Pointing and observation strategy

Euclid will survey in “step-and-stare” mode, with the telescope acquiring images and spectra over a large patch of the sky by pointing successively to adjacent fields of around 0.54 deg² (equivalent to two and a half times the apparent surface area of a full Moon, which is approximately 0.2 deg2). The observation strategy allows for the constraints of operating at the L2 Lagrange point and the satellite’s field of view—the Sun-Earth-satellite alignment moves one degree a day in the ecliptic plane—as well as maintaining thermal stability, which requires the pointing direction to be as far as possible perpendicular to the Sun-satellite alignment. This means that for a given day, the zone observed is mostly confined to a great circle perpendicular to the Sun-satellite alignment.

An instrument sequence is as follows: visible imaging (VISible Instrument (VIS)) and simultaneous imaging/spectra (slitless spectrometry with the Near Infrared Spectro Photometer (NISP) through grisms), then radiometric imaging through the three infrared filters in NISP’s Filter Wheel Assembly (FWA). Each 0.54-deg² field will be observed four times at 100-arcsecond intervals to eliminate artefacts and optimize the signal-to-noise ratio. The time taken to image a field is approximately 75 minutes.

Euclid will observe strips on a daily basis. These strips are adjacent sky fields along a great (scanning) circle of roughly constant ecliptic longitude (passing through the Sun). Each strip will cover an area on the order of 15 to 20 degrees. Six months into the mission, the satellite’s pointing direction will be reversed—along a great circle in the ecliptic plane—to survey sectors in the other hemisphere. The satellite can also be slewed around the axis of the telescope.

Euclid pointing strategy. Credit: J. Amiaux et al, Euclid Mission: building of a Reference SurveyMission: building of a Reference Survey

Euclid will survey the whole sky with the exception of the ecliptic plane (in which the planets in our solar system lie, polluted by zodiacal light) and the galactic plane (in which the stars in the Milky Way lie, polluted by galactic light), which is too bright and masks galaxies beyond. This means Euclid will see about 35% of the celestial vault.

The observation strategy is based on a wide survey mode and a deep survey mode:

• Wide survey: this will cover about 14,700 deg² of the extragalactic sky over the six-year mission, i.e. about 35% of the celestial sphere using the pointing strategy detailed above. The aim is to observe objects up to an apparent magnitude of 24.5, that is, 30 million times fainter than what the human eye can see. During the mission, the instruments will view some 30,000 fields and 10,000 calibration fields.
• Deep survey: this will involve regular observations—about 40—of three patches of the extragalactic sky at least 10 deg² deeper, up to two magnitudes deeper than in wide survey mode (150 million times fainter than what the human eye can see) to reach galaxies beyond z = 6, or even z = 7 (about 12 billion years back in time). These three patches are:
• A 20-deg2 field near the north ecliptic pole, in the Draco (Dragon) constellation
• A 10-deg2 field near the south ecliptic pole in the Fornax (Furnace) constellation
• Near this same pole, a 20-deg2 field in the Horologium (Pendulum Clock) constellation

As well as their cosmological value, these regular observations will be useful for spectral calibration of NISP and for assessing the satellite’s pointing stability.

Survey zones outside the galactic plane (centre) and ecliptic plane (inclined) for wide survey mode. Zones in yellow are those selected for deep survey mode. Credit: ESA/Gaia/DPAC; Euclid Consortium

Areas of sky surveyed for each of the mission’s six years (year 1 in green, year 2 in red, year 3 in blue, year 4 in orange, year 5 in mauve and year 6 in yellow). Credit: J.C. Cuillandre on Euclid Consortium backdrop/Planck collaboration/A. Melfinger