  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.
