GIANO: An ultra-stable IR spectrometer optimized both for low and high resolution

People involved at OAB: L. Origlia, M. Rossetti, P. Montegriffo, M. Lolli

The recent development of high sensitivity infrared (IR) spectrometers has opened a new window in astrophysics. The investigation of the 1 to 2.5 $\mu$m band has yielded the discovery of a wealth of diagnostic tools, both in terms of absorption features and emission lines, which are crucial for a thorough understanding of several hot topics of modern astrophysics, from faint solar system objects and extra-solar planets to stellar clusters and galaxies, up to the highest redshift quasars.

The much reduced extinction at these wavelengths allows IR spectrometers to pierce the dust embedding several Galactic and extragalactic objects, which are heavily obscured in the optical. At high redshift several emission and absorption spectral features, commonly exploited when studying local galaxies, are shifted into the IR. These and several other advantages of IR spectroscopy have led to a rapid growth of the community of astronomers, from essentially any scientific field, making use of these facilities.

GIANO is an optimized near IR spectrograph which can yield, in one shot, complete 0.9-2.5 $\mu$m spectra either at low ($\rm RS=500$) or high (up to $\rm R=50,000$ with a 0.5" slit) resolutions maintaining, in both modes, a very high stability and throughput throughout the whole spectral range.

This project is part of the Second Generation Instrumentation Plan of the Telescopio Nazionale Galileo (TNG) located at Roque de Los Muchachos Observatory (ORM), La Palma, Spain.

The core of GIANO is the spectrometer unit which uses, in double pass, the same optical elements (3 aspheric mirrors) as collimator and camera. These feed, through cross-disperser prisms, a commercial 23.2 ll/mm R2 echelle grating acting as high resolution disperser. All these elements and the 2048$^2$ Hawaii-2 array are fixed to an optical bench which will operate in vacuum at cryogenic and thermostated temperatures.

By simply inserting a flat mirror in front of the echelle the instrument ``changes its face'' (hence the name GIANO) and becomes a low resolution spectrometer with an unprecedented combination of spectral coverage, throughput and resolution. Measurements of circular and linear polarization can be obtained using a combination of beam splitters and super-achromatic retarders which can be inserted/rotated in the pre-slit optical system.

More details about the project can be find on the GIANO Web page.