The future of far-infrared observations rests on our capacity to reach sub-arcsecond angular resolution around 100 μm, in order to achieve a significant advance with respect to our current capabilities. Furthermore, by reaching this angular resolution we can bridge the gap between capacities offered by the JWST in the near infrared and those allowed by ALMA in the submillimeter, and thus benefit from similar resolving capacities over the whole wavelength range where interstellar dust radiates and where key atomic and molecular transitions are found. In an accompanying paper,1 we present a concept of a deployable annular telescope, named TALC for Thinned Aperture Light Collector, reaching 20m in diameter. Being annular, this telescope features a main beam width equivalent to that of a 27m telescope, i.e. an angular resolution of 0.92" at 100 μm. In this paper we focus on the science case of such a telescope as well on the aspects of unconventional data processing that come with this unconventional optical configuration. The principal science cases of TALC revolve around its imaging capacities, that allow resolving the Kuiper belt in extra-solar planetary systems, or the filamentary scale in star forming clouds all the way to the Galactic Center, or the Narrow Line Region in Active Galactic Nuclei of the Local Group, or breaking the confusion limit to resolve the Cosmic Infrared Background. Equipping this telescope with detectors capable of imaging polarimetry offers as well the extremely interesting perspective to study the influence of the magnetic field in structuring the interstellar medium. We will then present simulations of the optical performance of such a telescope. The main feature of an annular telescope is the small amount of energy contained in the main beam, around 30% for the studied configuration, and the presence of bright diffraction rings. Using simulated point spread functions for realistic broad-band filters, we study the observing performance of TALC in typical situations, i.e a field of point sources, and fields with emission power at every physical scales, taken to represent an extragalactic deep field observation and an interstellar medium observation. We investigate different inversion techniques to try and recover the information present in the input field. We show that techniques combining a forward modeling of the observation process and a reconstruction algorithm exploiting the concept of sparsity (i.e. related to the more general field of compressed sensing) represent a promising avenue to reach the angular resolution promised by the main beam of TALC.
TALC, Thin Aperture Light Collector is a 20 m space observatory project exploring some unconventional optical solutions (between the single dish and the interferometer) allowing the resolving power of a classical 27 m telescope. With TALC, the principle is to remove the central part of the prime mirror dish, cut the remaining ring into 24 sectors and store them on top of one-another. The aim of this far infrared telescope is to explore the 600 μm to 100 μm region. With this approach we have shown that we can store a ring-telescope of outer diameter 20m and ring thickness of 3m inside the fairing of Ariane 5 or Ariane 6. The general structure is the one of a bicycle wheel, whereas the inner sides of the segments are in compression to each other and play the rule of a rim. The segments are linked to each other using a pantograph scissor system that let the segments extend from a pile of dishes to a parabolic ring keeping high stiffness at all time during the deployment. The inner corners of the segments are linked to a central axis using spokes as in a bicycle wheel. The secondary mirror and the instrument box are built as a solid unit fixed at the extremity of the main axis. The tensegrity analysis of this structure shows a very high stiffness to mass ratio, resulting into 3 Hz Eigen frequency. The segments will consist of two composite skins and honeycomb CFRP structure build by replica process. Solid segments will be compared to deformable segments using the controlled shear of the rear surface. The adjustment of the length of the spikes and the relative position of the side of neighbor segments let control the phasing of the entire primary mirror. The telescope is cooled by natural radiation. It is protected from sun radiation by a large inflatable solar screen, loosely linked to the telescope. The orientation is performed by inertia-wheels. This telescope carries a wide field bolometer camera using cryocooler at 0.3K as one of the main instruments. This telescope may be launched with an Ariane 6 rocket up to 800 km altitude, and use a plasma stage to reach the Lagrange 2 point within 18 month. The plasma propulsion stage is a serial unit also used in commercial telecommunication satellites. When the plasma launch is completed, the solar panels will be used to provide the power for communication, orientation and power the cryo-coolers for the instruments. The guide-line for development of this telescope is to use similar techniques and serial subsystems developed for the satellite industry. This is the only way to design and manufacture a large telescope at a reasonable cost.
We illustrate the status of the international infra-red telescope IRAIT-ITM, a project developed thanks to an Italian- Spanish-French collaboration and now sited at the Dome C Antarctic base. The telescope and its subsystems were installed at DomeC by a team of Italian and French scientists. The 80 cm telescope is placed on a small snow hill next to a laboratory of astronomy. The operations started in January 2013, with the Nasmyth focal planes equipped with the midinfrared camera AMICA for 1.25 to 25 μm and the sub-millimetre camera CAMISTIC for observation of the sky noise at 200 and 350 μm using a bolometer camera. During 2013 the two winter-overs worked mainly on technological duties, learning how to operate the telescope, while temperatures decreased down to -80°C. The cryogenic systems could be operated respectively at 0.25K and 4K at all times, with satisfactory use of the heat from the compressors of the cryocoolers to the warm-up the laboratory through a closed loop glycol system. The lack of tests and reliability in extreme conditions of some components and difficult access to maintenance hampered regular observations below -50°C. Using the lessons of this first winter, the summer team improves the robustness of the failing systems and ease the access to maintenance. The winter 2014 is the first one with programmed observations. Because of power restrictions, the two instruments are used each one at a time by periods of 2 weeks. The Camistic camera continues to observe the stability of the sky at a fixed altitude in chopping mode and performs skydips. The TCS is being upgraded in order to prepare the next summer season with extensive observations of the sun with Camistic.
ArTeMiS is a camera designed to operate on large ground based submillimetric telescopes in the 3 atmospheric windows
200, 350 and 450 µm. The focal plane of this camera will be equipped with 5760 bolometric pixels cooled down at 300
mK with an autonomous cryogenic system. The pixels have been manufactured, based on the same technology processes
as used for the Herschel-PACS space photometer. We review in this paper the present status and the future plans of this
A prototype camera, named P-ArTeMiS, has been developed and successfully tested on the KOSMA telescope in 2006 at
Gornergrat 3100m, Switzerland. Preliminary results were presented at the previous SPIE conference in Orlando (Talvard
et al, 2006). Since then, the prototype camera has been proposed and successfully installed on APEX, a 12 m antenna
operated by the Max Planck Institute für Radioastronomie, the European Southern Observatory and the Onsala Space
Observatory on the Chajnantor site at 5100 m altitude in Chile. Two runs have been achieved in 2007, first in March and
the latter in November. We present in the second part of this paper the first processed images obtained on star forming
regions and on circumstellar and debris disks. Calculated sensitivities are compared with expectations. These illustrate
the improvements achieved on P-ArTeMiS during the 3 experimental campaigns.
Submillimetre astronomy is the prime technique to unveil the birth and early evolution of stars and galaxies in the local
and distant Universe. Preliminary meteorological studies and atmospheric transmission models tend to demonstrate that
Dome C might offer atmosphere conditions that open the 200-μm atmospheric windows, and could potentially be a site
for a large ground-based telescope facility. However, Antarctic climate conditions might also severely impact and
deform any telescope mirror and hardware. We present prerequisite conditions and their associate experiments for
defining a large telescope facility for submillimetre astronomy at Dome C: (1) Whether the submm/THz atmospheric
windows open from 200 μm during a large and stable fraction of time; (2) The knowledge of thermal gradient and (3)
icing formation and their impact on a telescope mirror and hardware. This paper will present preliminary results on
current experiments that measure icing, thermal gradient and sky opacity at Dome C. We finally discuss a possible
roadmap toward the deployment of a large telescope facility at Dome C.
Astronomical observations at sub-millimetre wavelengths are limited either by the angular resolution of the telescope or
by the sensitivity and field of view of the detector array. New generation of radio telescopes, such as the ALMA-type
antennas on Chajnantor plateau in Chile, can overcome these limitations if they are equipped with large detector arrays
made of thousands of sensitive bolometer pixels.
Instrumentation developments undertaken at CEA and based on the all silicon technology of CEA/Leti are able to
provide such large detector arrays. The ArTeMiS project consists in developing a camera for ground-based telescopes
that operates in two sets of atmospheric windows at 200-450 μm (channel 1) and 800-1200 μm (channel 2).
ArTeMiS-1 consists in grid bolometer arrays similar to those developed by CEA for the Herschel Space Observatory. A
prototype camera operating in this first atmospheric window was installed and successfully tested in March 2006 on the
KOSMA telescope at Gornergrat (Switzerland) in collaboration with the University of Cologne. ArTeMiS-2 will consist
either in antenna-coupled bolometer arrays or specific mesh bolometer arrays.
By the end of 2008, ArTeMiS cameras could be operated on 10m-class telescopes on the Chajnantor ALMA site, e.g.,
APEX, opening new scientific prospects in the study of the early phases of star formation and in cosmology, in the study
of the formation of large structures in the universe. At longer term, installation of such instrumentation at Dome-C in
Antarctica is also under investigation. The present status of the ArTeMiS project is detailed in this paper.