La Silla - Science Operation Department

Overview of HARPS

The instrument is built to obtain very high long term radial velocity accuracy (on the order of 1 m/s). To achieve this goal, HARPS is designed as an echelle spectrograph fed by a pair of fibres and optimised for mechanical stability. It is contained in a vacuum vessel to avoid spectral drift due to temperature and air pressure variations. One of the two fibres collects the star light, while the second is used to either record simultaneously a Th-Ar reference spectrum or the background sky. The two HARPS fibres (object + sky or Th-Ar) have an aperture on the sky of 1"; this produces a resolving power of 115,000 in the spectrograph. Both fibres are equipped with an image scrambler to provide a uniform spectrograph pupil illumination, independent of pointing decentering. The spectral range covered is 378nm-691nm, distributed over the echelle orders 89-161. As the detector consists of a mosaic of 2 CCDs (altogether 4k x 4k, 15 microns pixels), one spectral order (N=115, from 530nm to 533nm) is lost in the gap between the two chips.

HARPS produces a signal-to-noise ratio of 110 per pixel at 550nm for a Mv=6, G2V star, in 1 minute integration time (1" seeing, airmass = 1.2). When using the Simultaneous Thorium Reference Method (which is the standard mode for achieving the highest accuracy radial velocities) this signal-to-noise ratio should be sufficient to achieve a photon noise error on the radial velocity determination of about 0.90 m/s. Taking into account errors introduced by the guiding, focus, and instrumental uncertainties, a global radial velocity accuracy of about 1m/s RMS is achieved. This is attained for spectral type later than G and for non-rotating stars (v sin i < 2km/s).

In simultaneous Th-Ar mode, HARPS users should strictly follow the calibrations foreseen by the Calibration Plan, which includes a number of bias, flat field and Th-Ar exposures taken before the night.

HARPS is equipped with its own pipeline (installed at La Silla). This pipeline provides the visiting astronomer in near real-time with extracted and wavelength calibrated spectra in all observing modes. When the Simultaneous Thorium reference method is applied, the pipeline delivers precise radial velocities (RV, relative to the solar system barycentre) for late type stars whose RV is known within 1-2km/s, provided that a set of standard calibrations has been executed in the afternoon.

Since May 2004 the CES (Coude' Echelle Spectrograph) fiber is attached to the HARPS adapter, allowing switching between the two instruments in less than a minute and night sharing.

Up-to-date information and documentation on the instrument, including manuals and exposure time calculator are available on the HARPS web page at http://www.eso.org/lasilla/sciops/3p6/harps/