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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),Footnote 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.

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 HEALPIXFootnote 2 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 ListFootnote 3 (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.

We have extracted from the IGSL basic data some representative sky regions 11 × 11 deg2 in size, each sampling 10,000 independent FGS FOV areas of 7′ × 7′. Then, we have selected all the potential FGS reference stars complying with the requirements specified above. A color-coded stellar density map of each 100 × 100 FOV areas is a useful tool for identifying lack of stars, anomalous gradients, or peculiar features not obviously attributable to the real sky (see figures). We analyzed a few of these cases by overlaying the catalog objects onto the digitized astronomical images available from the ALADIN Sky Atlas (aladin.u-strasb.fr). Figures 15.1 and 15.2 show some results of the analysis of the Southern Ecliptic Pole (SEP) region centered at α = 90∘, \(\delta = -67^{\circ }\), and galactic latitude \(b = -30^{\circ }\).

Fig. 15.1
figure 1

Analysis of 100 × 100 FOVs in the SEP region. The color-coded map show the density of a sub-sample of “single” stars with astrometric accuracy better than 0. 4″ (the red, i.e., denser area is in the direction of the large magellanic cloud). The table reports the complete objects statistics before and after FGS selection criteria are applied

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Fig. 15.2
figure 2

 ≈ 14 × 10 arcmin fields from the SEP region; catalog objects (blue crosses) are overplotted to the Anglo-Australian Schmidt survey plate image. Left Panel: In spite of the high crowding, some candidate reference stars are available but we must consider the limited reliability of the sources provided by the current astronomical catalogues Right Panel: Typical example of false object detection caused by the diffraction spike of a very bright star from a Schmidt image

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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.