TWILIGHT ASTRONOMY FROM ANTARCTICA - Michael G. Burton
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Proceedings of Second ARENA Conference, Potsdam, Germany, Sept 17-21, 2007
Editors H. Zinnecker, H. Rauer & N. Epchtein
EAS Publications Series, 2008
TWILIGHT ASTRONOMY FROM ANTARCTICA
Michael G. Burton 1
Abstract. A description of the early days of infrared site testing at
the South Pole is given, with special attention paid to the data which
quantified the conditions during summer and the long twilight period.
Such times are also provide relatively benign conditions for humans
in Antarctica as well. We show that they are favourable for many
observing programs that require the thermal infrared, and then dis-
cuss a number of specific projects that could be undertaken. Such
projects might mark the first to be conducted with any new telescope
in Antarctica, as they would enable science to be carried out during
the commissioning period.
1 Introduction
The best conditions for ground-based observational astronomy across the visible
to mm-wave spectrum — the coldest, darkest, driest and most stable skies at-
tainable on Earth — occur during the depths of winter on the summits of the
Antarctic high plateau. At the same locations, superb conditions still occur for
a restricted range of science programs when the sky is not yet fully dark, in the
long hours of twilight, or even in the summer daytime period. At these times,
outside the harshness of winter, conditions for humans are far more benign. It is
relatively easy to work outside. Experiments can be constructed fairly readily. A
team can be on hand to commission and run them. Thus, twilight conditions are
when we might expect the first results to be obtained from new facilities built in
Antarctica. This article discusses what we can be expected during the twilight
period in Antarctica, and some of the experiments that might be undertaken as
“first light” experiments. It does so through a personal perspective on the early
infrared site testing measurements undertaken at the South Pole, to illustrate the
contrast between summer and wintertime conditions.
1 School of Physics, UNSW, Sydney, NSW 2052, Australia
!c EDP Sciences 2007
DOI: (will be inserted later)2 Proceedings of Second ARENA Conference, Potsdam, Germany, Sept 17-21, 2007
Fig. 1. Skydips obtained with the IRPS at the South Pole during commissioning in
January 1994. The mirror zenith angle changes from 0◦ (straight up) to 90◦ along the
horizon. Two scans, one with the Moon passing though the 4◦ beam of the instrument,
and the other when not, are shown. In addition, a curve for a “sec z” plane parallel
atmosphere is shown for reference.
2 Infrared Site Testing — as the sky gets dark
The first infrared site testing in Antarctica took place in the winter of 1994, using a
retired single element photometer from the Anglo Australian Telescope, the IRPS
or “InfraRed Photometer Spectrometer”. This had been a pioneering instrument
in the pre-array days of IR astronomy, for instance being used to make the first
images of the stars at the centre of the Galaxy (Allen, Hyland & Jones, 1983)
and to find windows to peer through the upper atmosphere of Venus (Allen &
Crawford, 1984). It was an ideal instrument for site testing, as the detector was
stable in DC mode, allowing the signal from the sky to be monitored without the
need to chop. The IRPS was used to quantify the sky brightness from 1-5µm, and
its variation with time, at the South Pole from 1994-96. The science results were
reported in Ashley et al. (1996) and Phillips et al. (1999). Here we show some
unpublished data from the commissioning phase in January 1994.
The IRPS worked by staring at a gold mirror, which could be rotated in ele-
vation along one azimuthal direction, from horizon to horizon. The first task was
to determine this alignment, and to verify that a sky signal was being measured.
In the bright sunshine of mid-summer at the South Pole, sky emission from theBurton: Twilight Astronomy from Antarctica 3
scattered sunlight swamps the detector at wavebands shortward of 3µm, orders of
magnitude stronger than its nighttime value. In the thermal IR, however, scat-
tered sunlight is of much less importance to the total signal. We figured that it
should have been possible to see the Moon directly in day light, through the 5mm
aperture of the IRPS in L–band (3.8µm). This thus became the first experiment
to be tried. A rough azimuthal angle for the IRPS was estimated, and the time
when the Moon would transit this azimuth found. Then the mirror of the IRPS
was rapidly scanned back and forth, from zenith to horizon, as the appointed hour
approached. For several minutes all that was seen was the “sec z” signal pro-
duced by the varying airmass with zenith distance. Suddenly, however, the signal
increased by a factor five-fold (see Fig. 1), as the Moon entered the 4◦ beam of
the instrument. We were now doing real infrared astronomy for the first time in
Antarctica, in broad daylight! Despite the rather modest scientific value of this
measurement, it rates as one of the author’s most exciting moments in astronomy,
finally observing a celestial source from Antarctica, several years after the idea to
conduct IR astronomy there had been conceived, a task that had looked infeasible
for a long time! From the point of view of the experiment, it of course told us we
had a working instrument, as well as calibrating its alignment from the time of
the Moon’s transit.
The Sun sets at the South Pole on the date of the vernal equinox (a northern
hemisphere-centric statement!), an event that lasts for about a day, as the Sun
rolls along the horizon. Twilight itself lasts over a month, before the sky gets
truly dark. This can be seen from Fig. 2. These show 1-2.5µm sky spectra sent
back during this period by the winter-over scientist, John Briggs. The H & K win-
dows themselves are clearly apparent, with the atmospheric absorption between
them causing the dip in signal from the reduced level of scattered sunlight. The
flux levels in the bands drops rapidly after sunset, with a distinct change in shape
apparent from early May. This was the appearance of the long-predicted “cos-
mological window”, where the combination of low thermal and negligible airglow
emission works together, resulting in sky fluxes two orders of magnitude lower
than at temperate sites. It was not until we saw the spectrum obtained at the
end of May that we knew that the lowest level had finally been reached. Notice
how the total sky signal has dropped by five orders of magnitude from the start
of twilight! The H & K bands are clearly not a good place to be doing twilight
science, unless the object of interest is of itself extraordinary bright (as it is for
planets, and this does provide one such an application).
In the 3–4µm L–band window it is, however, another story, as Fig. 3 illustrates.
Over the month of March the sky flux dropped by only a factor of 10, with the
lowest levels already reached by the end of that month. Notice how there is only
a small scattered component to daytime spectrum. The relatively small difference
between day and night corresponds to a change in the temperature from ∼ −40◦ C
to ∼ −60◦ C. This thus provides an opportunity for science in the thermal infrared
bands; i.e. from 3-30µm. Even the summer time background is lower than at
temperate latitude sites at very their best. These windows, therefore, provide
prime targets for conducting twilight science experiments through.4 Proceedings of Second ARENA Conference, Potsdam, Germany, Sept 17-21, 2007
Fig. 2. 1–2.5µm (H&K bands) CVF spectrum of the sky emission (1% spectral reso-
lution), obtained with IRPS during the twilight period of 1994. Note the 5 order of
magnitude drop in total sky flux, and how the exceptionally dark 2.4µm Kdark window
opens up after a month of twilight.
The question of how far the Sun must be below the horizon for true dark
time conditions to exist also raises an interesting question for Antarctica, as the
reduced levels of scattering in the atmosphere mean that the Sun can be closer to
the horizon than at a temperate site. Fig. 4 (from Phillips et al., 1999) illustrates
this, showing the continuum flux levels in the H, Kdark and M bands (1.6, 2.4
& 4.7µm), as a function of solar zenith angle. The two short-wave bands show
considerable scatter until the Sun drops more than 10◦ below the horizon. This
is when the Sun stops directly illuminating the high-altitude OH layer where the
airglow emission arises. In the case of the thermal M-band, no effect of zenith
distance is apparent once the Sun drops below the horizon; the fluctuations seen
are purely the result of changing weather conditions (i.e. cloud). The classical view
of astronomical dark time occurring when the Sun drops more than 18◦ below the
horizon clearly does not apply at the South Pole in the infrared!Burton: Twilight Astronomy from Antarctica 5 Fig. 3. 3–4µm (L band) sky spectrum obtained with a 1% resolution CVF in the IRPS during the twilight period of 1994. In comparison with Fig. 2, the total background reduction from summer to winter is only a factor of ∼ 10, and the darkest conditions are reached within two weeks of sunset. 3 Carina Revealed — daytime observations from the South Pole The final example we choose to illustrate twilight science comes from November 1998, when the IR site testing program at the South Pole had been completed, and instead a prototype IR telescope, the 60cm SPIREX, was in operation. This ran through the 98 & 99 winters using the Abu 1–5µm 1024 × 1024 array camera, with the primary purpose of verifying that front-line infrared astronomical observations were indeed possible during the Antarctic winter (e.g. see Burton et al., 2000). In Fig. 5 are shown two images of a 5" field around η Carina from Brooks et al. (2000). They illustrate some of the photodissociation regions that accompany this spectacular source, dense clumps of molecular gas being irradiated by its far–UV photons. The left image shows the 2.12µm H2 line emission, as mapped with a Fabry-Perot interferometer on the AAT, produced from a 4 × 4 mosaic of the field. The right image shows a portion of a single field obtained with SPIREX at 3.3µm, in the PAH emission band. It was obtained during bright sunshine! There is a direct correspondence between the two images, showing the PDR nature of the emission, the molecules fluorescently excited by the radiation field. Further
6 Proceedings of Second ARENA Conference, Potsdam, Germany, Sept 17-21, 2007
6000
4000 H
2000
0
90 95 100 105 110 115
1000
µJy/arcsec2
500
Kdark
0
90 95 100 105 110 115
3•106
2•106 M
1•106
0
90 95 100 105 110 115
Solar zenith angle/degrees
Fig. 4. Sky flux in the 1.6, 2.4 & 4.7µm (H, Kdark and M bands), as a function of zenith
distance, measured by the IRPS in the winter of 1996 (from Phillips et al., 1999). Note
that “astronomical darkness” is reached when the Sun is only 10◦ below the horizon in
H and Kdark bands, and as soon as the Sun drops below the horizon in M band. This is
in contrast to the “classical” value of 18◦ at temperate sites.
Fig. 5. Two images of a 5" field containing the spectacular massive star η Carina, from
Brooks et al. (2000). The image on left show the H2 2.12µm emission line mapped with a
FP-interferometer on the AAT. The image to right is of the 3.3µm PAH feature, imaged
in a single field with SPIREX at the South Pole. It was obtained on November 11, 1998,
in the broad daylight of an Antarctic summer!Burton: Twilight Astronomy from Antarctica 7
comparison of these images with dark clouds seen in the optical, and CO clouds
in the mm, actually allows the 3D geometry to be inferred. For instance the
prominent Keyhole nebula was not seen in the IR images, showing it to be a
foreground absorption cloud, not subjected to η Carina’s UV radiation field.
4 Sample Science Programs for Twilight Astronomy
The parameter space for twilight science is wide. We simply outline a few possi-
bilities here. The first two rely special characteristics of the source and site, with
the remaining ones depending on the low level of the infrared sky:
• Orbital Debris Tracking Polar orbits and an extended terminator period for
sun-glints, combined with the excellent seeing, make the Antarctic plateau
the best location for detecting and tracking much of the human-induced
orbital debris around the Earth, allowing objects to be followed over many
successive orbits.
• Global Atmospheric Monitoring of Venus Global circulation on Venus can be
followed through windows at 1.7 & 2.3µm that peer through to high-altitude
clouds, silhouetted against hotter layers below. The highest spatial resolu-
tion comes at closest approach to Earth, when Venus is visible in daylight.
Through the technique of lucky imaging, taking advantage of the excellent
daytime seeing to improve its efficiency, diffraction limited images will be ob-
tainable from Antarctica, allowing weather patterns to be followed on Venus,
with a spatial resolution down to ∼ 50km with a 2m-class telescope.
While not the subject of this article, sub-mm conditions are also superb all
year, with the worst-quartile precipitable water vapour on the plateau (in sum-
mer) comparable to the best-quartile at the best temperate sites. Indeed sub-mm
observing has been carried on most of the year at the South Pole with the 1.7m
AST/RO telescope, with some of the best data obtained over the 3 months of the
Antarctic “Spring”.
Thermal IR observations can also be conducted all year, as the discussion here
has indicated. A wide range of prospective projects beckon. This include:
• The Galactic Ecology, through spectral line imaging to study the interaction
between the molecular, neutral and ionized components of the ISM, as well as
the ices in the very coldest gas. The IR windows provide access to many key
diagnostic species, with minimal extinction to obscure their distribution. For
instance, ices of methanol, carbon dioxide, carbon monoxide and ammonia
are evident from spectral absorption bands in accessible windows. The pure
H2 rotational lines at 12.28 and 17.03µm provide access to molecular gas
warmer than about 100 K – i.e. to regions of turbulent heating, as well as to
shocks from outflows, in addition to fluorescence from far–UV photons. PAH
emission at 3.3µm follows the edges of photodissociation regions – the neutral
interfaces at the boundaries of molecular clouds. The ionized medium can be8 Proceedings of Second ARENA Conference, Potsdam, Germany, Sept 17-21, 2007
examined through the Brα line at 4.05µm, a fundamental transition for the
recombining H atoms, as well as in the exceptionally bright [NeII] 12.8µm
cooling line in HII regions.
• The Evolutionary State of Embedded Young Stellar Objects in the molecular
clouds could also be examined through broad band IR imaging, with disks
being readily apparent through an excess in the emission at 3µm and longer
over that from a star that is simply reddened by extinction.
• The First Light, searching for gamma rays bursts at high redshift, through
detection of their light in the 3.8µm L-band. For instance, a z = 20 object
would not be seen at shorter wavelengths. It might provide a signal from the
supernova collapse of the first generation of super-massive stars. Any twilight
monitoring program in Antarctica would need to be conducted in parallel
with a 1-2.5µm monitoring program at a temperate site, to enable sources
only appearing at the longer wavelength to be found. It would provide
greatly improved sensitivities for the 3.8µm band over other sites. In winter,
of course, the near–IR bands could be observed from Antarctica as well.
Acknowledgements
The IR site testing program at the Pole with the IRPS was a collaboration between
the US Center for Astrophysical Research in Antarctica (CARA), in which John
Bally, Al Harper and John Briggs played an exceptionally active role, and the
UNSW, where Michael Ashley, John Storey and then undergraduate student Jamie
Lloyd were also key players. The SPIREX/Abu project expanded the collaboration
to include the US NOAO, with Ian Gatley and Al Fowler. Without the tireless
contributions of all these people, and those of many, many others which space
precludes from directly acknowledging, those early IR projects at the South Pole
would not have achieved the successes they did.
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