2.4.2 The Spectrometric Instrument

The design of the spectrometric instrument, or Spectro, is still somewhat uncertain, partly due to the fact that its inclusion in the payload was decided relatively late in the course of the mission preliminary studies and partly because, as far as spectrometry was concerned, the experience of previous space missions gave little or no help in tackling the problems posed by the continuous scanning motion of the satellite. For this reason, even if it is generally agreed that the instrument will be used for radial velocity measurements and for medium band photometry, the instrumental parameters and the instrument's general design itself are provisional and will be subject to further discussion.

According to the provisional design, the three-mirror telescope features a $0.75 \times 0.7~\textrm{m}^2$ aperture, a focal length of $4.17~\textrm{m}$ and a field of view of $2 \times 1~\textrm{deg}^2$, over which the instrument provides diffraction-limited performance. A central part of size $1 \times 1~\textrm{deg}^2$ of the field of view is devoted to spectroscopic radial velocity measurements (Radial Velocity Spectrometer, or RVS), while the two regions making up the rest of the field are devoted to medium-band photometry in a number of bands depending on the photometric system that will finally be adopted (Medium-Band Photometer, or MBP). The first portion of the preceding region of the MBP actually works without filters (i.e. in the $G$ band) and is used as a dedicated sky mapper to detect objects crossing the field of view and to cross-identify them with those observed by the astrometric instruments. The RVS is a slitless spectrograph consisting of a collimating lens, a disperser and an imaging lens, working at unit magnification in a symmetrical configuration. The spectrum of the object crossing the field of view is spread across scan over about 600 pixels and the radial velocity is derived from the Doppler shift of some spectral lines in the wavelength range 850-875  nm. The CCDs covering the focal plane are operated in TDI, giving an integration time per scan of 3 s per band in the MBP and 30 s per spectra in the RVS. A CCD with a QE curve shifted towards the red with respect to that used in the Astros and with a square pixel of $10~\mu\textrm{m}$ side, the so called , is used in order to optimize the instrument for the wavelength range chosen for radial velocity measurements. The spectral and spatial resolution are thus about and 0.5 arcsec/pixel, respectively.