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The Joint Australian Centre for Astrophysical Research in Antarctica |
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| The Douglas Mawson Telescope A 2-m infrared telescope for Antarctica |
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DMT
science workshop
Afternoon tea |
DMT
science workshop |
Background to the Douglas Mawson Telescope
The Douglas Mawson Telescope (DMT) is a proposed 2-metre IR-optimised telescope to be built at Concordia Station, Dome C, Antarctica. It will be operated as a collaboration between Australia, France, Italy and the USA.The proposal to build a 2-metre IR telescope on the Antarctic plateau goes back many years, with a preliminary science case developed in 1994. However, it is only recently that the site-testing data has been available that makes the case for building such a telescope compelling. This work was in collaboration with the US Center for Astrophysical Research in Antarctica (CARA).
Strawman specifications of the telescope- 2.0m primary aperture,
- Cassegrain optical design,
- Infrared optimised with sub-mm capability,
- 30 arcmin unabberated field of view,
- 100 Hz SiC fast-tip-tilt secondary,
- Operating temperature -80C, survival to -100C.
An interesting question is whether the telescope mounting should be equatorial or alt-az. The alt-az advantage is cost, although this should not be such a factor at the Dome C latitude of -75 degrees. The disadvantage of alt-az is that you require an instrument rotator, which complicates the hardware and software, and leads to a rotating point-spread-function and difficulties in precision flat-fielding.
Concordia station
A major new research base, called Concordia Station is being constructed at Dome C by the French and Italian Antarctic programs. Concordia will open for year-round operation in 2003, and will support a winter crew of up to 15 people. Concordia is being built at Dome C, some 1600 km from the South Pole, and 1200 km from the Australian coastal station of Casey. Conditions for infrared astronomy at Dome C are extraordinarily favourable. Winter-time temperatures drop below -80C, while the wind remains extremely low with an average speed of just 2 m/s. A more general workshop on Astrophysics at Dome C will be held in Hobart on 28 - 29 June, 2001.
Since 1994, JACARA (the Joint Australian Centre for Astrophysical Research in Antarctica) has been actively exploring the potential of Antarctica for astronomy. The complete JACARA bibliography includes 21 papers in refereed journals, 30 conference papers, 19 popular articles, 11 poster papers, and 12 student theses, as of April 2001.
The Science
The DMT will have exceptional performance in the thermal infrared, with a sensitivity for wide field mapping and for observing extended objects that is comparable to or better than 8-metre class telescopes (with their narrower fields of view) at temperate sites.
Moreover, the DMT will also be equipped with sub-millimetre instrumentation, allowing it to build on the success of the 1.7 metre AST/RO telescope currently operating at South Pole.
The following table, from Burton et al (Pub. ASA, in press, 2001), shows sky backgrounds (in Jy/square arcsecond), relative signal-to-noise ratios, and sensitivities in magnitudes (5 sigma, 1 hour) comparing 4 telescopes in K, L and N bands for both wide-field (per square arcsecond) and point-source (i.e., diffraction-limited) imaging.
Telescope --> Mauna Kea 8m SSO 3.9m Antarctic 2m Antarctic 8m
Wide Point Wide Point Wide Point Wide Point
Field Source Field Source Field Source Field Source
Waveband
K (2.25-2.37um)
Sky background 0.003 0.003 0.00015 0.00015
Relative S/N 1.0 1.0 0.5 0.2 1.1 0.3 4.4 4.0
Sensitivity 21.5 23.1 20.5 21.3 21.2 21.0 22.8 24.1
L (3.65um)
Sky background 2 3 0.1 0.1
Relative S/N 1.0 1.0 0.4 0.2 1.1 0.3 4.5 4.5
Sensitivity 16.7 17.8 15.3 15.5 16.9 16.4 18.4 19.4
N (11.5um)
Sky background 200 1000 20 20
Relative S/N 1.0 1.0 0.2 0.1 0.8 0.2 3.2 3.2
Sensitivity 11.8 11.5 9.9 8.9 11.2 9.5 12.7 12.5
NOTE that care should be used in interpretting the above table. In particular, the point source sensitivities assume diffraction limited performance, which corresponds to 0.05 arcsecs at K for an 8-m telescope. If you are not diffraction limited, then the sensitivity will be correspondingly reduced, which will favour the smaller telescope. Also, if adaptive optics is used on the 8-m telescope, this will introduce a number of additional reflections from warm mirrors, with a corresponding increase in background.
The following figure shows model calculation of the atmospheric transmission at the South Pole across the thermal infrared, from 2-500um, corresponding to 164um of precipitable H2O, plus an aerosol visibility of 100km (these numbers are based on our site-testing data from the MISM instrument at the South Pole, see Hidas et al., PASA, 17, 260 (2000)).
New windows for ground based astronomy are opened in the mid-IR between 20 and 50um, and even windows at 200 and 220um may be accessible.
At optical and near-infrared (up to 2.5um) wavelengths, the DMT will have two advantages over other telescopes.
- The very low scintillation at the site will allow exceptionally good photometry [note: the hyperlink is to a PowerPoint presentation in French].
- Continuous observation of the many circumpolar sources will enable variability studies to be conducted that are impossible at other locations, and facilitate microlensing and other time-dependent observations.
It should be remembered that the DMT is a general purpose telescope and in fact will be the third-largest one to which Australian astronomers have significant access.
We should stress that the MNRF proposal is not envisaging an initial suite of instruments to make the DMT as flexible as, say, the AAT or 2.3m. However, the size of the telescope does make it practical for University consortia to realistically fund and build an instrument in Australia that is internationally competitive.
Gravitational Microlensing from the Antarctic - Philip Yock, U. Auckland
The following is an excerpt from "Observations from Australasia using the Gravitational Microlensing Technique", Phillip Yock, PASA, 17, 35. Bold emphasis has been added.
To realise the full potential of the gravitational microlensing technique it is necessary to monitor millions of stars with good photometric accuracy at a sampling rate of a few observations per hour in several passbands. The existing network of southern survey and follow-up telescopes (MACHO, OGLE, EROS, GMAN, PLANET, MPS and MOA) do a relatively good job of monitoring the Galactic bulge and the Magellanic Clouds. Further improvement may be expected to occur soon as new image subtraction techniques with better photometric accuracy are refined and incorporated.
A quantum leap might be realised with a telescope at the Antarctic.
The idea has been raised before (Sahu K., 1998, in Astrophysics from Antarctica, ed. G. Novak and R. Landsberg (ASP Conf. Series) 179; and Muraki, Y. et al. 1999 Suppl. Prog. Theor. Phys. 133, 233). Such a telescope could monitor southern fields essentially continuously, thus avoiding the not inconsiderable difficulties associated with combining data from different groups using different telescopes and different passbands, and working under different seeing conditions. Losses of data due to inclement weather would also be less serious. To monitor the complete peak of a typical high magnification event from the Antarctic would require good weather for a few days in one location only. Presently, good weather is required simultaneously in Chile, in Australasia and in South Africa. By observing in the infrared, and taking advantage of the exceptionally dry conditions at the Antarctic, one could extend the present measurements to include the centre of the Galaxy. Gould has pointed out this would increase the total event rate (Gould, A. 1995b ApJ 446, L71). A 2-m class telescope would be the preferred option to extend the current observations being made with 1-m class instruments. US and Australian groups have already made considerable progress towards the development of the Antarctic for infrared astronomy (Burton, M. 1996 PASA 13, 2). Observations have been made from the South Pole which confirm its excellent characteristics at infrared wavelengths, and site-testing is in progress at Dome-C, which promises to be even superior. In view of the above, Dome-C would seem to be a promising site for future development of gravitational microlensing.
And here is some additional information on microlensing from Burton et al 2001, Pub ASA, in press
Gravitational micro-lensing occurs if the geodesic from a star to us passes sufficiently close to a massive, foreground object that its path is bent, or lensed, splitting the light into multiple images (Paczynski, ApJ, 301, 503 (1986)). If there is a planet near one of the images an additional lensing effect can occur (Gould and Loeb ApJ, 396, 104 (1992)). The amplitude and light curve of such an event depends on the geometry of the orbit and mass of the planet, but typically will cause a perturbation on the microlensing light curve with a magnitude of a few percent for a few hours. If there is a planet present in the lensing system the probability of detecting a lensing signature from it is reasonably high if the sampling is frequent and the photometric accuracy high (Albrow et al., ApJ. 512, 673 (1999)).
To maximize the possibility of finding such events a dedicated telescope should continuously image the same region of sky where the stellar density is high. Nowhere is this more so than towards the Galactic centre. Furthermore, the Galactic centre becomes readily detectable at 2.4um (extinction precludes observation at much shorter wavelengths), the very waveband where the sky background is lowest in Antarctica. Moreover, the Galactic centre is always visible from the South Pole. For example, a 2-m telescope equipped with only a single 1024x1024 array with 0.6-arcsec pixels, mosaicing on a 4x4 grid, could image a 40x40 arcmin region roughly every 20 minutes, achieving a sensitivity of ~17.5 mags at 2.4um. Towards the Galactic centre every pixel would contain at least one star! As calculated by Gould (ApJ, 446, L71 (1995)), the optical depth for lensing is then unity; i.e., we would always expect to find at least one lensing event underway. Such a facility would be a powerful tool for exploring the incidence of planetary systems through the secondary lensing signature imposed on the micro-lensing light curve.
Optical stellar spectropolarimetry - Brad Carter, USQ
While the DMT is primarily envisaged as an infrared telescope, it has some interesting applications for optical stellar spectroscopy and polarimetry.
The DMT's principal advantage will be its ability to continuously monitor a star. Continuous and complete spectroscopic coverage of all rotational phases of an active star is:
- the simplest way to map stars with rotational periods around a day,
- the best way to map any active star with rapid active-region variability,
- the best way to measure starspot differential rotation (a key dynamo parameter), and
- provides matching ground-based observations for multi-wavelength around-the-clock observing campaigns utilising space observatories.
High photometric precision could also yield the first direct observations of starspots for moderately active sun-like stars, whose optical and infrared variability is very hard to detect, yet is directly relevant to the question of solar variability.
For optical stellar spectroscopy, the DMT could be instrumented relatively cheaply with an instrument perhaps similar to the high efficiency "ESPaDOnS" spectrograph/spectropolarimeter now under development by Jean-Francois Donati, Claude Catala and John Landstreet for the CFHT.
The high efficiency of ESPaDOnS on a 2m telescope would match the 4m AAT with UCLES, and if the 2m was at Dome C, the above-mentioned advantages would greatly enhance its potential.
For more information on the science possible with an ESPaDOnS clone, see this link
Measuring stellar oscillations from Antarctica - Tim Bedding, USyd
The aim of measuring stellar oscillations is to obtain a detailed picture of the insides of stars by measuring the frequencies at which they pulsate, in exactly the same way that seismologists have used earthquakes to probe the interior of the Earth.
Stars pulsate in many different modes simultaneously, each with a slightly different period. Click here to see some animations of pulsating stars. Each mode is a sound wave, so the periods give information about the sound speed inside the star.
For example, during its life, a star burns hydrogen into helium in its core. The speed of sound in helium is less than in hydrogen, so the pulsation periods of a star increase as it gets older (its voice deepens!). Thus, measuring pulsations and comparing with theory allows us to measure the ages of stars.
The study of starquakes, a field known as asteroseismology, has until now been almost impossible because of interference from the Earth's atmosphere. The twinkling of stars may be great for poets and lovers, but it is extremely frustrating for astronomers. It is caused by the movements of the air above us, and has nothing to do with variations in the stars themselves.
Antarctica offers the advantage of very much reduced scintillation. A telescope in Antarctica should therefore be able to measure stellar oscillations with a precision only surpassed by a space telescope. An additional advantage is that almost continuous observations will be possible. This is important when trying to disentangle the different oscillation modes. Telescopes at lower latitudes can only observe during fixed periods each day (i.e., night time), leading to aliasing effects.
Exo-solar planets - John Innis
The high-photometric precision expected of the proposed DMT, due to reduced scintillation, will have great applicability to photometric studies searching for and studying planets around stars other than our Sun. Potential work falls into three categories:
- Searching for new exo-planets by looking for the signature of the transit of the planet across the face of the parent star. A clear detection of the eclipse of the parent star by a 'hot-Jupiter'-planet from ground-based data has been reported for the star HD 209458 (Charbonneau et al., Ap.J., 592, L45-L48, 2000), where the eclipse depth was of order 0.01 magnitude. High-photometric precision is clearly needed. The excellent site characteristics of Dome C would allow searches to be extended for smaller planets (shallower eclipses).
- Obtaining additional, high-quality, multi-wavelength data of known eclipsing planet-star systems to observe the fine details of the transit, including limb-darkening effects at the different wavelengths. The extremely intriguing possibility exists for the detection of the secondary eclipse - i.e. the drop in light resulting from the disappearance of the planet behind the star - implying a direct detection of planet itself. Charbonneau et al (op cit) estimate that for HD 209458, at IR wavelengths, secondary eclipse may be 3 milli-magnitudes. High-precision photometry from an excellent site should enable this to be seen. As the time of primary eclipse will be known, along with the orbital period and eccentricity, the times of secondary eclipse can be estimated. A number of observations from the calculated times of secondary minima could be combined to further reduce noise. This appears to be an extremely exciting possibility, as multi-wavelength data could reveal information about the planetary properties directly.
- Obtaining high-quality photometric data on exo-planet systems suspected from radial-velocity data, in order to determine the parent star's rotation period. One of the major ambiguities in attempting to determine the mass of the unseen companion from radial velocity data alone is that the inclination of the orbit is usually not known. Hence there may be considerable uncertainty as to whether the companion is a planet, brown dwarf, or even a low mass 'normal' star (Han et al., Ap. J., 548, L57-L60, 2001). If the parent star's rotational period, P, can be measured, through the photometric detection of a rotational modulation due to the passage of sunspot-like activity over the disk (see Brad Carter's contribution to this page), and with the assumption that the orbital and rotational axes are parallel, or nearly so, the orbital inclination, i, can be estimated directly from: sin i = 0.02 P (v sin i)/ (R/R_sun), (Campbell and Walker, IAU Coll. 88, pp5-18, 1985) where v sin i is the rotational broadening (obtainable from the stellar spectra), R is the star's radius (which can be estimated from the spectral type, or from other methods), and R_sun is the solar radius. Hence, such data will provide additional constraints on the allowable range of exo-planetary masses.
The Environment of Star Forming Complexes - Burton et al 2001
While massive star formation is one of the most spectacular events in the Galaxy, paradoxically it is poorly understood. This is because of both the short timescales for the various stages of the process, and because of the many interacting phenomena for which it is hard to disentangle cause and effect. The environment of such star forming complexes, which dominate the southern Galactic plane, can be studied in the thermal infrared through the spectral features from ionized, neutral and molecular species that are present. HII and ultra-compact HII regions can be traced in the Br alpha 4.05um line, even when deeply embedded. Polycyclic Aromatic Hydrocarbons (PAHs), organic molecules that are fluoresced by far-UV radiation from the young stars and trace the edge of photodissociation regions, are visible through a spectral feature at 3.3um. They can be imaged at high spatial resolution, unlike other prime tracers of these regions, such as the far-IR [CII] 158um line. Excited molecular hydrogen emission, resulting from either shocks or UV-fluorescence, can be imaged in the v=1-0 Q-branch lines at 2.4um, which are both stronger and suffer less extinction than the commonly used 1-0 S(1) line at 2.12um. Several solid state absorption features are also present, for instance the ice band at 3.1um.
As an example of the potential for this kind of study, the following image shows shows an 18 x 18 arcminute region of the star forming complex NGC 6334, observed with the SPIREX/Abu camera from the South Pole (Burton et al. ApJ, 542, 359 (2000)), in the PAH and Br alpha features, as well as in the L-band continuum at 3.5mu. The pixel scale in this image is 0.5 arcsec, and combining the 1.5 arcsec diffraction limit with 1 hour of unguided tracking, the typical resolution achieved was ~3 arcsec. Shells of photodissociated gas surround bubbles of ionized gas in which embedded, massive protostars reside. Despite the modest size of the SPIREX telescope (just 60cm), these are the deepest images yet obtained at these wavelengths at this spatial resolution. The small aperture, however, also made possible the wide field of view with a similarly modest instrument.
Complete Population Census of Star Forming Regions - Burton et al 2001
A key goal for studies of star formation is to undertake a complete population census of star forming clouds in order to determine the number and types of stars that form in them, and how this varies between different complexes. To do so requires observations in the thermal infrared (beyond 3um). These wavelengths not only penetrate to the depths of cloud cores, but also allow us to distinguish between the embedded population and background stars. In simple terms, young stellar objects are surrounded by warm (few hundred K) disks which emit strongly at wavelengths greater than 3um, and thus are readily distinguished in infrared colour-colour diagrams (e.g., [1.65-2.2um] / [2.2-3.8um]) from reddened stars. Near-IR colour-colour diagrams (e.g., [1.25-1.65um] / [1.65-2.2um]), while relatively easy to construct because of the better sensitivities available, show only small IR excesses from the disks. These excesses are readily confused with reddening, and the surveys fail to identify the most deeply embedded sources.
The problem has been that at 3.8um sensitivities are typically 4-5 magnitudes worse than at 2.2um from most observing sites, thus limiting the work that has been done in this waveband. Needed are deep, wide-field surveys of comparable sensitivity to those conducted at 2.2um in order to determine the complete stellar membership of a star formation region. Such an opportunity is afforded by an Antarctic telescope through the greatly reduced thermal background at these wavelengths over temperate sites.
Brown dwarfs - cool sub-stellar objects - may also be identified through the deep absorption band at 3.4um, using narrow band filters on and off the band to determine "colours". Even cooler protostellar objects would be detectable in the mid-IR, for instance embedded sources within "hot molecular cores" (e.g., Walsh et al. MNRAS, in press, (2001)), suspected of being the first stage in the process of massive star formation. Imaging through narrow band (1um wide) filters at 8.5, 10.5, and 12.5um, where the background is at a minimum in the mid-IR window, will allow determination of spectral colours of these cooler objects, and thus help to place their evolutionary state.
Protogalaxies and the First Star Formation - Burton et al 2001
The star formation history of the Universe is being probed through deep pencil-beam surveys, of which the Hubble Deep Fields (HDF, Williams et al. AJ, 112, 1135, (1996)) are the most prominent examples. At the faint end of the samples the relative number of peculiar or disturbed galaxies rises dramatically, suggesting that processes to do with star formation (e.g., mergers, starbursts) are active in these sources. However, these galaxies also correspond to the most distant in the samples, with the highest redshift, and in the visible the rest frame being imaged is that of the far-UV. Here star formation is not at its most apparent, and dust absorption can be significant. An Antarctic telescope can search extraordinarily deeply in the 2.4um "cosmological window" to where, for example, the H alpha line is red-shifted at z=3. It could undertake the first high spatial resolution, wide-field surveys at 3.8um (L-band), where the visible light from z=5 galaxies would be observed. While the magnitude limit of the HDF (I ~ 28 mags.) will remain far deeper than that which an Antarctic 2m telescope will reach at 3.8um (L ~ 19 mags. in 24 hours), the colours of high-z galaxies are particularly red. For instance, an E/S0 galaxy at z=1.4 has an unreddened colour of V - L ~ 10. Thus a galaxy with V=28 and L=19, barely detectable in the HDF, would be detectable with an Antarctic 2m telescope in a day of integration. Moreover, redder and presumably more interesting galaxies, not seen in the HDF, would also be detectable.
Interferometry of Proto-Stellar Disks and Jovian Planets - Burton et al 2001
One of the great challenges facing astronomy, and the focus of major national programs such as NASA's Origins program, is the search for Earth-like planets. Several grand design projects have been envisaged towards this goal, for instance NASA's Terrestrial Planet Finder (Beichmann, Woolf and Lindensmith (editors), "The Terrestrial Planet Finder (TPF)", NASA JPL-publication 99003 (1999)) and ESA's Darwin (Penny et al., Proc. SPIE, 3350, 666, (1998)). These are space-based nulling interferometers, a suite of telescopes operating in mid-infrared where the unfavourable contrast between star and planet is least. Such facilities are not likely to be built before the middle of the 21st century, and many major technological issues remain to be addressed first. Several ground-based interferometers are now under construction, such as the Very Large Telescope, the Large Binocular Telescope and the Keck Telescopes, with the intermediate goal of imaging circumstellar disks, zodiacal dust and Jovian planets in nearby stellar systems. An Antarctic infrared interferometer (AII) is an obvious next step after a 2m class telescope, exploiting the reduced background, the improved sky stability compared to temperate sites, and the constant airmass of sources. We envisage the AII as a suite of 2m size telescopes, initially with just two connected interferometrically, but readily expanded for relatively low cost by the addition of more telescopes, to explore the optimal configuration for imaging other solar systems. It would provide the most powerful ground-based instrument for this purpose.
The Star Formation Rate in the Local Universe - Stuart Ryder, AAO
The DMT will be a powerful tool for surveying the true rate of massive star formation in nearby (z<0.03) galaxies, through measurement of the Br-alpha line flux at 4 microns. Traditionally, the star formation rate has been estimated from the H-alpha line in the optical (Kennicutt 1983, ApJ, 272, 54; Ryder & Dopita 1994, ApJ, 430, 142), but the variable amount of dust extinction leaves an uncertainty in the star formation rate of at least a factor of 2 for any one galaxy. Surveys at radio wavelengths do not suffer from extinction, but are subject to variable contamination from non-thermal emission sources (AGN, etc.) By going to 4 microns, the extinction is reduced to less than 10% that at H-alpha, with a consequent reduction in the extinction uncertainty. Although the atmospheric transmission in Antarctica is not significantly better than a site like Mauna Kea at 4 microns, the reduced thermal background from the sky and telescope makes the DMT easily competitive with an 8m on Mauna Kea, for the kind of wide-field survey proposed here (see Table above).
We would envisage using a tunable filter (akin to UNSWIRF) to image the Br-alpha line and nearby continuum for a large sample of galaxies out to a redshift of 7500 km/s (after which the atmospheric transmission drops off precipitously), but this would easily allow a much improved knowledge of the local current rate of star formation (and its variation with galaxy type, and environment) than is presently assumed. Such a survey can be extended to higher redshifts by observing the Pa-alpha line when it is redshifted away from its rest wavelength of 1.88 microns (where the atmosphere transmits poorly) into the region of the K-band beyond 2.25 microns (z>0.2), where the DMT excels.
Other infrared windows into the star formation properties of normal spiral galaxies that have recently been opened up by the ISO satellite include the 20-42 micron continuum (Dale et al. 2001, ApJ, 549, 215), the 7 micron continuum (Dale et al. 2000, AJ, 120, 583), and the so-called "aromatic features in emission" between 5.5 and 13 microns (Helou et al. 2000, ApJ, 532, L21). All of these windows are in regions of the spectrum where the DMT is expected to easily compete with much larger ground-based telescopes, and even with existing infrared satellites.
Observing gamma-ray bursts from Antarctica - Carl Akerlof, U. Michigan
Thirty years after their serendipitous initial discovery by satellites designed to detect clandestine tests of nuclear weapons, gamma-ray bursts (GRBs) are now understood to be incredibly powerful explosions occurring at cosmological distances. The extreme physical conditions that must exist in these events strain the most imaginative attempts to find a reasonable theoretical description. This is an area of science that is clearly data-driven. In light of the diverse behavior of GRB light curves and the large range in apparent luminosity, we are unlikely to reach a deep understanding of this phenomena until a large sample of these events can be observed in a broad range of wavelengths. The significant breakthrough in this effort has been the determination of precise X-ray coordinates by the Beppo-SAX mission, paving the way for optical observations and spectroscopic measurements of red shifts. After four years, there are still less than twenty GRBs that have been optically observed and a whole sub-class, short duration bursts, have not been seen at all. Of the X-ray observed events, only 50% have been correlated to an optical signal, leaving open the question of why this ratio should be so low. Finally, the extremely high intrinsic brightness of GRBs raises the possibility of using these events as probes of the early Universe as star formation became significant. The fact that GRB990123, at a red shift of 1.61, was one of the most intense bursts ever seen in gamma-rays and reached an optical brightness with mv < 9 shows that these events should be detectable at much higher red shifts, far deeper than we can probe with supernovae.
The proposed Douglas Mawson Telescope (DMT) has some unique characteristics that would greatly improve our ability to observe gamma-ray bursts at longer wavelengths. The location near the South Pole gives it one outstanding advantage - it is possible during the Antarctic winter to continuously monitor a specific burst for as long as patience will allow. This means that extensive light curves can be obtained from a single instrument without the systematic problems that beset present attempts. With imaging sensors that extends deep into the infra-red, this instrument is likely to find counterparts either hidden by dust or molecular clouds or at red shifts inaccessible with silicon CCDs. Although space-born telescopes have the advantage of working outside the glow of the atmosphere, such expensive missions cannot devote extensive observing time to one particular research program. If the DMT can be engineered with reasonably rapid slew, it will have another advantage - access to the optical burst phase such as viewed by the ROTSE project for GRB990123. Because of the torque requirements and the complexity of safely reorienting a spacecraft, this observing niche must remain solely in the domain of ground-based instruments. There are two new robotic 2-meter telescopes that have rapid slew capability; both of them lie in the northern hemisphere (La Palma and Hanle, India). The number of events that can be monitored per year can be estimated fairly reliably. The SWIFT mission which is expected to launch around 2004 should identify ~300 events per year with accuracies of the order of a few arc-minutes. With a target-of-opportunity observing program, at least 20 events should be accessible for prompt observations - this might be significantly higher since deep IR imaging can be performed under daytime conditions. The most exciting possibility is that the DMT will be able to detect bursts at distances that far exceed anything we know about today. Lamb and Reichart (Astrophysical Journal 536, 1-18) have calculated that the SWIFT GRB mission will be sensitive to red shifts in excess of 70. Coupled to estimates of the early star formation rate, it is likely that a significant number of bursts can be optically detected at red shifts between 5 and 10. Such a discovery would open a new window to understanding the evolution of the Universe.
Astrometric interferometry in Antarctica - James Lloyd & Ben Oppenheimer, U.C. Berkeley
Lower boundary layer turbulence at the South Pole degrades the seeing from the superb free atmosphere value. At sites such as Mauna Kea or Paranal the seeing is primarily caused by turbulence at altitudes of 10-20km.
Now, it happens that the mean square error for an astrometric measurement with a dual beam differential astrometric interferometer in the very narrow angle regime is proportional to the integral of h^2 C_n^2(h). Therefore, sites at which the turbulence occurs only at low altitudes offer large gains in astrometric precision.
Science programs that would benefit greatly from such an instrument include planet detection, microlensing by dark matter candidates, studies of the mass and dynamics of the galaxy, and fundamental astrophysical measurements such as stellar properties and the cosmic distance scale.
Integrating a Hufnagel-Valley turbulent atmosphere model that fits Rodney Marks' South Pole median low altitude data gives an astrometric error of 4 microarcsec for a 1 hr integration with a 100m baseline for stars separated by 1 arcmin. The same interferometer at Mauna Kea with Roddier's 1990 "typical" Scidar profile gives 50 microarcsec accuracy. This gives an Antarctic interferometer a factor of 12 increase in accuracy, or a factor of 144 in speed.
The Keck interferometer, SIM and the VLTI are all planning extensive astrometric science projects. Some scientific ighlights for an antarctic interferometer might be:
- Planet
Detection
(see http://sim.jpl.nasa.gov/science/planet.html). The important things to know about planet detection:
- Jupiter's signature at 10 pc is about 1 milliarcsec.
- The Earth's is about 1 microarcsec.
- Astrometric signatures decline as 1/d
- You will never see the Earth's orbit unless you model out Jupiter's (see the figure on the SIM web page). This means that a 5 year mission (e.g., SIM) would cover an insufficient time interval to fully model a solar-like system.
For planet detection, a factor of 12 in accuracy translates directly into:
- a factor of 12 in detectable mass, or
- a factor of 12 in distance at which a given system can be detected, or
- a factor of 1700 in volume to search!
- a 144 times increase in sky area from which to select ref stars
- Astrometry
of Microlensing towards the LMC
There is a controversial body of evidence that suggests that a substantial fraction of the dark matter in the Milky way is in the form of old white dwarfs. These may have been detected by microlensing, and direct surveys. Astrometry of the microlensing events is sufficient to determine the distance to the lens, and therefore determine whether the LMC microlensing lenses are in the Halo of our Galaxy. The LMC is also uniquely suited as a prime target for Antarctic telescopes.
- Measuring
the distance to the LMC, perhaps even by direct parallax.
Optical design of the DMT with near-IR imager - Peter Gillingham (AAO)
I've made a preliminary study of the performance to be expected from a Ritchey Chretien telescope with a 2 metre diameter primary mirror imaging directly onto an array in the K' and L infrared windows.
The telescope
The figure below shows the telescope. It has a primary focal ratio of f/2, a final focal length of 12m, and a back focus (distance from primary vertex to focus) of 700mm. The secondary is made undersized so that light from outside the primary is not reflected by the secondary anywhere within a 30 arcmin diameter field. The secondary diameter is 560 mm and the diameter used on the primary for any one point in the field is 1894mm, making the final focal ratio f/6.34. The scale is about 58 microns/arcsec.
With an optimum choice of primary and secondary asphericities (the Ritchey Chretien condition) coma-free images are formed on a surface with radius of curvature 1234 mm. For a plane detector, a reasonable compromise focus can be set for a field radius up to about 10.6 arcmin. The figure below shows the diffraction based images with their Strehl ratios for this case. The K' window is represented by wavelengths 2.3 and 2.6 microns and the L window by 3 and 4 microns. Note that the boxes are 100 microns on a side.
For a larger field with a plane detector, it is necessary to flatten the field optically. The figure below shows the layout with a meniscus field flattener of CaF2 just ahead of the detector.
The figure below shows that, with this arrangement, images with little degradation compared with the diffraction limits are obtained to the full 15 arcmin radius.
Near IR imager
Considering
- that the scale at the Ritchey Chretien focus already suits high resolution imaging with an IR array having pixels about microns square,
- that there is little requirement for spectroscopy, and
- that the telescope environs will be very cold,
it is likely (as suggested by Peter McGregor at the DMT Science Workshop on 4 May 2001) that re-imaging the telescope pupil onto a cold stop can be avoided. In the first figure above, the locations are indicated where a cold baffle (inside the IR dewar) and "warm" Narcissus mirrors might be put. With suitable choice of the locations and radii of curvature of these mirrors, it is possible to ensure that no part of the field is exposed to any radiation from telescope surfaces either directly or via reflection.
The figure below shows how effective Narcissus mirrors could be in the case where the field is 15 arcmin square (roughly filling a 2k x 2k array). Extraneous skylight, passing directly to the detector without reflection from the primary and secondary mirrors, is limited to about 20% addition to the unavoidable sky background, without vignetting the field.
Efficacy of Narcissus mirror in excluding extraneous sky
My example above, giving ~ 20% additional sky radiation, was for a 2k x 2k x 18um array. In theory, the excess radiation would halve for a 1k x 1k array (and double for 4k x 4k). One could reduce the percentage of excess radiation by accepting some vignetting; e.g. for the 2k case, making the narcissus mirror aperture match the circular beam profile for the on-axis case would result in vignetting and excess radiation each grading from 0 at the field centre to about 6% at the corner of the field. For a 1k array especially, the sky exclusion would, I think, be very little inferior to that of a system with imaging onto a cold stop. Its efficiency, freedom from ghosts, and cost would be very favourable.
Guiding
I imagined offset guiding would be done using either small CCDs or fast IR arrays set next to the sides of the IR array, inside the Dewar.
Large arrays
I note from the 13 Oct '98 letter to Gatley from Greenhouse (NGST Deputy Project Scientist) appended to the Report to the OIR Panel of the Decadal Committee..., "Infrared Astronomy at the South Pole" that NGST were meaning to use 4k x 4k InSb arrays. Depending on its pixel size, the diagonal of such an array might cover about 30 arcmin at 12 metre focal length.
Alt-az versus equatorial
I think an equatorial telescope at latitude 75 deg, ie. an alt-az with its az axis tilted 15 deg., would be little more complicated than an untilted alt-az. Avoiding the necessity for an instrument rotator and having a non-rotating psf seem worthwhile. However, I think it would be highly desirable to have fine adjustments of the polar axis tip and tilt motorised (somewhat like the polar axis elevation of the UK Schmidt, but for different reasons). I suspect the ice foundation, even if it's not built up 150 m above the plateau, will drift in angle sufficient to need correction several times a year for the most critical "wide field" imaging.
Conclusions
A Ritchey Chretien optical system with a field flattener can give virtually diffraction limited performance in K' and L across a flat field 30 arcmin diameter with the aid of a simple field flattening lens.
For a 2k array about 38mm square, no field flattener is needed and Narcissus mirrors can limit extraneous radiation directly from the sky to about 20%, without vignetting the field. This leads to a very simple system for direct imaging.
Frequently Asked Questions and Common MisconceptionsHow does the DMT compare with other planned and proposed facilities?
The DMT, as a 2m size telescope, is a relatively cheap facility, costing less than 5% that of large optical telescopes, or the airborne and space-facilities being planned by other countries. Nevertheless, the unique environment of Antarctica allows it to undertake a range of science that is competitive with these facilities.
At wavelengths beyond 2.3 microns the DMT is as sensitive as an 8m telescope on an excellent site like Mauna Kea, for imaging extended sources (ie when measuring flux per unit area). It is, however, considerably easier to image large fields of view. Thus, for wide-field thermal infrared imaging an Antarctic 2m would out-perform existing telescopes. It can therefore attempt a range of projects that are complementary to those larger facilities (for instance, surveying wide areas surrounding fields imaged by the larger facility), as well as many that would not be undertaken with an 8m (for instance, mapping the Galactic ecosystem).
For point-source imaging, if the diffraction limit can be achieved then invariably the larger the telescope used the greater the sensitivity achievable, because the background noise signal is obtained from a smaller region of sky. If, however, the full-diffraction limit of the telescope is not achieved by the larger telescope that gain is rapidly lost over an Antarctic telescope, where the background levels are typically 20 times lower in the infrared.
Planned facilities such as SIRTF, SOFIA and NGST, while they will each achieve superb performance in their own areas, still remain virtual facilities that are subject to descoping. SOFIA has recently been delayed a further two years due to budgetary considerations. NGST has just had its long wavelength functionality considerably descoped. SIRTF has had its specifications fixed for some time. It is yet to be launched and its operations then entail some considerable risks. While in areas of overlap a ground-based facility cannot achieve the sensitivities of a cryogenic space telescope, SIRTF is a relatively small facility with a spatial resolution of, at best, of 2.4 arcseconds. In the science wavebands of relevance to the DMT, SIRTF has just four fixed broad band filters, at 3.6, 4.5, 5.8 and 8 microns. There are no line filters, for instance, to measure molecular hydrogen, PAHs or ionized hydrogen.
SIRTF will be a superb facility, nevertheless. It is going to open up new areas of study, particularly at far- infrared wavelengths, which cannot be accessed from the ground. Rather than complete all science that is possible at thermal infrared wavelengths, SIRTF is going to stimulate great demand for follow-up projects, projects for which the time available with SIRTF was insufficient to undertake and projects for which its instruments were incapable of attempting. Historically, the age of space observatories has resulted in greatly increased demand for ground-based follow-up. HST has stimulated the demand for optical 8m telescopes, for instance. In the thermal-IR there are few other facilities anyway, so that demand for those that exist is expected to be intense once SIRTF is launched.
Australian access to SIRTF, and later to SOFIA and NGST, will be extremely limited in any case as we have no national involvement in these facilities. The DMT, as one of the few facilities available which allows follow-up projects to be conducted, therefore provides Australia with an opportunity to trade time for access to the other facilities.
Why work in the thermal infrared?
To date there has been little work done in Australia in the thermal infrared part of the spectrum. In large, this is due to the difficulties of working in this waveband, the Australian mainland not having any really good sites for working beyond 3 microns. However, even at major observatory sites like Mauna Kea, working beyond 3 microns is still hard, and relatively few groups have concentrated their science there. The scientific potential for infrared investigations, however, is not questioned. The birth of planets, stars and galaxies can only be seen in these wavebands. The importance that has been attached to observing in the infrared can be judged by the fact that several nations are now investing in major infrared facilities like SIRTF, SOFIA and NGST, facilities that can cost over two orders of magnitude more to build than the DMT.
As the frontiers of astronomy keep advancing we need to move forward too if we wish to participate at its leading edge. A century ago cutting-edge science involved measuring the orbits of double stars. A decade ago it was spear-headed by the 4m optical telescopes like the AAT. In the coming decade it will involve the infrared.
Why work in the sub-millimetre?
The reasons are similar to those given above for the infrared. It is impossible to work in this regime from Australia. It remains difficult to observe in it from Mauna Kea. Yet the first signatures of proto-galactic and proto-stellar collapse are given out in the sub-millimetre. Technological advances in receiver and telescope design now make it feasible to consider interferometric arrays operating in the sub-millimetre, the most notable being the one billion dollar ALMA project (planned for the Atacama in Chile). Australia currently has no expertise in this area even if we wished to participate. The Antarctic plateau does, however, provide the best locations for sub-millimetre astronomy on the Earth. It would be particularly suitable for a single- dish telescope, complementing ALMA, and even observing in some windows that will remain closed to ALMA (eg at 200 microns).
Australia is also rapidly developing skills in millimetre astronomy, through both the Mopra telescope and the forthcoming millimetre interferometer on the ATCA. Sub-millimetre astronomy in Antarctica provides a natural confluence connecting the interests of the optical/IR and radio communities in Australia, and will also provide us with the skills and ability to contribute in the future to ALMA.
How important is high spatial resolution?
Spatial resolution is, of course, exceedingly important for a wide range of astronomical observations. The DMT, by virtue of its size, would not be able to compete with an 8m telescope in this regard. This is why the initial scientific focus will be on wide-field studies, where areas of sky typically two orders of magnitude greater than observable with an 8m could be studied.
Australians requiring the highest spatial resolution for their projects would continue to use Gemini, though they only have a handful of nights available each year to do so, and so any such projects are somewhat restricted in their scope.
Ultimately the best resolution will be obtained by an interferometer, not a single telescope. The superior phase stability of the sky makes Antarctica an attractive location for a mid-infrared interferometer, needed to resolve proto-planetary disks and zodiacal clouds around stars.
How does the seeing affect performance?
The ice-level seeing at the South Pole is relatively poor, about 1.5 arcseconds in the visible, comparable to Siding Spring. It is however, confined to a narrow inversion layer, some 200m thick. This leads to an isoplanatic angle of about 1 arcminute, some 30 times greater than achievable on Mauna Kea, where the turbulent layer arises in the jet stream. This greatly facilitates adaptive optics correction, with longer coherence times, most of the sky containing stars bright enough to be used for AO correction, and of course a one arcminute corrected field of view. Moreover, there is every expectation that at high plateau sites such as Dome C the boundary layer will be even thinner, raising the possibility that a telescope could be placed on a raised tower above it, avoiding most of the seeing altogether!
Is accurate photometry possible?
The proximity and narrowness of the turbulent boundary layer also implies that scintillation will be considerably reduced from temperate-latitude sites. Since scintillation noise is a major limitation to precision photometry (ie at the milli-magnitude level), this opens up several fields where this level of precision is required, for instance exo-solar planet detection via occultations and stellar seismography.
How much better can we do microlensing experiments in Antarctica than Australia?
Lensing experiments in the optical have obviously been done exceedingly effectively in Australia, notably from Canberra, Hobart and Perth. However it is not possible to obtain a unit depth to lensing within a field (i.e., to have enough stars in the field so that a lensing event is always going on) unless viewing the Galactic centre, and this necessitates infrared observations in order to reduce the extinction sufficiently for the stars to be seen. At 2.35 microns all the stars towards the Galactic centre can be seen, and moreover this is the very wavelength where the background is lowest, between the thermal emission at longer wavelengths and airglow emission to shorter wavelengths. For planet searches it is essential to have both fine time resolution (the secondary events due to planets might only last minutes), over extended periods of time (so the source must be continuously visible) and enormous numbers of sources to work on (requiring a sight line towards the Galactic centre). All these conditions can be uniquely met from Antarctica.
How does the DMT maintain our ability in astronomical technology?
Antarctica is a challenging environment, and operating in it requires novel solutions to some engineering problems, and attracts the best engineers to solve them. Moreover, innovative engineering solutions often have commercial spin-offs in unrelated areas, which enhances their value considerably.
As optical telescopes have got larger the size of their instruments has expanded considerably. The number of institutions that can hope to build them has correspondingly diminished, and they are now beyond the capabilities of all but the largest groups. In Australia only at Mount Stromlo and the AAO are there groups capable of undertaking instrument projects for an 8m telescope. A 2m telescope, such as the DMT, requires a relatively small instrument, presenting a project which is within the capability of university groups. Research groups will once more be able to contemplate building an instrument to focus on a particular area of interest. This will contribute to Australia maintaining a vibrant research community across a range of institutions.
How does global warming and the shrinking polar ice caps affect the DMT?
- There will be no significant change to the polar ice caps in the next ten years. Icebergs come and go along the coast, but then, they always did.
- Most models predict an increase in global hunidity, which harms the sites in Chile and Hawaii. The relative humidity at Dome C is already 100% (although the absolute humidity is, of course, miniscule), so the increased global humidity won't affect the atmospheric transmission.
- The increased global humidity will cause an increase in the height of the polar cap (because it is such an efficient cryopump). Dome C will get even higher.
So, 100 years from now, Dome C will be even better and Chile and Hawaii even worse. Meanwhile, both Sydney and Melbourne will be under 10 feet of water, and infrared astronomy will be the least of our concerns...
How difficult is it really to work in Antarctica?
There is no doubt that Antarctica provides a challenging environment in which to work in. However much of difficulty is perceived, and arises from the fear of the unknown. Australia now has a decade of experience working on a range of astronomical instrumentation at the South Pole, and has worked closely with the USA over that time, sharing many ideas along the way. The main problems are thermal issues, and these are now well understood. In fact Antarctica has many features that make designing telescopes easier than at temperate sites. These include:
- Very low wind speeds on the plateau
- Humidity is not a problem
- There is no need for a dome to keep the rain off
- There is no dust
- The telescope can be left open to the elements throughout the year
In addition to the AASTO and the SPIREX telescope that Australia has had direct involvement in, there are a number of other successful telescopes with sophisticated instrumentation operating at the South Pole, which show that the challenges can indeed be met. These include the VIPER and DASI CMBR experiments, the AST/RO sub-millimetre telescope and the AMANDA neutrino telescope.
How much time could the telescope be used for? Can it be used in the summer months?
Optical experiments could obviously only be undertaken in winter months, and there are only about 4 months for which the sky is truly dark. However, in the thermal infrared there is little difference between day time and night time conditions, and the longer the wavelength the less the difference is. For instance, the published image of the PAHs emission in Carina (Brooks et al, MNRAS, 319, 95-102 (2000)) was obtained during November, ie the South Pole summer. Thus the primary science the DMT would perform could be undertaken at any time of the year. In practice, of course, during the summer time would also be devoted to telescope maintenance and instrument upgrades, but once the telescope is fully operational only instrument changes would limit full-time observation.
What science is driving the desire for an interferometer?
Origins themes, such as the search for another Earth, are increasingly driving the directions of several major national programs. To meet their objectives it is necessary to construct a mid-IR interferometer, where the signatures of an Earth-like planet are believed to be clearest. Grand-design experiments, such as NASA's TPF or ESA's Darwin, have been proposed for this. These are tremendously challenging projects, and will require a vast amount of preparatory work. Foremost is the challenge of constructing a nulling interferometer. The reduced sky emission and the improved phase stability offered by Antarctica make this the most suitable location for developing these techniques. Along the way an Antarctic interferometer would also be able to undertake much valuable science, from imaging proto-planetary disks to detecting zodiacal clouds as a star evolves towards the main sequence, to possibly directly detecting Jovian-sized planets orbiting several AU from their parent star.
Where does the DMT lead to?
While the DMT provides a range of fascinating science projects that can be undertaken, it is also just an entry point to Antarctica that lets us contemplate some truly exciting science that could be undertaken with major facilities. These include both large optical/IR telescopes, sub-millimetre telescopes and interferometers. The DMT can be augmented through a range of instrumentation and through construction of further 2m-sized telescopes, each devoted to single major projects. This is possible because the cost of the telescope no longer dominates the cost of the project - a major instrument could indeed cost more than the telescope itself. Further on, a large optical/IR telescope might be built, and/or several 2m telescopes connected together to form a mid-IR interferometer. A possible scenario might be as follows:- The DMT is built with a 3-5 micron thermal infrared camera and a 1K array.
- University groups contribute individual instruments to the telescope. These might include a mid-IR camera, 1-2.5 micron and 3-5 micron cameras with their 30 arcminute focal plane tiled with arrays (for lensing studies, monitoring projects and surveys), an optical echelle spectrograph mapping stellar surfaces, and a sub-mm bolometer array. Each instrument might be used for focussed in-depth investigations (ie lasting 1-3 months), before the instrument is changed.
- A 6.5m telescope, the Australian Large Telescope, Antarctic (ALTA), is built. Its performance beyond 2.4 microns would exceed that of the Keck for all possible observations. This could be built in as little as 5 years after the DMT, as the requisite expertise is now commercially available.
- A suite of 2m-sized telescopes are connected together to form a mid-infrared interferometer, used both as a testbed for developing the technology of nulling interferometry, and to study the evolution of proto-planetary disks into zodiacal clouds as planetary systems form around young stars.
Without the DMT none of these projects will be possible. It truly is an entry point for a range of competitive projects that will allow Australia to maintain its reputation as a leading nation in astronomy and the associated high technology. But if the DMT is not built the impetus and initiative that Australia has established over a decade of effort in Antarctica will be lost, as we watch others capitalise on our pioneering endeavours.
On 29 July 2001, during the last day of the highly successful Dome C workshop in Hobart, we learned that the DMT funding proposal had been culled from further consideration.
We note that our proposal included support from three industrial partners and had operating and logistical costs for the telescope guaranteed for 10 years by the French and Italian governments.
Despite this setback, and because of the strong support the proposal has generated from other quarters, we are continuing to pursue funding possibilites through other channels.
Last updated 10-Aug-2001
DMT / Michael Ashley / mcba@newt.phys.unsw.edu.au
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© Department of Astrophysics, UNSW, Sydney, AUSTRALIA
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