Optics

This section describes the baseline optical design of GIANO.

The spectrometer consists of a canonical three mirrors anagstigmat (TMA) combination used in double-pass, which acts both as collimator and camera. The collimated beam is 100mm and the system focal length is 420mm. The dispersing element is a commercial 23.2 ll/mm R2 echelle working at a fixed position in a quasi-Littrow configuration with an off-axis along the slit of 5. A system of prisms from high dispersing IR optical materials acts as cross-disperser (CRD). As the entrance image of this system lies close to the detector, we also included a 2-mirrors (FR1-FR2) system which re-images the slit at a convenient place far from any critical element. The optical layout can be summarized as follows.

The entrance slit, illuminated by the beam from the pre-slit optics, feeds the conic FR1-FR2 mirrors which re-image the focal plane at F/4.2 on a side of the detector, 37 mm from its edge and about 8mm from the edge of the array carrier.

The light then enters into the TMA which creates a 100mm collimated beam feeding the cross-dispersing prisms and the grating. The rays reflected from the grating pass again through the prisms and are focused onto the array by the same three mirrors. Two flat mirrors (PM1, PM2) are added to conveniently position the array, the input slit and the FR1-FR2 mirrors.

An additional imaging mode (IMA) is obtained by inserting a flat mirror before the first cross disperser. In this configuration the TMA works as a 1:1 re-imaging system. This mode is useful during instrument maintenance and for checking the slit positioning.

The optical performances of the system are excellent, with > 80% en-squared energy within one pixel over most of the array area.

Two possible configurations for the the warm pre-slit optics feeding the GIANO spectrometer are under final consideration: a standard re-imaging system which covers a sky-projected f.o.v. of ∅ 30 arcsec and a fiber-fed system with a sky-projected angle of 1 arcsec.

This optical system is designed to create a parallel and collimated beam of ∅ 8.8 mm diameter where a super-achromatic Quartz/MgF₂ /4 or /2 retarder, a Wollaston polarizer (MgF₂ prisms with 15° opening angle) and an infrasil dove-prism for field de-rotation are in sequence located. All these elements are mounted on separate rotating stages which must be synchronized to maintain the wave-plate retarder at a fixed sky-projected angle, the Wollaston at a fixed angle relatively to the retarder, and the two images from the Wollaston aligned along the spectrometer slit.

This unit consists of a set of lamps for wavelength calibration (Th-Ar-Ne) and for flat-fielding (halogen) which illuminates the spectrometer input slit through a commercial integrated sphere mounted on the pre-slit bench in between the absorption cell and the HR/LR module.

The presence of a cold pupil stop before the spectrometer slit guarantees an illumination of the slit through the same solid-angle as the light from the telescope.

Taking advantage of the multiple non- destructive read-out of the IR array, one can obtain simultaneous arc lamps measurements during a scientific exposure, for precise radial velocity measurements.

Optical layout and ray-tracing of GIANO in its high resolution mode.

Spot diagrams at selected positions on the array. The squares represent the pixel size (18 micron) and the numbers on top are the percent of total energy falling inside the pixel area. Spot diagrams are grouped in 3 x 3 grids. The 3 rows correspond to the longest, central and shorter wavelengths of each order, respectively. Each column shows the spots at three positions (top, center, bottom) along the 6.5" slit.

Pictures of some optical elements of the GIANO spectrometer. The grating is a commercial 23.2 ll/mm R2 echelle with gold coating. The TMA1 mirror is a huge (30 cm) off-axis elliptical aluminum mirror. The cross dispersers are large (20 cm) custom prisms of infrasil (CRD1) and ZnSe (CRD2).

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