The Construction of a Reference Star Catalog for the Euclid Mission
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Optimization of scientific throughput from the ESA Euclid mission imposes stringent requirements on the performance of satellite absolute pointing. This will be achieved by the on-board Fine Guidance Sensor (FGS) with the aid of a specific stellar catalog, which must comply with well-defined, FGS-driven astrometric and photometric properties. By means of the OATo Star Catalog Database, used for the production of the Initial Gaia Source List, we present a preliminary assessment of the methods of construction of such a reference catalog.
KeywordsEuclid Mission Reference Star Catalog Fine Guidance Sensors (FGS) Catalog Database Absolute Pointing Error
15.1 Satellite Attitude Requirements and the FGS Reference Star Catalog
The ESA Euclid mission will investigate the nature of dark energy, dark matter and gravity by detecting weak gravitational lensing and barionic acoustic oscillations. Euclid will cover the extragalactic sky at | b | > 30∘ (with foreseen extension to the entire sky) by observing strips of adjacent fields along great circles of roughly constant ecliptic longitude. To meet the demanding pointing performance, the satellite Attitude and Orbital Control System will be endowed with a Fine Guidance Sensor instrument (FGS). The FGS must achieve an Absolute Pointing Error of < 2. 5″ (1 \(\sigma\)), and a Relative Pointing Error over 700 s of < 0. 025″ (1 \(\sigma\)) in the X and Y telescope axes. In Absolute Tracking Mode, targets will be selected using an input star catalog, which will be derived from the FGS Reference Star Catalog, complying with three main requirements: (a) 5 (with a minimum of 3) or more stars brighter than R FGS = 19 mag per Field-of-View (FGS),1 the limit operating magnitude; (b) astrometric accuracy of 0. 4″ absolute position (per coordinate) at epoch 2025; and (c) more than 2″ distance from nearest neighbor, or with a difference in brightness of more than 2 mag.
15.2 Selection and Analysis of Test Fields from the OATo Database
The catalog database hosted at OATo, a compilation of the major astronomical catalogs presently available, represents a resource for various scientific and technical activities. The cross-matched object list is mapped onto the celestial sphere using a level-6 HEALPIX2 scheme; in practice, the sphere is pixelized in 49,152 equal areas of 0.84 deg2 each. This database provided the basis for the production of the Initial Gaia Source List3 (IGSL) that is being used as starting point for the Initial Data Treatment of Gaia’s observations, and includes the following parameters, when available: positions, object classification, proper motions, magnitude and color information of all objects brighter than Gaia magnitude 21 over the entire sky.
15.3 Conclusions and Future Work
This preliminary assessment of the catalogue extracted from the OATo Database has highlighted some deficiencies, which must be cured in order to build an operational Reference Star Catalog for the Euclid FGS. We foresee better performances by including the new proper motions from GSC2.3 material (Qi Z. et al. 2015, “Absolute Proper Motions outside the Plane (APOP), A step towards GSC2.4”, Astron. Journal, 150:137) and, also, by paying special attention to critical sky areas. Moreover, the integration of novel data coming from ongoing ground-based CCD surveys such as PAN STARSS, Sky Mapper, VST, and ultimately Gaia, will help improve count completeness as well as astrometric and photometric accuracy.
This work has been partially supported by contract TASI – OATo, purchase order no.1520037410