thanks go to Tony Cook for taking the time and trouble to make readings during
his holiday and for compiling this report.
researched and compiled by Tony Cook, Vice President of the Leeds
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)
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.
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
(UT) Bmpsas VarianceNELM
1. 31/03/2008 22:30
Sky conditions. High complete cloud cover – visibly reflecting
light from Barnstable/Okehampton.
01/04/2008 23:55 21.61
Sky conditions. Very low complete cloud cover, bordering on ground
mist. Suppressed nearly all ground sourced light.
3. 02/04/2008 23:50
Sky conditions. Clear skies horizon to horizon, hint of mist at ground
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
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.
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