STARS4ALL Night Sky Brightness Phot= ometer
Jaime Zamorano1,*, Cristóbal García2, Rafael González2,
Carlos Tapia1, Alejandro Sánchez de Miguel1, S=
ergio Pascual1,
Jesús Gallego1,
Esteban González3, Pablo Picazo1, Jaime Izquie=
rdo1,
Mireia Nievas1, Lucía García1, Oscar Co=
rcho3, and
The STARS4ALL consortium
[1]<=
/a> 1 Universidad
Complutense de Madrid, 28040 Madrid, Spain
2 Agrupaci&oacu=
te;n Astro
Henares, 28820 Coslada, Madrid, Spain
3 Universidad P=
olitécnica
de Madrid, 28660 Boadilla del Monte, Madrid, Spain
Received 21 November 2016, Accepted 12 Decem=
ber
2016
Abstract
We present the main features of TESS-W, the first version of a series of
inexpensive but reliable photometers that will be used to measure night sky
brightness. The bandpass is extended to the red with respect of that of the=
Sky
Quality Meter (SQM). TESS-W connects to a router via WIFI and it sends
automatically the brightness values to a data repository using Internet of
Things protocols. The device includes an infrared sensor to estimate the cl=
oud
coverage. It is designed for fixed stations to monitor the evolution of the=
sky
brightness. The photometer could also be used in local mode connected to a
computer or tablet to gather data from a moving vehicle. The photometer is
being developed within STARS4ALL project, a collective awareness platform f=
or
promoting dark skies in Europe, funded by the EU. We intend to extend the
existing professional networks to a citizen-based network of photometers.
Keywords: Light pollution, Photometry, Sky brigh=
tness,
Instrumentation
1.
Introduction
Light pollution (the introduction by humans, directly or indirectly=
, of
artificial light into the environment) is a major issue worldwide. One=
of
the effects of the light pollution is the brightening of the night sky. The
increase of the nocturnal sky glow prevents citizens from observing a dark
starry sky. Although the light sources are located mainly in urban are=
as
it is necessary to travel far from these places to find a sky with unpollut=
ed,
natural sky [1], [2].
The professional astronomical observatories are being pushed to rem=
ote
areas as far as possible from the light pollution sources of the urban areas
and efforts are made to prevent the increase of light pollution in the
observatory surroundings. Moreover, the night sky brightness is one of the =
key
parameters of the sky astronomical quality. The measure of the sky brightne=
ss
as a proxy of light pollution has been the task of the astronomers to monit=
or
the increasing light pollution that threatens their astronomical observator=
ies.
The methods of the astronomical photometry, using the stars as stan=
dard
sources, are lengthy and complicated. During the last decade there have bee=
n an
increasing number of astronomers and interested citizens who have been
measuring the night sky brightness using portable photometers. Their
measurements were performed both under dark and bright skies. The Sky Quality Meter (SQM) from
Unihedron is by far the most used [3]. Thousands of measures made with hand=
held
SQM or with computer linked photometers have provided the data needed to bu=
ild
the world atlas of the sky brightness [4].=
STARS4ALL, a collective awareness platform for promoting dark skies=
in
Europe, funded by the EU [5], aims to push citizen science in light polluti=
on.
We are trying to involve citizens within the several light pollution
initiatives (LPIs). One of them is related to development of a European
Photometer Network. Interested citizens will help us in monitoring the light
pollution of European villages with a sensor network. The network will exte=
nd
the current existing professional photometer networks to a citizen-based
network of photometers.
2. N=
ight Sky
brightness photometer
The TESS-W photometer is the first version of a series of cheap but
reliable photometers that will be used to measure night sky brightness. TESS
stands for ¡®telescope encoder and sky sensor¡¯ and has been developed by t=
he
amateur astronomer and expert engineer Cristóbal García as pa=
rt of its remote
observatory system (http://www.observatorioremoto.com/). The TESS-W photome=
ter
uses WiFi communication.
This version has been designed to monitor the sky from fixed places,
but could also be used from a moving vehicle. This device also measures the
ambient and sky temperature to record the cloud coverage. It communicates t=
o a
router via a WIFI module and sends automatically the brightness values to o=
ur
data repository using IoT (Internet of Things) protocols. TESS-W does not n=
eed
an additional enclosure since the box (80x55x35 mm) is weatherproof. The
hardware, the software and the data will be open.
2.1. Hardware
The sky brightness detector is a TSL237 photodiode that convers lig=
ht
to frequency. It is the same sensor used by the SQM photometers. However, t=
he
bandpass is more extended to the red range with the use a dichroic filter. =
The
light from the sky is collected with the optics that includes a dichroic fi=
lter
to select the bandpass (see Figure 1). The filter fully covers the collector
(1); the sensor (not seen in Figure 1) is located on a printed circuit board
along with the custom-made electronics (2); the WiFi module (3) with an ant=
enna
inside the box that extends the WiFi range; a near-infrared sensor (4) is u=
sed to
measure sky temperature; finally, the heater (5) is switched on when needed=
to
get rid of condensation on the window (6). The field of view (FoV) is FWHM=
=3D17
degrees.
(Right) TESS-W photometer field of view (FoV) is similar to a gaussian func=
tion
of FWHM=3D17 degrees.
2.2. Software
The photometer is configured using a web browser to connect via WiF=
i to
a router (SSID, password, name, interval between measures etc.). In normal =
use
the reading of the sensors are received by the MQTT broker and stored. This=
is
the method for monitor the sky brightness at fixed stations. The user can a=
lso
read the data of its device in local mode with a computer or Android device.
This method is useful for measures from a moving vehicle to map the sky
brightness of extended regions. The GPS data is provided by the Android dev=
ice
that is connected to the photometer.
Fig. 2. (Left) Co=
mmunication
schematic. Local mode (Right)
2.3. Spectral response
Although the sensor employed is the same that uses SQM, we use a dich=
roic
filter instead the color filter. The overall response is thus different and
fully includes the lines of the High Pressure Sodium (HPS) lamps since the
spectral response is extended to the red. We show in Figure 3 a comparison =
of
the spectral responses of SQM and TESS-W photometers. Typical night sky
spectrum from Calar Alto observatory [6] and Madrid (UCM observatory) [7] a=
re
shown for reference.
Fig. 3. (1) Spect=
ral
response of the TSL237 sensor. (2) Transmission of the HOYA CM-500 (BG39) u=
sed
by Unihedron
SQM and that of the dichroic filter used in TESS. (3) Combined respon=
se
of filter and sensor. (4) 3000K LED spectrum.
(5) Calar Alto Sky spectrum [6] and (6) Spectral response over spectrum of
Madrid (SAND spectrometer [7])
The IR sensor allow us to estimate the cloud cover at the same time as
the sky brightness is being measured. This is very important to discard
measures of sky brightness affected by the light reflected into the clouds =
and
to get a better statistical estimate of the sky brightness evolution [1],[2=
].
3. F=
ield
tests
The first 12 working units of the TESS photometer have been assembl=
ed
and they are being tested under both bright and dark skies. Most of the
locations are fixed stations of the Spanish Network of Light Pollution Stud=
ies
which provides SQM readings for inter comparison. The absolute calibration =
will
be performed on LICA-UCM optical lab and the astronomical observatory of the
Universidad Complutense de Madrid.
We show in Figure 4 the measures of night sky brightness at zenith
taken from UCM observatory (Madrid) using AstMon astronomical camera, the S=
QM
and TESS photometers. AstMon is an all sky camera that perform astronomical
photometry using standard stars. It provides all sky maps in the Johnson B,=
V
and R photometry bands. Preliminary results of the field test show that TES=
S-W
photometer is around 0.5 magnitudes more sensible than the Unihedron SQM
photometer on polluted skies as Madrid: using a zero point ZP=3D20.38 for T=
ESS
the curves of SQM (ZP=3D19.9) are similar.
The graphs of Figure 5 show the dependence of this offset with (B-V)
and (V-R) colors (first two panels) and the variation of the color of the s=
ky
in Madrid along this night. The color of the sky changes along the night in
Madrid as the bluer ornamental lights are being switched off. The offset
between SQM and TESS depends on the color of the sky. This an expected resu=
lt
since the TESS spectral response is extended to the red to cover the High
Pressure Sodium (HPS) lines (around 590 nm) that dominates the Madrid spect=
rum
(Figure 3, panel 6), while the SQM bandwidth decreases at the wavelength of=
the
HPS lines.
We also tested the TESS photometer taking measurements from a moving
vehicle. The results are comparable to those obtained with an SQM measuring=
at
the same time. This method is useful to build night sky brightness maps as =
the
one made around Madrid using SQM photometers [8].
4. C=
ontinuing
Work
Up to now, the hardware and communication software of the TESS-W
photometer are working without any problem. After the successful test the n=
ext
70 units are being assembled. The photometers will be cross calibrated on t=
he
LICA-UCM optical lab. We hope that the photometers will be ready for
distribution in the next months. We are developing more low cost devices th=
at
could measure night sky brightness [13].
Measuring night sky brightness with photometers pointing to the zen=
ith
is a first estimate of the light pollution effect at this location. However=
, it
is a simple and reliable method to monitor the evolution of the sky brightn=
ess.
Analyzing the data taken along the night, every night along extended period=
s it
is possible to obtain statistical results of the variation of the sky
brightness with atmospheric conditions [1],[2],[12].
More detailed information about the color and location of the sourc=
es
can be obtained with all-sky images taken with cameras fitted with fish-eye
lenses that are more complicated and time consuming. For a complete descrip=
tion
of the methods it is recommended to read the review of A. Hänel [9]. A=
ll-sky
brightness maps can also be built using simple photometers and the NixNox p=
roject
method [10][11].
Interested citizens will help us in monitoring the light pollution =
of
European villages using fixed photometers pointing to the zenith ('European
Photometer Network'). The aim is to extend the photometer network of =
the
Spanish Light Pollution research network [12] to European countries. We pla=
n to
joint efforts with other networks in Europe with similar monitor stations
(Italy, Switzerland, etc.). The data collected will be archived in open
repositories. The citizens, associations, and schools will be able to
purchase and install low cost night sky brightness photometers, which will =
be
sold in the STARS4ALL marketplace.
Ackn=
owledgements
STARS4ALL is a project funded by the European Union
H2020-ICT-2015-688135. This work has been partially funded by the Spanish
MICINN (AYA2012-30717, AYA2012-31277 and AY2013-46724-P), by the Spanish
program of International Campus of Excellence Moncloa (CEI), and the Madrid
Regional Government through the SpaceTec Project S2013/ICE-2822, The suppor=
t of
the Spanish Network for Light Pollution Studies (Ministerio de Econom&iacut=
e;a y
Competitividad (MINECO) AYA2015-71542-REDT) is also acknowledged.
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