covers the wavelength range 640—1050 nm.

Sometimes, BP and RP are collectively referred to as XP.

Optical coatings deposited on the prisms, together with the telescope transmission and detector quantum efficiency, define the bandpasses.

The prisms are located in the common path of the two telescopes, and mounted on the CCD cold radiator, directly in front of the focal plane.

Both photometers are equipped with a dedicated strip of seven CCDs each, which cover the full astrometric field of view in the across-scan direction (see Sect. 3.3.2 for details on the photometric CCDs).

This implies that the photometers see the same (number of) transits as the astrometric instrument.

The prisms disperse object images along the scan direction and spread them over ∼45 pixels (for 15-mag objects): the along-scan window size is chosen as 60 pixels to allow for background subtraction (and window-propagation and windowplacement quantisation errors).

The spectral dispersion, which matches the earlier photometric-filter design described in Jordi et al. (2006), results from the natural dispersion curve of fused silica and varies in BP from 3 to 27 nm pixel−1 over the wavelength range 330—680 nm and in RP from 7 to 15 nm pixel−1 over the wavelength range 640—1050 nm.

The 76% energy extent of the along-scan line-spread function varies along the BP spectrum from 1.3 pixels at 330 nm to 1.9 pixels at 680 nm and along the RP spectrum from 3.5 pixels at 640 nm to 4.1 pixels at 1050 nm.

For the majority of objects, BP and RP spectra are binned on-chip in the across-scan direction over 12 pixels to form one-dimensional, along-scan spectra.

Unbinned, single-pixelresolution windows (of size 60 × 12 pixels2) are only used for stars brighter than G = 11.5 mag; this is often in combination with temporary TDI-gate activation, during the period of time corresponding to the bright-star window length, to shorten the CCD integration time and avoid pixel-level saturation.

The object-handling capability of the photometric instrument is limited to 750 000 objects deg−2 (Sect. 8.4); only the brightest objects receive a window in areas exceeding this density.

The data quality, however, is already affected at lower densities by contamination from the point spread function wings of nearby sources falling outside the window (degrading flux and background estimation) and by blending with sources falling inside the window (leading to window truncation and necessitating a deblending procedure; Busso et al. 2012).
3.3.7.

Spectroscopic instrument The spectroscopic instrument, known as the radial-velocity spectrometer (RVS), obtains spectra of the bright end of the Gaia sample to provide:
1. radial velocities through Doppler-shift measurements using cross-correlation for stars brighter than GRVS ≈16 mag (Sect. 8.4; David et al. 2014), which are required for kinematical and dynamical studies of the Galactic populations and for deriving good astrometry of nearby, fast-moving sources which show perspective acceleration (e.g. de Bruijne & Eilers 2012); 2. coarse stellar parametrisation for stars brighter than GRVS ≈ 14.5 mag (e.g.

Recio-Blanco et al. 2016); 3. astrophysical information, such as interstellar reddening, atmospheric parameters, and rotational velocities, for stars brighter than GRVS ≈12.5 mag (e.g.

Recio-Blanco et al. 2016);
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4. individual element abundances for some elements (e.g.

Fe, Ca, Mg, Ti, and Si) for stars brighter than GRVS ≈11 mag (e.g.

Recio-Blanco et al. 2016),
where GRVS denotes the integrated, instrumental magnitude in 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.