the spectroscopic bandpass (defined below).

The spectroscopic instrument (Cropper & Katz 2011), like the photometric instrument (Sect. 3.3.6), is highly integrated with the astrometric instrument, using the same telescopes, the same focal plane (using a dedicated section of it), and the same sky-mapper (and AF1) function for object detection (and confirmation).

The actual (faint-end) selection of an object for RVS, however, is based on an onboard estimate of GRVS that is generally derived from the RP spectrum collected just before the object enters RVS.

The RVS is an integral-field spectrograph and the spectral dispersion of objects in the fields of view is materialised through an optical module with unit magnification, which is mounted in the common path of the two telescopes between the last telescope mirror (M6) and the focal plane.

This module contains a blazed-transmission grating plate (used in transmission in order +1), four fused-silica prismatic lenses (two with flat surfaces and two with spherical surfaces), and a multilayer-interference bandpass-filter plate to limit the wavelength range to 845–872 nm.

This range was selected to cover the Ca ii triplet, which is suitable for radial-velocity determination over a wide range of metallicities, signal-to-noise ratios, temperatures, and luminosity classes in particular for abundant FGK stars, and which is also a well-known metallicity indicator and stellar parametriser (e.g.

Terlevich et al. 1989; Kordopatis et al. 2011).

For early-type stars, the RVS wavelength range covers the hydrogen Paschen series from which radial velocities can be derived.

In addition, the wavelength range covers a diffuse interstellar band (DIB), located at 862 nm, which traces out interstellar reddening (e.g.

Kuˇcinskas & Vanseviˇcius 2002; Munari et al. 2008).

The dispersed light from the RVS illuminates a dedicated area of the focal plane containing 12 CCDs arranged in three strips of four CCD rows (see Sect. 3.3.2 for details on the spectroscopic CCDs).

This implies that an object observed by RVS has 43% (1 −4/7) fewer RVS focal plane transits than astrometric and photometric focal plane transits.

The grating plate disperses object images along the scan direction and spreads them over ∼1100 pixels (R = λ/∆λ ≈11 700, dispersion 0.0245 nm pixel−1); the along-scan window size is 1296 pixels to allow for background subtraction (and window-propagation and window-placement quantisation errors).

For the majority of objects, RVS spectra are binned onchip in the across-scan direction over 10 pixels to form one-dimensional, along-scan spectra.

The onboard software (Sect. 3.3.8) contains a provision to adapt this size to the instantaneous, straylight-dominated background level (Sect. 4.2), in view of optimising the signal-to-noise ratio of the spectra, but this functionality is not being used.

Single-pixel-resolution windows (of size 1296 × 10 pixels2) are only used for stars brighter than GRVS = 7 mag.

The object-handling capability of RVS is limited to 35 000 objects deg−2 (Sect. 8.4); in areas exceeding this density, only the brightest objects receive a window.

As for the photometers, however, the data quality will be severely compromised in dense areas by contamination from and blending with nearby sources.

3.3.8.

Video processing unit and algorithms Each CCD row in the focal plane (Sect. 3.3.2) is connected to its own video processing unit (VPU), essentially a computer in charge of commanding the CCDs and collecting the science data and transmitting it to the onboard storage (Sect. 3.3.9).

The VPUs run the video processing algorithms (VPAs; Provost et al. 2007), which are a collection of software routines configurable through a set of parameters that can be changed by telecommand.

The seven VPUs are fully independent although each one runs the same set of VPAs albeit (possibly) with different parameter sets.

Parameter updates are possible but require a transition from VPU operational mode to VPU service mode, which means a loss of science data of a few dozen seconds.

The VPUs and VPAs have a large number of functions such as CCD command generation, including deriving the TDI-line signals from the spacecraft master clock (Sect. 3.3.10) for the synchronisation of the CCD sequencing.