The Scientific Context
Continuous progress in understanding the Universe arise from improvements in our capacity to scrutinize the sky to greater depth, in previously unexplored wavebands, with better photometric and astrometric accuracies, or with improved spatial, spectral, or time resolution. Thanks to the drastic progress in computing science and technology and the dramatically increasing capacity of data archiving, future sky surveys will completely change our future perception of the Universe. The next-generation instruments, and the surveys carried out with them, will maintain this progress.
The pivotal astrophysical problems of the next decade, soundly discussed and synthesized in the recent prospect surveys carried out in Europe and the USA (ESA Cosmic Vision, ASTRONET, US Decadal Survey, etc...) consists of the determination of the nature of dark energy and dark matter, the study of the evolution of galaxies and the structure of our Galaxy, the opening of the time domain to single out faint variable objects and transient events, the deep exploration of the small bodies of the Solar System, and the identification of habitable extra-solar planets. All require wide-field frequently repeated deep imaging of the sky in a wide range of spectral bands.
There are plenty of wide field imagers currently available at visible wavelengths, each with varying strengths in field of view, spatial resolution and depth. Nevertheless the top priority of the 2010 US Decadal survey is the Large Synoptic Survey Telescope (LSST), a 6.5m wide-field telescope equipped with a ~ 10 square degrees camera. From about 2018 the LSST will be available to repeatedly survey the sky with deep short exposures. Thanks to recent developments in the industrial production of larger infrared arrays (currently 4 but soon up to 16 Mpixels), it has been possible to develop facilities for similar surveys in the near infrared. The leading facility is the VISTA telescope at ESO (Paranal) which started operations in 2009 and is equipped with a 64 MPixel camera and a 1.2°x 1.2° field of view. It is currently undertaking a mixture of shallow wide area and deep small field surveys over wavelengths from 0.8 to 2.3 μm.
Since the new arrays can operate up to 5 μm, the next logical step is to investigate the thermal infrared wavelengths from 2.3 to 5 μm. Apart from narrow field largely spectroscopic observations, this is not practical from ground based mid-latitude observatories like Paranal because of the drastic rise in the thermal sky background emission which swamps any signal. Consequently, up to now this has essentially been the domain of space borne missions such as Spitzer, AKARI, and WISE. These have indeed produced compelling results in the thermal infrared, but at modest angular resolution and thereby with limited astrometric precision and therefore suffer from confusion limitations in crowded areas. Moreover, they do not explore systematically the time domain. In the coming decade, the James Webb Space Telescope, the E-ELT and other extremely large telescopes will partially operate in this range with unprecedented sensitivity and angular resolution, but will not be practical for repeated observations of large areas of the sky. Therefore the need for a synoptic survey able to cover thousands square degrees comparable to LSST and VISTA at longer wavelengths is fully justified. in complement to the recent space missions.
The Polar Large Telescope (PLT)
The PLT is an internationally supported facility primarily aimed to perform deep imaging and possibly spectro-imaging surveys of large sky areas (several thousands square degrees) in the near thermal infrared range (2.3-5 µm) at the diffraction limit of a 2-3 m aperture telescope at 2 µm (~250 mas). Every sky area will be observed at least 20 times a year to identify transients and monitor variable objects. PLT is aimed at extending the VISTA survey at longer IR wavelengths and at complementing the LSST in the infrared.
It is intended to be built at the French-Italian CONCORDIA station at Dome C in Antarctica to benefit from the exceptional polar atmospheric conditions.
First light at Dome C is foreseen before 2020, for a 10-year exploitation period.
PLT mainly focuses on deep high angular resolution surveys of large areas of the sky at wavelengths not easily accessible from the ground even in the best sites, namely, beyond 2.3 µm. It will fill the gap between the deep surveys that VISTA will undertake up to 2.2 µm (Kshort) and mid/far-IR surveys carried out by space missions (such as Spitzer, AKARI, Herschel, and WISE,... ).
Since the basic optical configuration of the telescope will offer 2 Nassmyth foci, a second instrument could be set up simultaneously, for instance, a spectro-imaging camera covering the mid-infrared range (5 - 40 µm) that could be specially dedicated to large scale mappings in the rotational lines of molecular hydrogen.
To comply with logistics limitations, the proposed aperture of the primary mirror of the PLT is ~2.5 meters, but the final aperture size will range between 2 and 3 meters according to the conclusions of the conceptual design study (CDS).
The CDS will determine the characteristics of the telescope, the camera, the data pipeline, the strategy of observations, civil engineering, logistics, and operations in Antarctica, in consideration of the science cases priorities, the overall cost, the final site assessment results and the current logistics capability that can be reasonably offered by the Polar operators (the French IPEV and the Italian PNRA at Dome C)
A working group has been set up to support and prepare a management plan and to seek funding for a phase B study to be undertaken in the period 2012-2014 on the basis of the PILOT phase A study made in Australia (AAO+ UNSW) in 2007-2008.