For submillimeter astronomy, particularly at 200m, the ARENA working group has proposed a 25 m telescope at the For submillimeter astronomy, particularly at 200µm, the ARENA working group has proposed a 25 m telescope at the Concordia station on Dome C. Issues related to this suggestion are reviewed

Chamberlin

Devlin

Dole

Hauser

Herter

Holland

Lynch

Padin

Radford

S.J.E. Radford

Thaddeus

Von Hoerner

Wang

English

Crossref

Submillimeter Astronomy

For submillimeter astronomy, particularly at 200$\,\mu$m, the
ARENA working group has proposed a 25 m telescope
at the Concordia station on Dome C.
Issues related to this suggestion are reviewed

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Radford, S. J. E.

Caltech Authors - Main

:grd ARENA Conference: An Astronomical Observatory at C01VCORDIA 
(Dome C, Antarctica) 
L. Spinoglio and N. Epchtein (eds) 
EAS Publications Series, 40 (2010) 405-410 
5UBM1LLIMETER ASTRONOMY 
S.J.E. Radford 1 
Abstract. For submillimeter astrononlY~ particularly at 200 pITL the 
AREN A working group has proposed a 25 m telescope at the Concordia 
station on Dome C. Issues related to this suggestion are reviewed. 
1 Submillimeter Astronomy 
T,'venty five years ago~ astronomy at subnlillinleter ,vavelengths was just beginning. 
Observations since then have advanced our understanding of star fonnation, cir-
cumstellar envelopes. astrocheIllistry, debris disks~ the structure and evolution of 
the Galactic interstellar 11lediurl1, and the solar systelll. In extragalactic research~ 
two results are particularly notable. First. a population of optically inconspicu-
ous but subnlil1imeter lunlinous galaxies \vas discovered ,vith the SCVBA camera 
(Holland et ala 1999). The rest frarue spectral energy distributions of these galaxies 
peak around 100 pnl. At longer wavelengths. their steep dust spectra conlpensate 
for distance as redshift increases. so their subnlillinleter fhD~:es are alnlost redshift 
independent and t.hey are visible at great distances. Representative of this popula-
tion is Goods 8.50-5~ a distant starburst systern that is undetected in deep optical 
and near IR itnaging \vith HST and Subaru but is bright in the subnlillimeter 
("rang et al. 2009). Its redshift~ z = 4, has been deternlined by measurenlent 
of a CO line~ putting its luminosity. 1013 L:z : alnong the greatest kno\vn (Daddi 
et al. 2009). These objects provide dues to understanding the fonnation and 
evolution of galaxies in the early universe. including the rapid genesis of metals 
and the effect of collisions and mergers on galaxy assembly. Second was the recog-
nition the integrated intensity of the far IR and subnlillinleter radiation in the 
universe equals the intensit:~l at optical 'wavelengths (Hauser & D'wek 2001: Dole 
et al. 2006). 'Vhereas the optical radiation is direct starlight prinlarily emitted 
by relatively nearby galaxies. the FIR and submm radiation is starlight that has 
been absorbed and reenlitted by more distant dusty galaxies. R,eeent observations 
1 California Institute of Technology, Pasadena~ eSA 
© EAS, EDP Sciences 2010 
DOl: lO.1051/eas/1040056 
406 3rd ARENA Conference: An Astronomical Observatory at CONCORDIA 
Fig. 1. Left: historically, the sensitivity of bolometers for millimeter and submillimeter 
wavelengths has doubled every two years. Right: likewise, the size of (almost) back-
ground limited array cameras has doubled a bit faster than every two years. (Courtesy 
J. Zmuidzinas). 
confirm galaxies at z > 1 generate the majority of the FIR and submm background 
light (Devlin et al. 2009). 
New instrumentation is now poised to capitalize on these discoveries and fur-
ther our understanding. Perhaps the most notable project, ALlVlA, is the epitome 
of heterodyne interferometry. vVith many antennas adding up to an unsurpassed 
collecting area, flexible configurations extending to long baselines, sensitive re-
ceivers, and a powerful correlator, AL:NIA \vill provide high quality of individual 
objects images with exquisite spatial and spectral resolution (Testi 2009). Despite 
its capabilities, however, even AL:NIA cannot do everything. In particular, it is 
not efficient at wide field surveys. As P. Thaddeus (1996) quoted J. Ostriker (\vith 
apologies to V. Lombardi), ~'Surveys aren't the most important thing in astron-
omy, they're the only thing." Several groups have noted this opportunity and have 
proposed telescopes designed for surveys and outfitted with array cameras (Stark 
2003; IIerter et al. 2004; I-Iolland et al. 2004). The ARENA \vorking group has 
advanced the idea of a 25 m telescope at the Concordia station on Dome C (NIinier 
et ale 2008, 2009). 
2 Submillimeter Cameras 
Bolometer array cameras, which are \vell suited to high sensitivity, wide field sur-
veys, are fundamentally complementary to the heterodyne technology used in in-
terferometers such as ALlVIA. Over the past 50 years, the sensitivity of bolometers 
for submm wavelengths has shown a remarkable improvement, doubling every two 
years. Around 1990, the sensitivity of individual detector elements reached the 
background limit for ground based observations at a good site. Since then, array 
sizes have doubled a bit faster than every two years (Fig. 1). Extrapolating these 
trends, such cameras will soon outstrip the capabilities of existing telescopes. On 
S.J .E. Radford: Submillimeter Astronomy 407 
a 25 m telescope at a good site, the per pixel sensitivity of a background limited 
bolometer array observing at 350)-tID is about the same as ALJ\J'lA, but the survey 
speed would be many times faster because of the large instantaneous field of view. 
Such an instrument would reach the background confusion limit in about an h01U. 
lVlegapixel cameras might be anticipated about 2025, which is about mid career 
for any major telescope now under development. At 350 J..Lm on a 25 m telescope, a 
megapixel camera would Nyquist sample a field of view more than 3D' in diameter. 
Clear ly any new telescope design must consider the instantaneous field of view and 
be designed with such cameras in mind. 
Furthermore, non heterodyne spectrometers using gratings or other dispersive 
elements and direct detectors are undergoing rapid development. vVhile they offer 
only relatively modest spectral resolution, they can cover extremely wide band-
widths and are limited by background rather than heterodyne quantum noise, 
making them well suited for extragalactic surveys (Nikola et ala 2008). Laboratory 
studies of flexible dielectric waveguides point the way to multiobject spectrometers. 
3 Site Conditions 
In order to achieve the science goals and to justify the investment of a major sub-
millimeter telescope, excellent observing conditions are necessary. Submillimeter 
radiation is strongly attenuated by the atmosphere, in particular by water vapor. 
As a result, ground based observations are only possible from a handful of very 
dry locations. For many years, Ivlauna Kea has been the premiere site for submil-
limeter astronomy. In northern Chile, the 5000m Chajnantor plateau was selected 
for ALIvIA and other telescopes because it enjoys excellent observing conditions 
(Radford & Holdaway 1998). Even better conditions exist at 5600m on higher 
peaks in the vicinity (Ivlarrone et al. 2005; Radford et ala 2008). The potential of 
Antarctica has been recognized for some time (Chamberlin & Bally 1994) leading 
to the installation of major telescopes at the South Pole station, most recently the 
10 m diameter SPT (Padin et al. 2008). Conditions at the South Pole and on the 
Chjnantor plateau are comparable (Radford & Chamberlin 2000; Radford 2001). 
On the basis of meteorological data, it has been expected that Dome C would 
have better observing conditions, in particular, better transparency, than the 
South Pole. Recent measurements at 200 J..Lm indicate this is indeed the case 
('Iremblin et ala 2009). Although these preliminary measurements are encourag-
ing, comparison with other sites should be approached carefully as cross calibration 
of different instruments can be difficult. Additional transparency measurements, 
for example with a 183 GHz water line radiometer, and the planned measurements 
of sky brightness stability with CAlVIISTIC on lRAIT would corroborate these 
preliminary results. 
In site selection, sky coverage should not be neglected. The good zenith trans-
parency of an Antarctic site would be quickly nullified by observing low declination 
fields. At a tropical site more of sky may be observed at smaller zenith angle than 
at a polar site. Although the distant Universe can be observed in any direction, 
the wider astronomical community has placed tremendous resources into the study 
408 3rd ARENA Conference: An Astronomical Observatory at CONCORDIA 
of several deep survey fields, i. e., Goods~ Cosmos, X:\.Jl\I-LSS~ etc. These fields 
are (mostly) equatorial to allow observations by telescopes in both hemispheres. 
The observations of these fields, including major allocations of spacecraft time. 
are unlikely to be repeated at other places. Similarly, solar system objects and 
the Galactic centeL targets of some interest, are inconveniently 10'''" ,vhen viewed 
from Antarctica. 
4 200 J..Lm Observations 
The possibility of frequent time for observations at 200 pm is an attractive feature 
of Dome C. In the rest frame, the spectral energy distribution of starburst galaxies 
peaks close to 100 pm. At shorter \vavelengths~ the luminosity drops rapidly. 
Hence 200 )lm surveys \viII preferentially detect gala.xies at z < 1. For more distant 
galaxies, including the most active epoch of galaxy formatation at 1 < z < 2~ the 
200)lm flux 'will drop out. Indeed this may be ver:r usefuL in conjunction ,,,ith 
longer V\~avelength observations~ as a selection strategy for high redshift objects. 
5 TeJescope 
At short submillimeter 'wavelengths~ especially at 200 )lm. an excellent telescope 
will be essential. Otherwise the quality of the observations will be deternlined by 
the telescope perfornlance l rather than natural limitations such as the atInosphere. 
For high aperture efficiency~ the half \vavefront error of the entire systeln should 
be better than 1/20 of a \vavelength, or 10 pm rms for observations at 200 pill. 
Commensurate pointing knowledge is required, better than 1/10 of the diffraction 
beam. or better than 0.2" for observations at 200 /Jm with a 25 TIl telescope. These 
are challenging requirements (Fig. 2), unmet by any existing telescopes. Although 
design work is underway for CCAT ('Voody et al. 2008), the technical chal1enges 
are not yet solved for a telescope at a temperate site. The size and specifications 
of the SPT and the A.L~'1A antennas are similar enough the 8PT was able to use a 
design based on the \Tertex AL1.1A antenna. Antarctic conditions pose additional 
challenges (Durand et al. 2008)~ for example the fonnation of frost on exposed 
surfaces. Heating the telescope surface to remove this frost rnay cOlllprolnise the 
thernlal stability needed to maintain the surface precision. 
6 Logistics 
In considering a major telescope in Antarctica~ the logistics must be carefully 
considered. Although Antarctic bases are operated in support of scientific research~ 
there are considerable political reasons behind their establishment. Hence while an 
excellent science case is certainly necessary to justify construction and operation 
of a major telescope in Antarctica~ it ma~y not be sufficient. Full support fronl the 
polar agencies v.rill be essential to success of such a project. 
S . .J .E. Radford: Submillimeter Astronomy 409 
100 # 
cJ / ~/¢ 
~~ ,,/j 
~0 'tty 
E '" / 
,.: / (J) 
..-(J) 
C8AT E 
.!Q 
'"0 
~ 
:J 
t 
Q) 
10 c. 
<C 
10' 102 103 
Surface error, J-lm 
Fig. 2. Surface precision of radio telescopes and natural limits imposed by material 
properties. The eso and GBT have open loop active surfaces; all others are passive 
designs. (After von Hoerner 1967.) 
The costs of an A.ntarctic telescope nlust be acknowledged. A rough break-
down of t.he 2010 budget for the US KSF Office of Polar Progra.ms (lrlWW. nsf. gov) 
suggests the total annual cost of operating the South Pole station. including the 
proportional share of the supply chain as ':\lell as the research activities~ is about 
90 lllillion CSD. This amount supports~ of course~ several science activities at the 
Pole. but indicates the scale of the required expenditures. To support a major 
telescope at DOlne C. proportionate operating costs must be foreseen. This w'ill 
require a lllajor cOlnnlitment by the funding agencies. 
In considering the prospects for a major 8ubmillimeter telescope at Dome C, 
it Inay be instructive to revie\v the history of astronomy at the South Pole, \vhere 
SUlllmer experiments in the late 1970s and through the 19808 paved the ,vay for the 
first all year telescope, AST /RO~ in 1995. and ultimately the SPT ten years after 
that (Lynch 1998). Up to now. only summer experiments and site characterization 
lneasurelnents have been carried out at Dome C. A 25 m telescope \vould be a big 
step requiring significantly enhanced logistic support. 
1 Summary 
Subnlillimeter astronomy has deyeloped significantly in recent years and, \vith the 
advent of ALl\IA~ is nnw poised for further discoveries. An important complement 
410 3rd A.REKA Conference: An Astronomical Observatory at CO~CORDIA 
to AL1IA will be wide field surveys with large array cameras: which are rapidly 
increasing in size and nlapping speed. Observing conditions at Dome C promise 
considerable observing tinle at 200 11m: suggesting an emphasis on this wavelength. 
The technical and ]ogistics issues~ ho\vever. involved in constructing and operating 
a nlajor telescope at Dome C are considerable. 
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