In September 2014 the ING Board issued a call for letters of intent to provide innovative instrumentation on the INT in exchange for substantial allocations for telescope time. The Terra Hunting Experiment (THE) replied with a proposal to install HARPS3 on the INT to carry out a monitoring survey of stars, or the Core Survey, lasting 10 years, with the goal of detecting Earth mass exoplanets in the habitable zone. The proposal included provisions to deploy a new telescope control system on the INT that would allow for robotic operation of HARPS3.

The HARPS3 spectrograph is a stabilised high-resolution spectrograph capable of delivering velocity precision of 1m/s. Its performance is expected to be similar or better than those of HARPS on the La Silla 3.6m telescope or HARPS-N on the TNG. The instrument will be placed in the INT large Coudé room. The HARPS3 Cassegrain fibre pick-off system uses the straight Cass INT focus.

The instrument control software delivered with HARPS3 includes an automated scheduler to allow for unassisted operation at the telescope during the night, with the goal of making observations robotic. The scheduler delivered with HARPS3 allows the real-time scheduling of nights, which combine survey observations and other scheduled observations with HARPS3.

The THE is a collaboration (hereinafter the Consortium) between Cambridge University, Exeter University, Netherlands Research School for Astronomy (NOVA), Instituto de Astrofísica de Canarias (IAC), Geneva University and Uppsala University, all represented by the University of Cambridge.

The ING provides the Consortium with guaranteed observing time (GTO) on HARPS3 to carry out its Core Survey. The GTO consists of two components:

1. Survey time – acknowledging the value to the ING community of the HARPS3 instrument and the INT upgrade, ING allocates to the HARPS3 Survey an amount of time equal to 50% of the time available, averaged over the 10 years of operation. The time available is defined as 365 nights, minus discretionary nights, minus ITP time. The exact amount of telescope time dedicated to the HARPS3 Survey is negotiated well in advance of each semester by the Consortium and ING. In any given semester, the survey time is limited to 60% of the available time. Over any 10 consecutive semesters, it is limited to 56%. On average over 10 years it will be limited to 50%.

2. Contribution time – acknowledging the work to maintain and operate the instrument, for each three hours of observing time with HARPS3 granted to observers external to the Consortium by the various time allocation committees, the HARPS3 Consortium receives one hour.

Together, the Survey and the Contribution time form the GTO. Excluding engineering nights, it should never be less than 4 hours during every night. Scheduling of the GTO nights respects the ING scheduling principles of allowing equal access to all right ascensions and all moon phases.

Open time is available for use by astronomers from the ING communities either with or without an explicit collaboration by the Consortium. The existing observing time allocation procedures are followed for time requested by astronomers from the ING communities with any additional procedures dictated by the restriction that only robotic observing mode may be offered and that community requested time is compliant with the GTO requirements. All open-time observations go through a phase-2 system. The Consortium provides and maintains the Phase 2 submission tool, and ING organises the submission by open-time applicants of Phase 2 to the observations database.

The raw data files produced by HARPS3 are in standard FITS format. A data-reduction software pipeline (DRSP), specific for the processing of the data, is available. The Consortium provides scientific data products as produced by the DRSP to all users of HARPS3. Its end data products include a radial velocity and its photon-noise error, as well as extracted, wavelength- and flat-field-calibrated, two-dimensional echelle spectra.

The open-time applicants own their HARPS3 data, following ING standards. The data are available to the Consortium but only for the specific purpose of data quality control checks. Public availability for HARPS3 raw and DRSP processed data starts one year after observations, except for those data obtained for the Consortium Core Survey.