whole image as it passed through the centre! - that's if each images was processed the same way.

[Mike73]

My 22.77 reading was under a cloudy moonless sky - with no light lights illuminating the underside of said clouds! When the sky is clear I 'only' get 21.85 max, to get 22+ readings you need to be somewhere very dark - like in an unlit room at night!

As I said cloud underside readings are a good measure of local light pollution. Light pollution travels a long way, to escape light pollution you need to be 100+ miles away from big cities- I can pick up LP from cities 80 miles to the south of me on high contrails.

Fair enough but I dont think that a 22.77 reading should be quoted as the best Elan valley reading (not by you btw) when it was taken on a cloudy night.

I'd gladly travel up from Cornwall for 22.7 cloudless skies, I'd be a bit disappointed if when I got there it was nowhere near that dark.

[Beulah]

Cornwall is pretty dayum dark, judging by some of your sketch stats, Mike. Don't need to go anywhere else.  

[Mike73]

Cornwall is pretty dayum dark, judging by some of your sketch stats, Mike. Don't need to go anywhere else.  

I cured my 'aperture fever' a while ago, its the 'dark sky fever' that is kicking in now!

I think Elan has slightly darker skies than Cornwall so I'II be on the road to Wales in the Autumn.