Euclid preparation. I. The Euclid Wide Survey

Euclid Collaboration; Scaramella, R.; Amiaux, J.; Mellier, Y.; Burigana, C.; Carvalho, C. S.; Cuillandre, J. -C.; Da Silva, A.; Derosa, A.; Dinis, J.; Maiorano, E.; Maris, M.; Tereno, I.; Laureijs, R.; Boenke, T.; Buenadicha, G.; Dupac, X.; Gaspar Venancio, L. M.; Gómez-Álvarez, P.; Hoar, J.; Lorenzo Alvarez, J.; Racca, G. D.; Saavedra-Criado, G.; Schwartz, J.; Vavrek, R.; Schirmer, M.; Aussel, H.; Azzollini, R.; Cardone, V. F.; Cropper, M.; Ealet, A.; Garilli, B.; Gillard, W.; Granett, B. R.; Guzzo, L.; Hoekstra, H.; Jahnke, K.; Kitching, T.; Maciaszek, T.; Meneghetti, M.; Miller, L.; Nakajima, R.; Niemi, S. M.; Pasian, F.; Percival, W. J.; Pottinger, S.; Sauvage, M.; Scodeggio, M.; Wachter, S.; Zacchei, A.; Aghanim, N.; Amara, A.; Auphan, T.; Auricchio, N.; Awan, S.; Balestra, A.; Bender, R.; Bodendorf, C.; Bonino, D.; Branchini, E.; Brau-Nogue, S.; Brescia, M.; Candini, G. P.; Capobianco, V.; Carbone, C.; Carlberg, R. G.; Carretero, J.; Casas, R.; Castander, F. J.; Castellano, M.; Cavuoti, S.; Cimatti, A.; Cledassou, R.; Congedo, G.; Conselice, C. J.; Conversi, L.; Copin, Y.; Corcione, L.; Costille, A.; Courbin, F.; Degaudenzi, H.; Douspis, M.; Dubath, F.; Duncan, C. A. J.; Dusini, S.; Farrens, S.; Ferriol, S.; Fosalba, P.; Fourmanoit, N.; Frailis, M.; Franceschi, E.; Franzetti, P.; Fumana, M.; Gillis, B.; Giocoli, C.; Grazian, A.; Grupp, F.; Haugan, S. V. H.; Holmes, W.; Hormuth, F. et al.
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Astronomy and Astrophysics

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Euclid is a mission of the European Space Agency that is designed to constrain the properties of dark energy and gravity via weak gravitational lensing and galaxy clustering. It will carry out a wide area imaging and spectroscopy survey (the Euclid Wide Survey: EWS) in visible and near-infrared bands, covering approximately 15 000 deg2 of extragalactic sky in six years. The wide-field telescope and instruments are optimised for pristine point spread function and reduced stray light, producing very crisp images. This paper presents the building of the Euclid reference survey: the sequence of pointings of EWS, deep fields, and calibration fields, as well as spacecraft movements followed by Euclid as it operates in a step-and-stare mode from its orbit around the Lagrange point L2. Each EWS pointing has four dithered frames; we simulated the dither pattern at the pixel level to analyse the effective coverage. We used up-to-date models for the sky background to define the Euclid region-of-interest (RoI). The building of the reference survey is highly constrained from calibration cadences, spacecraft constraints, and background levels; synergies with ground-based coverage were also considered. Via purposely built software, we first generated a schedule for the calibrations and deep fields observations. On a second stage, the RoI was tiled and scheduled with EWS observations, using an algorithm optimised to prioritise the best sky areas, produce a compact coverage, and ensure thermal stability. The result is the optimised reference survey RSD_2021A, which fulfils all constraints and is a good proxy for the final solution. The current EWS covers ≈14 500 deg2. The limiting AB magnitudes (5σ point-like source) achieved in its footprint are estimated to be 26.2 (visible band IE) and 24.5 (for near infrared bands YE, JE, HE); for spectroscopy, the Hα line flux limit is 2 × 10−16 erg−1 cm−2 s−1 at 1600 nm; and for diffuse emission, the surface brightness limits are 29.8 (visible band) and 28.4 (near infrared bands) mag arcsec−2.