Objectives

The primary objective of Gaia's Radial Velocity Spectrometer (RVS) instrument is the acquisition of radial velocities. These line-of-sight velocities complement the proper-motion measurements provided by the astrometric instrument. To this end the instrument will obtain spectra in the narrow near infrared band [847-874 nm] with a resolution of 11 500.

The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets. For these late-type stars, the RVS wavelength interval displays, besides numerous weak lines mainly due to Fe, Si, and Mg, three strong ionised calcium lines (at around 849.8, 854.2, and 855.2 nm). The lines in this triplet allow radial velocities to be derived, even at modest signal-to-noise ratios. In early-type stars, RVS spectra may contain weak lines such as Ca II, He I, He II, and N I, although they will generally be dominated by Hydrogen Paschen lines.

Design and Operations

The RVS instrument is a near-infrared, medium-resolution, integral-field spectrograph dispersing all the light entering the field of view. It is integrated with the astrometric and photometric functions and uses the common two telescopes.

RVS uses the Sky Mapper function for object detection and confirmation. Objects will be selected for RVS observation based on measurements made slightly earlier in the Red Photometer. Light from objects coming from the two viewing directions of the two telescopes is superimposed on the RVS CCDs.

The spectral dispersion of objects in the field of view is achieved by means of an optical module physically located between the last telescope mirror (M6) and the focal plane. This module contains a grating plate and four dioptric, prismatic, spherical, fused-silica lenses which correct the main aberrations of the off-axis field of the telescope. The RVS module has unit magnification which means that the effective focal length of the RVS equals 35 m.

Location of the RVS optical module and detectors. Image courtesy: EADS Astrium

Spectral dispersion is oriented in the along-scan direction. A dedicated passband filter restricts the throughput of the RVS to the desired wavelength range.

The RVS-part of the Gaia focal plane contains 3 CCD strips and 4 CCD rows. Each source will typically be observed during ~40 field-of-view transits throughout the 5-year mission. On the sky, the RVS CCD rows are aligned with the astrometric and photometric CCD rows; the resulting semi-simultaneity of the astrometric, photometric, and spectroscopic transit data will be advantageous for variability analyses, scientific alerts, spectroscopic binaries, etc. All RVS CCDs are operated in TDI (time-delayed integration) mode.

The RVS spectra will be binned on-chip in the across-scan direction. All single-CCD spectra are foreseen to be transmitted to the ground without any further on-board (pre-)processing. For bright stars, single-pixel-resolution windows are foreseen to be used, possibly in combination with TDI gates. It is currently foreseen that the RVS will be able to reach object densities on the sky of up to 40 000 objects deg^-2.

On-ground Data Processing

Radial velocities will be obtained by cross-correlating observed spectra with either a template or a mask. An initial estimate of the source atmospheric parameters derived from the astrometric and photometric data will be used to select the most appropriate template or mask. Iterative improvements of this procedure are foreseen. For stars brighter than ~15th magnitude, it will be possible to derive radial velocities from spectra obtained during a single field-of-view transit. For fainter stars, down to ~17th magnitude, accurate summation of the ~40 transit spectra collected during the mission will allow the determination of mean radial velocities.

Atmospheric parameters will be extracted from observed spectra by comparison of the latter to a library of reference-star spectra using, for example, minimum-distance methods, principal-component analyses, or neural-network approaches. The determination of the source parameters will also rely on the information collected by the other two instruments: astrometric data will constrain surface gravities, while photometric observations will provide information on many astrophysical parameters.