Note: this article deals nearly only with
environmental measurements made during the astronomical
trip in Namibia of August 2011. Look also at the photographic results here.
Any amateur astronomer knows that the night sky of Namibia
is at a top level, just as a few other locations in
the world such as the Atacama desert in Chile, the
Australian outback, or the Hawaii mountaintop. But how dark
is that dark sky? Any subjective description can be
folkloristic and nice, but it fails to objectively depict
the subject, and can be misinterpreted. Objective measurements, such as
the ones made with a SMQ-L
sensor (Sky Quality Meter with Lens), are much better.
Besides the measurements of the dark sky, climate
measurements such as temperature and relative humidity can
help to understand the "life" of an amateur observer under such
a dark sky.
"Namibia 2011" trip
This article describes the experience that I've lived in my
first trip to Namibia, with the only goal to observe and
image the night sky. Four friends of mine and I (Lorenzo Comolli,
Giosuè Ghioldi, Emmanuele Sordini and Marco Cardin)
moved to Tivoli Farm for ten
nights under the Southern sky, from 24 August to 3 September
. New Moon was on 28 August 2011. For the
schedule of sunset/sunrise and astronomical twilight, please
refer to the graph below. For the location, this is deep
winter season, corresponding to our late February. Well,
from the temperatures shown in the figures below, it does
not resemble to an Italian winter...
Tivoli Farm is located nearly exactly on the Tropic of
Capricorn, at 23° South (website
). Over there the ground is completely flat and the
environment is a kind of dry Savanna, on the Western edge of
the Kalahari desert. The Schreiber family, owner of the
farm, gave us a wonderful hospitality, and we immediately
realized that our holiday was going to become a true dream.
Upper raw: a
couple of temperature and relative humidity
measurement made during the afternoon at Tivoli Farm.
Yes, this is winter!
Raw below: the "Namibia 2011" team composed of Lorenzo
Comolli, Luigi Fontana, Giosuè Ghioldi,
Emmanuele Sordini and Marco Cardin.
A graph showing the time for sunrise/sunset and
astronomical twilight for Tivoli Farm. August and
September 2011, time is local UT+1. In the blue box
our stay. Generated using Perseus software.
During ten night and eleven days at Tivoli Farm we've never seen a single
What can be better?
Maybe we've been quite lucky, but this is not far from the
average of the season with nearly 80% of perfect days. Even
if each day was identical from a Sun illumination point of
view, some differences arose. The first day was quite cold,
due to chilly air coming from South. But after that day,
diurnal temperatures became quite similar, topping to about
35°C in the first afternoon. The relative humidity was
impressively low, at least for us living in the humid and
foggy Po Valley, Italy. During the day we measured an
average RH of 10-15%, topping down to 9% on a particularly
dry day. I had never experienced before such a single digit
humidity!!! The weather forecasts were completely useless,
as you can see from the example below.
A typical forecast
for Tivoli Farm: that's an easy job for
meteorologists! Not a single cloud from now to the end
of the long term forecast... Note that this is only
a part of the story. The situation of the preceding and following weeks
has been exactly the same. An unbelievable dream for an
Italian amateur astronomer used to the foggy Po
Valley. This is a screen shot from the MeteoBlue.ch
service. Tivoli Farm is in their database!
CLICK ON THE IMAGE
During the night the temperature dropped very fast. We got
minimum temperatures in the range from -1 to +9°C, with
an average difference from
day to night of 30°C !
This was a completely
new experience for all of us, used to temperatures nearly
constant during the nights of the Italian Alps and
Apennines. Besides the need to be dressed with many layers
of clothes as the temperature drops, the most dramatic effect
happening on our instrumentation was the focus shift. A refocusing was
needed every 1-2 hours, and you can clearly see a
worsening of the star shapes by naked eye on the monitor,
from one shot the the next! At home I never need to refocus because I've
never had this kind of problems.
Moreover, we needed to remove the camera during twilight to
allow the fresh air to enter the telescope tube and
better cool it down. This trick allowed us to improve a little
over the focusing problem.
The FWHM worsened
as the temperature dropped. Images from Pentax 75 and
Canon 5D (scale 3.38"/pix), while imaging Antares at
zenith. After the worsening of image 10, a refocusing
was done, but image 17 was again de-focused, a little
more than after an hour.
and relative humidity during the night
I've recorded the environmental data during each night at
about 2-3 hours intervals and the resulting graphs are shown here
below. From the RH measurements you understand why I felt like a
stupid to have carried with me many dew remover strips.
Not each night was
equal. The first one was the coldest and, together
with the last one, it reached freezing temperatures.
humidity shows some differences. The lowest night
topped a 28% while the highest at 62%. Anyhow, very
low values for my standard.
Minimum temperature and
maximum RH during each of the ten nights.
The dark sky of Namibia is not simply a night sky, it's an
experience! Any tourist visiting Namibia and spending at
least a night in the desert is amazed by that sky. You can
read dozens of reports of "standard" people that were
shocked by this show.
Each amateur astronomer should observe such a night sky at least once in a lifetime
Please consider this: I've a quite large experience of
observing from very dark locations in the Alps and I can
testify that Namibian sky is completely another matter.
That's a huge step forward. Not only for the darkness at
zenith, but also for the completely dark and transparent
horizon. I simply wish you to have a trip under a similar dark sky.
Your conception of dark sky will be completely redefined.
To get an objective measurement of how dark was the dark
sky, I've used a SQM-L device from UniHedron. The
measurements were gathered in the darkest part of the sky
and many trials were made each time to get the most reliable
measurement. The most impressive value is definitely the
22.20 mag/arcsec^2 value we got on the night of 29-30 August
2011 at about 3 a.m. That's a value largely higher respect
even to my best hope.
Just consider that my best night before was a 21.84 at Colle
dell'Agnello, 2700 m altitude in the Italian Alps, in 2010.
Standard values for this site range from 21.5 to 21.6. The
worser night in Namibia, the first, was "only" 21.75, nearly
equal to my best night before. Every other night was largely
A clear trend is noticeable in the graph. The darkness
increase in the second part of the night. This is mainly due
to the Milky Way going down from the zenith. At 3 a.m. the
MW contribution was at minimum, being the Southern galactic
pole at zenith.
The SQM-L measurements.
Not each night was equal. The first one was the the
Maximum darkness during
each of the 10 nights. Only 3 of 10 nights have not
exceeded 22.0 mag/arcsec^2.
all the nights where not equally dark?
That's a good question and I've no good reply. If you've any
idea, please write me. From a naked eye point of view, each
night was equally dark, without clouds, veils or whatever.
Also the many timelapses
that no disturbance was present.
While I've no clear answer, I can try to guess some possible
- Humidity: the first night was noticeable brighter
respect the other, and also relative humidity was
higher. Maybe also in the higher layers of atmosphere
the humidity was high, and a better reflection of the
distant lights of Windhoek was obtained. But also the
fourth night was bright, while the RH on the ground was
low. And the seventh night was relatively humid, but the
sky was really dark.
- Auroral activity: a well known source of natural
brightness is the airglow of the Oxigen layer at 90 km
altitude, excited by the solar wind particles, and
emitting mainly at 558 nm (green-yellow). This glow is
particularly noticeable in the night shots made by
astronauts on the ISS. I've tried to test the
relationship between the measured brightness and the
auroral activity, looking at the NOAA POES satellite
data, but no clear correlation is found, as the figure
- Moon light: especially first and second night, the
Moon was rising early in the last part of the night and
so it was preventing me to gain the best from the sky,
when the MW was low.
activity can be found from the data of NOAA POES
satellite data. The evidenced yellow bands correspond
to the 2nd part of each of my 10 nights in Namibia. No
clear correlation between these data and the night sky
brightness above can be found.
dark is the horizon?
The darkness at zenith is impressive. Even more impressive
in the darkness on the horizon. No light pollution is
present in this location, down to zero altitude. No
obstruction from mountains was present and so no possible
light pollution can be hided. The lights of the capital,
Windhoek, at about 130 km toward NW, are the only exception.
A very small cap of light was noticed, nearly 10° wide
and 3° high. It's superficial brightness was by far
lower respect to Milky Way or zodiacal light. Moreover this
light cap was visible only during 3 of 10 nights, when the
air was exceptionally transparent. In the other nights the
transparency on the horizon was "only" great and the cap was
completely invisible. So the influence of this light cap on
observations and images is simply zero. That's a dream for
each amateur astronomer!
To get a quantitative measurement of the brightness down to
the horizon I've made readings at 90°, 60°, 30°,
10° and 0°. Note that the field of view of SQM-L is
+/-10°, so the 0° reading include part of the dark
horizon. The worst measurement at 10° was 21.63, while
at zenith it was 22.05, not particularly dark for the
standard of this location. This measurement was done at 3
UT, when no Milky Way was in the sky (South galactic pole at
zenith). Another measurement at nadir, toward the ground,
These values fit very well with theoretical models including
the airmass increase, that predict an increase of about 0.4
in red band from zenith to 30° altitude (airmass 1 to
2). Refer, for this topic and much more, to F. Patat, Night sky brightness during
sunspot maximum at Paranal
, The Messenger, ESO
Brightness as a function of
the altitude on the horizon. 29 Aug 2011, h3 UT.
A few days before our departure from Italy, I was contacted
by Massimo Alessandria (SQM
) for a possible ride of an SQM-LE sensor
for Tivoli Farm. I was obviously happy to contribute to
expand this network and I offered also for help in
configuring the network during spare time. SQM-Network
is an Italian non-profit organization, part of GADS
association, based in Brescia, Italy. Its main aim is to
collect measurements of the sky brightness using SQM-LE
sensors all over the world. I contribute myself with a
sensor in my home observatory (see "meteo" page). Installing
such a sensor in the middle of nowhere can be tricky. Thanks
to the help of Massimo Alessandria in Italy and to Emmanuele
Sordini at Tivoli, we've been able to get it work. Now you
can look at real time measurements on the SQM-Network
here for direct link
. Be aware that looking at these
values may have side effects...
Installation was not easy, being the nearest electronics
store at 170 km drive... but Reinhold provided a long
ethernet cable and manufactured a long power supply cable.
The box was placed right above the library, so that it can
be connected to the local area network, and so also to the
internet. Yes, internet in the middle of the savanna!
Reinhold has a microwave radio link with good speed.
But the hardest part was the configuration. We needed to
understand how to configure the router, that showed a
strange behaviour. Moreover during configuration, internet
suddenly went down and we lost a lot of time trying to
figure out if this was our fault. Fortunately for us the
problem was on the internet provider side and the link was
reestablished the next day.
The last night we were able to get real time measurements
both from the SQM-LE sensor and from out hand-held SQM-L. A
comparison of that entire night is here below, together with
photos of the installation.
The SQM-LE was
installed by Reinhold above the library.
comparison between the fixed SQM-LE and the hand-held
SQM-L; the readings were made gathering the numbers
via the SQM-Reader software. Yes, my PC was gathering
images from my Canon 5D, and was connected wireless to
the internet, under such a great sky!
Below: the data was collected remotely also from the
SQM-Network, that produced this graph. The comparison
shows very similar values.
SQM measurements are only single point measurements. It
would be interesting to look at the brightness distribution
all over the sky, but no commercial instrument is available
with this feature. Nevertheless an All-Sky image can be
calibrated to get such a result. We've gathered such images
with a Canon 5D and a Peleng 8 mm lens, obtaining these
Milky Way. Calibration was
conducted using the SQM reading as a reference. Being the
digital sensor of a DSLR linear, the values all over the sky
can be obtained. I've produced the next images using Matlab
and a custom made script.
The images show three situations:
- Namibia, Milky Way at
zenith. This was for us at the start of the
night. So at the same time zodiacal light was present on
the West horizon. Nearly no area is free of "pollution"
from MW or ZL!!! The darkest area is 21.7.
- Namibia, Milky Wat on
the horizon. This was for us at about 3 a.m.
local time. The only "pollution" is due to zodiacal band
and gegenschein. The darkest part of the sky is 22.15.
This is not the darkest night we got, but quite close.
- Colle dell'Agnello,
Italy, Milky Way at zenith. This location at
2700 m altitude in the Alps is the best one I've
experienced in my life, before Namibia. The image was
taken during an extremely dark night for the Italian
standards. Even if most of the horizon is covered due to
nearby mountains, the sky below 30° is light
polluted. This was quite a dark night for the location,
at 21.5. I've no comparison from Italy without MW
because this image was intended just to image MW!
The most interesting regions are these.
- Galactic center
(first image). The center of MW measures 20.6, while
brightest parts of Cygnus are 21.2 and Crux is 21.0.
Southern MW is brighter!
- Zodiacal light (first
image). The brightness was impressive by naked eye. The
first night I was threatened! The brightest part is
20.9, so it is brighter respect to Cygnus MW and only a
little dimmer respect to MW bulge.
- Zodiacal band
(second image). The zodiacal band clearly split the sky
in two darker areas. The darkest part of the zodiacal
band is 21.90, that is 0.25 brighter respect to other
areas at similar altitudes. That's why you need a truly
dark sky to see the ZB.
- Gegenschein (second
image). The brightness is a little higher respect to ZB,
brightness is 21.70, that is 0.45 more respect to the
other areas at the same altitude. That's relatively easy
to see from Namibia, but from Italy I've observed it
only a single time, and it was a hard job.
- Namibian horizon
(second image). The horizon is incredibly dark.
Excluding the regions polluted by MW and ZL &C, the
brightest part of the sky is at about 10° altitude,
at 21.7-21.8. In Italy such a value at zenith can be
considered a very uncommon and exceptional night, that
can be obtained from only a few remote locations on
mountaintops and in a few days when the light pollution
from the cities is hidden by the fog. At altitudes lower
than 10° the sky is darker because of atmospheric
- Italian horizon
(third image). At the altitude of 10°, the sky of
Colle dell'Agnello shows 20.7, in the darkest region
(SE, toward mountains), and 20.0 in the most polluted
region (NW, toward Torino, about 1 million inhabitants,
70 km distance). The darkest area of the sky, at 21.5,
is brighter respect to the Namibian horizon... no
All-Sky maps of the night sky brightness. Blue is
brighter, red is darker. Labels in the hires shows the
values in mag/arcsec^2. North a top.
CLICK ON THE IMAGE FOR THE
At the end of this article I feel two sentiments, that also
you may feel: first I would immediately be teletransported
to Namibia, and second I feel sad for our "bright dark" sky.
I hope in this page to have given an idea of the environment
of the Namibian sky at Tivoli Farm, both from a
temperature/humidity point of view, and for the night sky
I frequently hear descriptions of "truly" dark skies, but
now I think that any description is void if you've never
observed a "really true" dark sky like the one I got in
Namibia. The problem is that even experienced observers, but
observing only from good Italian skies, can testify that a
particular night was very dark. In a case I was present
under the same sky of such a claim, and I've measured 21.2,
a full magnitude per square arcsecond brighter respect to my
Namibia! That's a really huge difference. The same
difference experienced from a moderately polluted sky of a
small city in the Po Valley (like the subhurbs of Varese,
Italy) and the supposed dark sky in the Apennines. Two
lessons: don't trust in any desciption, require objective
measurements; and let's go to see again that dark sky!
Acknowledgments: I'd like
to thank my friends Luigi Fontana, Giosuè Ghioldi,
Emmanuele Sordini and Marco Cardin for sharing this great
experience. Thanks also to Marco Cianci for addressing me
the paper by F. Patat. And again thanks to Marco Cardin for help in reviewing my bad English.