sqm readings

[scarp15]

It would be interesting to gauge a reading up in Sutherland and Caithness, though perhaps in the autumn as that far North will be starting to gain in too much light by mid May.

[jetstream]

It is amazing to see how much light is reflected by certain clouds and the impact it has on sky darkness.I can lose a magnitude this way at my dark site closest to my small town(13 miles).With these same clouds I can easily see the towns light dome 30 miles away,from a 21.7 mag sky.I can only imagine how far city light would go under the same circumstances.

[faulksy]

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.

that will be dobfest 2 then

[jetstream]

that will be dobfest 2 then

Did you guys dobfest 1 yet?how did it go?

[Beulah]

that will be dobfest 2 then

That'll be me begging for an invite then.. 

[Wyvern]

Regarding the "highest reading" of 22.77 achieved at the Elan Valley.  I have not  stated that this was obtained under cloudy skies. As far as the International DarkSky Association are concerned, it is totally against their guidelines -

Quote "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".

This "high" reading was obtained by one of the Elan Rangers working under the above guidelines, some time in Jan-Feb of this year before we took on the DarkSky application process. Readings at other sites taken, (I assume), at the same time - same hand and pen on sheets - were also slightly higher. Maybe, just maybe someone was out there on one of those jaw dropping, crystal clear moments of true clarity that we all wish for.

All readings will be submitted to the IDA showing the highest, lowest and average reading at each of the 12 sites that we are monitoring across the Elan landscape. - in line with their guidelines!

Regards to all,

Les

[acey]

As I mentioned in my earlier post, the manufacturers Unihedron state that they don't think a reading higher than 22 is possible, so the Elan Valley measurement of 22.7 sounds wholly implausible to me. It's nearly a whole magnitude darker than the observatories at Mauna Kea, Paranal etc. If the same observer was making consistently high readings then it suggests the device was being used improperly (e.g. he/she was not pointing directly at zenith or was inadvertently shading part of the instrument with his/her body) or that the sky was hazy and not truly clear. A "jaw dropping, crystal clear moment of true clarity" can be discounted since clear-sky darkness is limited by natural airglow. To get a reading far higher than 22 you'd need to switch off not only all the man-made light pollution but also most of the 578nm OI emission in the atmosphere (not to mention integrated background starlight).

If these figures are being used as part of the dark-sky validation process then it emphasises the need for proper rigour and control in the process. But rather than argue the toss over a past measurement, a better strategy would be to get a fresh set of measurements that can be considered more reliable.