Montage of the HARPS spectrograph and the 3.6m telescope at La Silla. The upper left shows the dome of the telescope, while the upper right illustrates the telescope itself. The HARPS spectrograph is shown in the lower image during laboratory tests. The vacuum tank is open so that some of the high-precision components inside can be seen
The HARPS can attain a precision of 0.97 m/s (3.5 km/h), with an effective precision of the order of 30 cm s−1,[relevant? – discuss] making it one of only two instruments worldwide with such accuracy. This is due to a design in which the target star and a reference spectrum from a thorium lamp are observed simultaneously using two identical optic fibre feeds, and to careful attention to mechanical stability: the instrument sits in a vacuum vessel which is temperature-controlled to within 0.01 kelvins. The precision and sensitivity of the instrument is such that it incidentally produced the best available measurement of the thorium spectrum. Planet-detection is in some cases limited by the seismic pulsations of the star observed rather than by limitations of the instrument.
Since October 2012 the HARPS spectrograph has the precision to detect a new category of planets: habitable super-Earths. This sensitivity was expected from simulations of stellar intrinsic signals, and actual observations of planetary systems. Currently, the HARPS can detect habitable super-Earth only around low-mass stars as these are more affected by gravitational tug from planets and have habitable zones close to the host star.