E S.Cruz de La Palma, Spain; ABSTRACT

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1 GIANO: an ultra-stable IR echelle spectrometer optimized for high precision radial velocity measurements and for high throughput low resolution spectroscopy E. Oliva 1,3, L. Origlia 2, R. Maiolino 3, S. Gennari 3, V. Biliotti 3,E.Rossetti 2,C.Baffa 3, F. Leone 4,P.Montegriffo 2,M.Lolli 2, F. D Amato 5,P.Bruno 4, S. Scuderi 4, F. Ghinassi 1, M. Gonzalez 1,M.Lodi 1, G. Falcini 3,E.Giani 3, G. Marcucci 6, and M. Sozzi 7 1 Istituto Nazionale di Astrofisica (INAF) Centro Galileo Galilei, calle A. de Abreu 70/1, E S.Cruz de La Palma, Spain; 2 INAF Osservatorio di Bologna, via Ranzani 1, I Bologna, Italy; 3 INAF Osservatorio di Arcetri, largo E. Fermi 5, I Firenze, Italy; 4 INAF Osservatorio di Catania, via S. Sofia 78, I Catania, Italy; 5 Istituto Nazionale di Ottica Applicata, largo E. Fermi 6, I Firenze, Italy; 6 Università di Firenze Dip. di Astronomia, largo E. Fermi 5, I Firenze, Italy; 7 INAF IRA, largo E. Fermi 5, I Firenze, Italy ABSTRACT GIANO is an infrared ( µm) cross-dispersed echelle spectrometer designed to achieve high throughput, high resolving power, wide band coverage and high accuracy radial velocity measurements. It also includes polarimetric capabilities and a low resolution mode that make it a very versatile, common user instrument which will be permanently mounted and available at one of the Nasmyth foci of the Telescopio Nazionale Galileo (TNG) located at Roque de Los Muchachos Observatory (ORM), La Palma, Spain. GIANO was selected by INAF as the top priority instrument among those proposed within the Second Generation Instrumentation Plan of the TNG. More information on this project can be found at the web page Keywords: Ground based infrared instrumentation, infrared spectrometers 1. INTRODUCTION The main aim of the GIANO project is to build, in the shortest possible time, a high resolution IR spectrograph with superb spectral stability, very wide spectral coverage and the highest possible throughput. The primary scientific target is the search of rocky planets (down to a few Earth masses) around low mass stars by means of radial velocity measurements with accuracies of a few m/s, i.e. comparable to those presently achieved at optical wavelengths. Complementary to this highly specific scope, the instrument must also offer top-level observing capabilities of interest for the very broad community of solar-system, stellar, galactic and extra-galactic astronomers who greedily access the TNG telescope. These apparently contradicting requirements are satisfied by the GIANO instrument which takes its name from the double-faced God of the ancient Romans. Its front face is that of high resolution (up to R=50,000) cross-dispersed echelle spectrograph delivering a quasi complete µm spectrum in a single shot, while the back face shows a long slit low resolution (R=500-1,000) spectrometer covering the full µm range and ideal for the study of very faint continuum objects. The capabilities of GIANO are summarized in Table 1. The expected throughput of the system (including pre-slit optics, see below) is 18% and 23% in the high and low resolution modes, respectively. The corresponding limiting magnitudes, adopting a very conservative estimate of the array performances (i.e. 17 electrons of noise with a maximum on-chip integration of 30 minutes and a maximum array efficiency of 60%), are summarized in Table Ground-based Instrumentation for Astronomy, edited by Alan F. M. Moorwood, Masanori Iye, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 Table 1. Overview of GIANO capabilities Observing mode a slit-length RS b spectral coverage High-res (HR) , µm Low-res (LR) µm Very-low-res (VLR) µm HR long-slit c 30 25,000 single order Integral Field Mode c 4 x4 d µm Slit widths: 0.5 (2pix), 0.75 (3pix), 1.0 (4pix), 1.5 (6pix) Limiting magnitudes HR mode, R=50,000 slit=0.5 seeing=1.0 Band S/N=100 in 1hr S/N=10 in 1hr 1µm & J H K Limiting magnitudes LR mode, R=500 slit=1.0 seeing=1.0 Band S/N=100 in 1hr S/N=10 in 1hr 1µm & J H K a All modes can be used with the polarimetric unit b Resolving power for 1 slit c Optional, not included in the baseline d Eight slices of 0.5 width 2. DESCRIPTION OF THE INSTRUMENT GIANO is designed with the specific requirement that all its components are commercially available or can be easily built using standard technologies. These constraints, essential for a fast-track instrument, imposes a compromise between spectral resolution and spectral coverage. We adopt a solution which achieves a quite high resolving power (R=50,000 with a 2 pixels slit) with a quasi complete spectral coverage of the near-infrared wavelength range. Specifically, the spectrum is 100% complete from λ=0.9 µm toλ=1.8 µm, and about 80% complete in the K-band ( µm). The resulting spectral format is displayed in Fig. 1. The optical design follows the concept of other compact, high efficiency instruments developed for optical high resolution spectrographs 1, 2 taking advantage of the very high reflectivity of gold-coated mirrors in the wavelength range of interest. The spectrometer optics consist of a classical three mirrors anagstimat (TMA) used in double-pass which acts both as collimator and camera. The mirrors all off-axis conical surfaces and the collimated beam is 100 mm. The dispersing element is a commercial 23.2 lines/mm R2 echelle working at a fixed position in a quasi-littrow configuration with an off-axis along the slit of 4. A system of prisms from high dispersing IR optical materials (infrasil and ZnSe) acts as double-pass cross-disperser. The use of prisms as cross-dispersers allows obtaining a simultaneous coverage of the µm range maintaining a very high throughput at all wavelengths. The optical layout of the instrument is shown in Fig. 2. The 2-mirrors focal reducer (FR) is used to re-image the input slit of the TMA at a convenient place far from any critical element. The spot diagrams are displayed in Fig. 3, the image quality is very good with 80% ensquared energy within onepixelovertheentirearray. Proc. of SPIE Vol

3 The detector is a Hawaii x2048 HgCdTe array controlled using a custom electronic and software system. 3 The use of two buttable Hawaii-2 RG arrays could provide a complete coverage of the µm band. However, this is incompatible with the very limited budget presently available to the project. The switching between high and low resolution modes is performed by inserting a flat mirror just after the second prism (see Fig. 2). A lower resolution mode (VLR) can be also obtained by inserting another mirror in between the two prisms and using only the infrasil prism as disperser. This observing mode could be very useful for the study of the near infrared spectral energy distribution of very faint objects down to the short wavelength cutoff of the detector, i.e. 0.7 µm (see the spectral format in the bottom panel of Fig. 1). The spectrometer is included in a custom-designed cryostat made of welded steel plates with suitable reinforcing ribs which supports the cold optical bench by a hexapod which compensates for the relative contraction and avoid permanent deformation of the bench. All the optics are enclosed in a radiation shield in thermal contact with the cold bench. The 400 kg and 170x100x80 cm dewar is cooled to a thermostatted temperature of 90K by two close cycle coolers one of which is also used to cool the detectors to a lower temperature adjustable between 65K and 90K. The spectrometer will be mounted at a fixed position and fed by a warm pre-slit optical system which primarily consists of two lens groups. The first lens re-images the F/11 telescope focal plane at a conveniently longer focal ratio (about F/25) while the second lens, which lies just before the spectrometer dewar window, focus the rays at F/15 onto the cold slit-wheel and creates, a few cm before the slit, an image of the TNG pupil which matched by a fixed Lyot stop. Before the second lens is a optical derotator consisting of three flat mirrors. The field of view accepted by the pre-slit optics is 30. A viewing/guiding camera, either a commercial CCD camera or a small IR camera with dedicated custom optics, can be fed by suitable beam-splitters inserted just after the optical derotator. Polarization analyzers and absorption gas cells can be inserted in the light path close to the intermediate, warm focal plane. The polarimetric system consists of a λ/2 and a λ/4 super-achromatic plates mounted on commercial rotatory systems. It also includes a beam-splitter, which produces two images at a sky-projected distance of 3 arcsec along the HR slit, and a de-polarizer. Each of these elements can be separately inserted in the beam using commercial slide systems. The absorption cell is cooled to 25 C inside a separate dewar to ensure that the lines at λ>2.1 µm are seen in absorption rather than in emission. The overall stability of the instrument is guaranteed by the fact that the spectrometer does not have any moving dispersing part and works in vacuum at a fixed thermostatted temperature. The accuracy on radial velocity measurements will therefore solely depend on the statistical limits imposed by the s/n ratio of the spectrum and on the number and spectral distribution of lines from the absorption gas cell. Preliminary simulations indicate that a few hundred lines, well spaced across the µm range, can be obtained using a mixture of HCl, HBr and HI gases. REFERENCES 1. McCarthy, J.K.; Sandiford, B.A.; Boyd, D.; Booth, J.; The Sandiford 2.1-m Cassegrain echelle spectrograph for McDonald Observatory - Optical and mechanical design and performance, Astronomical Society of the Pacific, Publications, vol. 105, no. 690, p , Pallavicini, R.; Delabre, B.; Pasquini, L.; et al.; The AVES adaptive optics spectrograph for the VLT: status report, Instrument Design and Performance for Optical/Infrared Ground-based Telescopes Iye, M.; Moorwood, A.F.M. eds; Proc. of the SPIE, Vol. 4841, pp , Baffa, C.; Biliotti, V.; Checcucci, A.; et al.; The Fasti Project, ADASS XII ASP Conference Series, Payne, H.; Jedrzejewski, R.; Hook, R. Eds., Vol. 295, p.355, Proc. of SPIE Vol. 5492

4 Figure 1. Spectral format in the high resolution mode (top, slit length is 6.5 ) and low resolution mode (bottom, slit length is 30 ) Proc. of SPIE Vol

5 1278 Proc. of SPIE Vol Figure 2. Optical layout of the GIANO spectrometer

6 Figure 3. Spot diagrams of the GIANO spectrometer in the high resolution mode (top panel) and low resolution mode (bottom panel). The squares are 18µm and correspond to the pixel size of the Hawaii-2 array detector Proc. of SPIE Vol