A Fabry-Perot interferometer for ground-based millimetric spectroscopy

Astrophysical observations at millimeter and sub-millimeter wavelengths are one of the most useful tools to understand the history of the Universe. Although ground-based observations are affected by the presence of the Earth atmosphere, space telescopes remain limited in size offering only modest angular resolution not suitable to detect small spatial features of so compact objects. To date the ground-based solution seems to be the only feasible way to perform high angular resolution deep sky surveys. In this context a Fabry-Perot Interferometer (FPI) has been developed in order to improve the capabilities of MAD (Multi Array of Detectors), a 4-channel photometer optimized for multi-frequency observation of Sunyaev-Zel’dovich effect that will operate at Millimetre and Infrared Testagrigia Observatory (MITO), a 2.6-m in diameter telescope located in the Alps in Val d’Aosta (3480 m a.s.l.). The transmission response of an innovative system composed of two resonant metal grids in a FPI configuration is explored. A semi-empirical approach to perform an analysis of atmospheric transmission and emission at Dome C (Antartic Plateau) is suggested. The spectrometer CASPER2 (Concordia Atmospheric SPectroscopy of Emitted Ra- diation - MITO version) has been developed specifically to record atmospheric spectra assisting cosmological observations with the 2.6-m in diameter Cassegrain telescope at MITO

Atmospheric monitoring in the mm and sub-mm bands for cosmological observations: CASPER2

Cosmological observations from ground at millimetre and sub-millimetre wavelengths are affected by atmospheric absorption and consequent emission. The low and high frequency (sky noise) fluctuations of atmospheric performance imply careful observational strategies and/or instrument technical solutions. Measurements of atmospheric emission spectra are necessary for accurate calibration procedures as well as for site testing statistics. CASPER2, an instrument to explore the 90-450 GHz (3-15 1/cm) spectral region, was developed and verified its operation in the Alps. A Martin-Puplett Interferometer (MPI) operates comparing sky radiation, coming from a field of view (fov) of 28 arcminutes (FWHM) collected by a 62-cm in diameter Pressman-Camichel telescope, with a reference source. The two output ports of the interferometer are detected by two bolometers cooled down to 300 mK inside a wet cryostat. Three different and complementary interferometric techniques can be performed with CASPER2: Amplitude Modulation (AM), Fast-Scan (FS) and Phase Modulation (PM). An altazimuthal mount allows the sky pointing, possibly co-alligned to the optical axis of the 2.6-m in diameter telescope of MITO (Millimetre and Infrared Testagrigia Observatory, Italy). Optimal timescale to average acquired spectra is inferred by Allan variance analysis at 5 fiducial frequencies. We present the motivation for and design of the atmospheric spectrometer CASPER2. The adopted procedure to calibrate the instrument and preliminary performance of the instrument are described. Instrument capabilities were checked during the summer observational campaign at MITO in July 2010 by measuring atmospheric emission spectra with the three different procedures.Comment: 11 pages, 9 figures, 2 tables, Accepted for publication in MNRA

Site-testing and continuous atmospheric monitoring at mm wavelength band with CASPER 2

Ground-based cosmological observations need a detailed knowledge about atmospheric transmission. A continuous monitoring of the transmission in the FIR/mm wavelength range is needed mainly due to H2O high frequency variations along the line of sight of a telescope. CASPER 2 (Concordia Atmospheric Spectroscopy of Emitted Radiation) is a spectrometer devoted to measurements of atmospheric emission in the millimetric region (4-12 cm -1, 120-360 GHz) with a spectral resolution of 0.2 cm-1. The instrument allows a continuous monitoring of the atmospheric transmission of the same sky region explored with MITO (Millimeter and Infrared Testagrigia Observatory, 3480 m asl, Italy) telescope, during cosmological observations. © 2010 EAS, EDP Sciences