Astronomy Course of ETH Zürich at Diavolezza

~ Astronomy Course of ETH Zürich at Diavolezza ~
By Christian Monstein, Andrea Banzatti and Leon Dedes
Institut für Astronomie
Section 1. Introduction
As in the past couple of years, a group of students in the physics department of ETH Zurich was
selected to attend a one-week course in astronomy at Diavolezza in Switzerland’s mountains.
The selection criteria for all candidates were accepted proposals about an astronomical
experiment. The course took place from January 10th until 15th at the high-altitude Berghaus
Diavolezza Hotel, 3000 meters above sea level. In total, 23 young, motivated students of the 5th
semester in bachelor-course participated in many different optical and radio experiments. See
figure 1. The students were supervised by five assistants (doctor-students and post-docs). In
addition we could count on an apprentice taking care of and maintaining the instrument
hardware. The students were split into six groups, one group per experiment:
1.
2.
3.
4.
5.
6.
Detection and confirmation of neutral hydrogen in our galaxy at wavelength, O= 21 cm +
Determination of diameter of the Sun by radio-interferometer at wavelength, O = 13 cm +
Observation and analysis of variable stars
Observation and analysis of planetary nebulae
Observation and analysis of mass of planet Jupiter and Saturn
Optical polarization measurement on Moon and Taurus
+
The results of these two experiments are described in Sections 2 and 3 of this article
This year, the gods of weather were not very sympathetic with us. Almost every night we got
snowfall and heavy winds. We have never checked the weather satellite and weather forecast as
often as this year, hoping to get better conditions for optical observations. As soon as the
instruments were set up, clouds or snowfall made optical observations impossible. Most of the
time students were totally busy stowing the instruments in the hotel and setting them up again
outside. This must not be seen as a negative aspect. It was very useful for the students to get
acquainted with the instruments. Anyway, this is fully compatible with the idea of propaedeutic
education. However, the individual motivation decreases rapidly during the week if one never
gets any useful data from the stars.
The radio astronomers among the students were in a much more comfortable situation. They
could observe day and night in all weather conditions, even during snowfall. Radio astronomical
observations in L- and S-band are still possible in extremely bad circumstances. After a few
hours the radio astronomers could present their first results derived from their collected data. The
experiments are described in the following two sections. Optical experiments shall be presented
in a future article, assuming the students can get anything out of the data they collected during a
week of bad weather conditions.
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In addition to our improved radio telescope for 21 cm wavelength observations, which has a
somewhat larger parabolic dish of 120 cm diameter, we could set up and test a new 2-element
total power swept frequency radio interferometer at 13 cm wavelength. The interferometer is
based on the CALLISTO spectrometer. Both instruments were manufactured and improved by
our apprentice Tobias Kittelmann (physics laboratory assistant in third year of education). On
Friday afternoon each group was persuaded to give a 10-minute presentation about their
experiment and, if available, first results. Optical students finally could do some observations
during Saturday night because the weather conditions improved from time to time. The students
are required to produce a report after our ‘Astro-Week’ is finished about their experiment (in
English) as part of the advanced lab course. If their report is accepted then they get two credit
points for their bachelor course.
Figure 1: Twenty-three still happily looking students and seven assistants in front of hotel Diavolezza. In the
background from left-to-right are mountains Piz Palü, Bella Vista, Crast Agüzza and Piz Bernina around 4000 m
above sea level. Author Christian HB9SCT is in second row, sixth from left with red/white jacket and black cap.
Photo: © ETH Zürich/ Heidi Hostettler.
Section 2. Galactic Spiral Structure from the HI 21cm Line
The disk of the Milky Way galaxy, as with many other galaxies, exhibits a prominent spiral
structure. It is the result of density waves created by perturbations of the galactic gravitational
field. Thus, in the spiral arms region, material is compressed leading to increased density of gas
and the formation of young stars. In stark contrast the regions between the spiral arms usually
have densities 3- to 4-times lower. Three students, Carina Stritt, Helene Stachel, and Philip
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Kerpen, decided to conduct an experiment to probe the galactic spiral structure using the 21 cm
emission line of neutral hydrogen, designated HI.
The existence of the neutral hydrogen emission line was predicted by van der Hulst in 1944. The
line was first detected by a number of radio astronomers in 1951. It is produced by the spin-flip
transition of the hydrogen atoms at the laboratory frequency of 1420.40575177 MHz (O =
21.10611405413 cm). While this transition is forbidden in the laboratories (t = 107 years), the
large amount of neutral hydrogen in the interstellar medium and collisions which reduce the
transition's lifetime make the line easily observable. The 21 cm line is ideal for the study of the
galactic structure. Firstly and most importantly, neutral hydrogen is distributed throughout the
galaxy in the form of a diffuse gas. It exists in two phases, a cold phase with a temperature of 80
K and a warm phase with a temperature of 8000 K. Because of its low density hydrogen acts as
a "tracer" of the galactic gravitational potential and thus follows easily the galactic spiral
structure. In addition, the HI line is largely unobstructed by dust absorption so we can use it to
probe regions of our galaxy which are obscured in the optical regime. Finally, using the HI line,
we gain kinematical information due to the Doppler shift; for example, we can determine the
velocity of a cloud of hydrogen gas that is moving toward or away from us by measuring the
frequency of the HI-line emitted from the cloud. This is very important because assuming the
differential rotation of our Galaxy, we can use velocities to estimate the distance of objects from
our position. Thus, by constructing a map of velocity versus galactic longitude, called a positionvelocity diagram, we can gain an idea of the HI density fluctuations throughout the galaxy and,
thus, probe the spiral structure.
The aim of this experiment was to create a position-velocity diagram by measuring the spectra,
or intensity versus velocity diagram, for a number of positions along the galactic plane. The
students used the 1.2 m ETH radio telescope (figure 2) fitted with an L-band (21 cm) receiver
and coupled with a state-ofthe art FFT spectrometer
(16384 channels every 10
ms over 1 GHz bandwidth).
This radio telescope was
assembled
in
the
observation
room
at
Diavolezza. Taking into
account that the angular
resolution of the telescope
is ~15°, a 7.5° grid was
selected to get the full
sampling. Not all the
galactic plane is visible
from Diavolezza, so the
mapping was limited to
within a galactic longitude l
between 30° < l < 220°.
Figure 2: 120 cm parabolic dish mounted on a parallactic mount, remote controlled from the observatory inside of
hotel Diavolezza. Christian Monstein is checking the double-quad feed. In the background two antennas of the 13
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cm interferometer. Farther in the background are mountains Bella Vista, Crast Agüzza and Piz Bernina 4049 m
ASL. Photo: © ETH Zürich/ Heidi Hostettler.
The first day in Diavolezza was used to set-up and test the instrument and to determine the
optimal observing mode. It was finally decided to use the frequency switching technique with a
shift of 1 MHz. In the following days, despite the bad weather, which included a snowstorm, the
students started to observe the different targeted regions in the sky. As the first spectra were
observed, an extensive data reduction was applied to them that producing a spectrum similar to
figure 3. When all the observations and the data reduction were finished, the next step was to
convert the frequency-axis in the spectra into velocity-axis with respect to Diavolezza. A final
conversion to the velocity was applied to convert it with respect to the local standard of rest.
Using all this spectra measurements, the students produced the position-velocity diagram seen in
figure 4. This is a diagram of the line-of-sight velocity versus the galactic longitude. It is
essentially a plot of the galactic plane HI emission with respect to the local standard of rest. As
seen there are negative velocities, around -50 km/s, and from a galactic longitude 100° - 150°,
there exists an intensity enhancement. Similar enhancements can be seen at positive velocities
around longitude 50° and close to longitude 220°. All these are attributed to the galactic spiral
structure, which as mentioned earlier, creates the density and, thus, intensity enhancement in the
map. If the spiral structure was absent, we would observe a uniform distribution of gas with
velocity.
Figure 3: Single measurement of the
Milky Way galactic spectrum
Figure 4: Velocity
galactic longitude
map
along
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Section 3. Estimation of Sun’s Diameter by Interferometry
In observational astronomy, interferometry is a powerful technique to reach high angular
resolution, or sharpness of our view of the sky. The basic principle is the same as that seen on a
liquid surface when two waves meet – the superposition of the two waves produces an
interference pattern (also called fringe pattern) where maxima and minima overlap, giving
points of constructive (where they sum) and destructive (where they cancel out) interference.
The same principle applies when we use two antennas together to observe a common source in
the sky. The electromagnetic waves caught by the two antennas are made to interfere
electronically and, as a result, we obtain the interference pattern of the source. The angular
resolution, which for a single telescope is given by the diameter of its dish (the larger the dish,
the better the angular resolution), now is given by the distance between the two antennas, the socalled baseline. It is clear that by using this technique we can build (in principle) arbitrary large
interferometers and get the sharpest view of the sky. On the other hand, the construction of big
single dishes is hindered by extremely challenging technical/mechanical problems. But, as usual,
advantages have disadvantages. While a simple 6-cm refractor telescope has enough resolution
to explore the details of the Moon surface, or the paths of the Jupiter’s satellites (following
Galileo’s observations of 400 years ago), with a simple interferometer we do not obtain pretty
images, but only interference patterns. Only with
arrays of telescopes it is possible to reconstruct ALMA is the Atacama Large Millimeter
a radio image of the observed sky. Still, these Array which will start operations in 2012, see
images do not compete with the beauty of the http://science.nrao.edu/alma/index.shtml
images given by, for example, the Hubble Space
Telescope. Therefore, interferometry is less attractive for amateur astronomers and is usually
restricted to those who really need it for scientific purposes. The enormous effort taken by the
worldwide community to build the new interferometer ALMA in Chile (sidebar), though, clearly
states the importance of this technique for the present and the future of worldwide radio
astronomy.
As part of the advanced laboratory at the ETH, interferometry was performed during the
“Astrowoche” (Astro-Week) using a simple swept frequency adding interferometer made with
two antennas and the ETH-made CALLISTO spectrometer (receiver), observing at wavelengths
from 11.1 cm up to 13.6 cm. The antennas and some associated instrumentation are see in figure
5. Four students of the physics department, Christian Stieger, Sandra Jenatsch, Giada Rutar and
Björn Beyer chose this rather challenging experiment. The goal was to obtain an introduction to
this technique while performing simple observations and exploring the properties of interference
patterns. In other words, we learn how to wear “special glasses” which show the information
hidden within the fringes. The main target was our Sun.
The method is very simple: we point the antennas toward a position along the celestial path of
the Sun and let it produce its fringes while passing through our beam. The interferometer sees the
Sun as an extended source because the instrumental angular resolution is much higher than the
visible angular size of the Sun (~ 1/2 degree). This affects the fringe pattern in a way that the
angular size of the source can be derived from the measured fringes. One example of our
observations can be seen in figures 6 and 7. At the wavelengths where the interferometer is
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working, the Sun appears bigger than in the optical range because radio emissions are coming
from outer layers in the corona above the solar photosphere, where the puzzle of the increasing
solar temperature happens. Therefore, the radiation we detect comes from a region that is
invisible to our eyes and more than 10 times hotter than the surface we can see with our eyes!
Figure 5: Two simple, low-cost. commercial parabolic grid antennas normally used for WiFi, each mounted on a
Celestron polar mount. In the center below the green cover are a Wilkinson power combiner and an S-band downconverter. The IF-output of the converter fits into the frequency range of Callisto which is inside of the observatory.
Mountains in the background: Crast Agüzza and Piz Bernina. Photo: © ETH Zürich/ Heidi Hostettler.
Figure 6: Fringe pattern of a swept
frequency adding interferometer in the Sband captured with a simple CALLISTO
spectrometer. Frequency gap in the center
was due to radio frequency interference
from the nearby WiFi. Intensity scale
(although not perfectly calibrated) is in
solar flux units [SFU]. The higher the
frequency the narrower the fringes.
The Sun is not the only source we
can observe, though. Other
astronomical (strong radio sources
in our galaxy) as well as
“terrestrial” (artificial satellites)
objects can be detected. The
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angular size of satellites is small enough to be seen by the interferometer as a point source, and
hence they produce a
different fringe pattern. An
example is shown in
figures 8 and 9 (to be
compared to figures 6 and
7). Satellites emit at
“single” frequencies, thus
are to be observed in
narrow spectral ranges,
and they are much faster
than the angular velocity
of the Sun as seen from
the Earth, thus the peaks
of the fringes are closer
and the entire observation
is over in a few minutes.
Figure 7: Sixteen integrated light curves
derived from the spectrum in figure 7 (green).
Black line shows averaged light curves based
on Fourier transform, filtering and inverse
Fourier transform. The raised minimum at
time = 70 minutes can clearly be identified.
Minimum and maximum values are used to
calculate Sun disc diameter. The radio
interference level was unexpectedly low at
the time of these observations.
Figure 8: Night spectrum taken with
CALLISTO while a military satellite passes
the antenna beam. Fringes allow us to
determine angular velocity of the satellite.
Figure 9: Light curve of two military
satellites derived from the night spectrum
in figure 7. The satellite is extremly strong
while passing the antenna beam. The
amplitude of the fringe plot saturates the
spectrometer
analog-digital
converter
(ADC) at time = 7 minutes. The fringe
pattern show no raised minimum, therefore
we know it must be a point source
(satellite).
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