Our thanks go to Tony Cook for taking the time and trouble to make readings during his holiday and for compiling this report.

Report researched and compiled by Tony Cook, Vice President of the Leeds Astronomical Society.

Equipment used: Unihedron “Sky Quality Meter” – see http://unihedron.com/projects/darksky/ - The meter belongs to the society.

This meter measures sky brightness by counting a set number of captured photons and timing the interval required to reach this number. This is then correlated to the sky brightness in units of “brightness magnitude per square arc second” (Bmpsas - a system used by professional astronomers). The meter produces repeatable results often varying only by +/-0.01 units when the sky conditions are steady. The meter is precalibrated by the manufacturer and independent analysis suggests the meters are accurate to better than +/-0.05 Bmpsas (http://unihedron.com/projects/darksky/sqmreport_v1p4.pdf)

“magnitude per square arc second” is a measure whereby the sky is divided up into “square” areas each side being 1 arc second in separation. If only one star of magnitude M is placed at the center of each of the areas then the whole sky brightness is said to be “M magnitudes per square arc second”. This system is not familiar to most amateur astronomers so it is useful to convert this to the more familiar Naked Eye Limiting Magnitude (NELM). The equation I have chosen for this report is:

NELM=7.93 - 5 * log(10 ^ (4.316 - (Bmpsas / 5)) + 1)

(see http://members.csolutions.net/fisherka/astronote/plan/tlmnelm/html/NELM2BCalc.html )

Method: The meter is pointed at the zenith. Five readings are taken and the average value calculated. The variance is also noted. The sky conditions are described.

A range of conditions are selected – complete cloud cover (to represent possible worse case conditions if the clouds scatter ground sourced light) and clear dark skies to represent near ideal conditions

When the sky is completely cloud covered, sky brightness is also affected by cloud altitude. Clouds act as diffusing reflectors (they are collections of very small water droplets) and thus altitude determines the limiting distance over which ground sourced light is reflected/scattered to the measurement site. It was not possible to measure cloud altitude other than qualifying that measurements were taken under high or low cloud.

Site characteristics:

Kimworthy is located in the centre of a large expanse of farmland in north Devon to the east of the A39 and about 7 miles from the sea. The town of Bideford and larger town of Barnstable are 12 and 22 miles away to the north east and Okehampton is 23 miles to the south east. The small coastal town of Bude is 7.5 miles to the south west and the market town of Holsworthy is 6 miles to the south. There are a scattering of small villages, homes and farms within a 5 mile radius. (distances approximate)

Under dry clear sky conditions (no mist in the air) the site is superbly dark. The transition between the hedges and sky at the edge of the observing field can not be made out under the best conditions with dark adapted eyes.

Results:

DateTime (UT) Bmpsas VarianceNELM

1. 31/03/2008 22:30 20.86 +/-0.01 6.04
Sky conditions. High complete cloud cover – visibly reflecting light from Barnstable/Okehampton.

2. 01/04/2008 23:55 21.61 +/-0.01 6.44
Sky conditions. Very low complete cloud cover, bordering on ground mist. Suppressed nearly all ground sourced light.
Very dark.

3. 02/04/2008 23:50 21.61 +/-0.01 6.44
Sky conditions. Clear skies horizon to horizon, hint of mist at ground level.

Conclusions:

Measurement 1 represents near worst conditions. Any presence of high cloud allows light from the remoter towns to be readily scattered across the sky. However a reading of 6.04 NELM is better than the clear sky readings in the vast majority of locations in mainland England!
For comparison the sky brightness in North Yorkshire at the best sites outside the Dales National Park (those with balanced clear sky frequency and darkest skies. E.g. near Kirby Malzeard) ) read at no better than 6.00 NELM.

Measurement 2 was under extremely low cloud and can be interpreted as conditions that suppress the scattering of ground sourced light from the remoter towns. Any light reaching the meter has most likely come from the nearest sources, local homes, farms and villages and most probably represents the near upper limit in darkness at this particular site – 6.44 NELM.

Measurement 3 was representative of perfect astronomical conditions and had a reading identical to the light suppressing low cloud conditions (measurement 2). A reading of 6.44 NELM is very impressive and represents nearly the best you might achieve in England. Only excursions to the centre of the Kielder Forest in Northumbria are likely to be better!

It should also be noted that the Milky Way is detectable by the Sky Quality Meter and zenith readings in August/September are limited by the light from the Milky Way band. However in March/April the Milky Way is near the horizon across the northern sweep of the sky and thus has least impact of sky brightness measurement at this time of year.


Kind regards, Tony Cook