3.2.3 The Attitude Star Catalogue (ASC)

The source catalogues used and their order of inclusion were:

• $\bullet$

  Hipparcos (Perryman et al. 1997): The photometry from the original Hipparcos Catalogue and the astrometric parameters from the update by van Leeuwen (2007b) when published. Initially, all entries were included regardless of the known errors, e.g., also for entries that are considered erroneous. Since inclusion in the ASC requires an estimate of the $G$-mag, the unreal entries were excluded as part of the cleaning phase.

• $\bullet$

  Tycho-2 (Høg et al. 2000): This catalogue forms the backbone of all the major ground-based catalogues currently available. It was made from a combination of the Tycho star mapper observations on the Hipparcos satellite (Høg et al. 1997), the Astrographic Catalogue, and 143 other ground-based catalogues.

• $\bullet$

  Sky2000 (Myers et al. 2001): The SKYMAP Star Catalogue System is a list of all stars with either measured Johnson blue or visual magnitudes brighter than 9.0. The version used here had $\mathrm{299\hspace{0.17em}167}$ entries of which 212 were not in the combined Hipparcos + Tycho-2 catalogues. Sky2000 provides positions at J2000, proper motions, and a blue and visual magnitude. We assumed positions errors of 100 mas, proper motions error of 10 mas yr${}^{-1}$, and ASC magnitudes derived from Sky2000 entries to have errors of 0.6.

• $\bullet$

  UCAC4 (Zacharias et al. 2013): The USNO CCD Astrograph Catalogue version 4 was the most precise all-sky astrometric catalogue (at the time of constructing the catalogue) in the range $V=10$–16 mag currently available. There are no original standard magnitudes in this catalogue.

• $\bullet$

  GSC2.3 (Lasker et al. 2008): The Second Guide Star Catalogue version 2.3 forms the bulk of the photometry and defines the red and blue magnitudes ($B_J$ and $R_F$) as this has the largest sky coverage with a homogeneous photometric system. The only variation with the public version is that we removed the multiple entries discussed in Section 4.2 of Lasker et al. (2008). This was done by insisting that only one entry from any objects with position differences of less than 10 mas were kept, selecting Tycho-2 or Sky2000 over other entries.

• $\bullet$

  PPMXL (Roeser et al. 2010): The Positions and Proper Motions ‘Extra Large’ (XL) Catalogue was produced from a combination of the USNO–B (Monet et al. 2003) and the Two Micron Sky Survey point source catalogue (Epchtein et al. 1999). This catalogue was included to provide magnitudes for those entries that did not have them in the previous catalogues.

In addition, any objects in the Washington Double Star catalogue (Mason et al. 2010) or the Tycho Double Star Catalogue (Fabricius et al. 2002) were indicated as probable members of a binary system.

The first version of the ASC was a subset of the IGSL derived using the procedure outlined in Smart and Nicastro (2014). In summary, we produced a master list of objects starting with the large faint catalogues, progressively adding other catalogues and increasing the master list as entries from new catalogues were unmatched. The catalogues of bright objects were then matched to a large master list which resulted in mismatches of the bright objects to noise or faint objects near the true bright objects. Also it was found that the large Schmidt catalogues in the overlap region between plates often had many multiple entries of the same objects. This can be seen in the sky plot of the PPMXL. These multiple entries, if bright enough, were included as ASC sources.

The first on-ground attitude reconstruction of Gaia is described in detail in Section 3.4.5. The goal of this attitude reconstruction was to provide the attitude with an accuracy of 50 mas for the first year, when the ASC was the primary source of reference objects. Later in the mission, it is planned to replace this catalogue with one produced by Gaia with an expected accuracy of 5 mas. This reconstruction requires at least one 2D measurement per second and per field of view, which equates into a minimum density of 75 stars per square degree.

To be automatically assigned a 2D window on-board Gaia, a star must have $G$ 13 mag but this does not provide enough calibration sources, especially near the galactic poles. The compromise was to provide a list of faint calibration stars to a $G$ =13.4 mag which sets the limiting magnitude of the ASC. Note that, originally, the limit of the ASC was set to $G$ =14.0 mag. However, a change of the procedure allowed a relaxation of that requirement to $G$ =13.4 mag.

The cross-match radius in the first on-ground attitude reconstruction was around 20–30${}^{\mathrm{\prime \prime }}$. Hence, we conservatively require that all ASC sources are isolated at the level of 40${}^{\mathrm{\prime \prime }}$. This would potentially allow up to 8000 ASC entries per square degree and not violate the isolation criteria. Following this consideration, in the original subset of the IGSL that constituted the ASC, we just lowered the magnitude limit to reduce the number of stars to be less than 1000. We then assumed the isolation criteria would always be met when there were this many objects per square degree. However, because of the multiple entries, uncatalogued binary systems and a general non-uniform distribution, it was found that the isolation requirement of the ASC subset was violated.

To address the isolation and duplicate entry issues, the ASC was regenerated from scratch using the catalogues listed above in the order given. The production of the ASC starts with all objects in the Hipparcos catalogue as a master list; the other catalogues are input and matched to this master list with a matching radius of 5${}^{\mathrm{\prime \prime }}$. All entries from the input catalogue not matched are included as new master list objects. If more than one entry from the input catalogue matches the master list, only the closest is considered matched and a new entry is generated for the others.

In this way, the master list grows with each included catalogue. Since the first catalogues are composed of bright objects, they are sparse and the chances of a mismatch between the input catalogues and the master list was reduced. The confusion at the bright end of the master list was in this way minimized. When the large, dense Schmidt plate based catalogues are included, there is still the possibility that non-real entries are matched to bright objects and that real bright objects in the GSC23/PPMXL enter as new entries. However, the Schmidt data is only used to provide photometric information and to clean up the ASC list, we drop any objects that are not in either UCAC4, Tycho-2, Sky2000, or the Hipparcos catalogues, under the assumption that the union of these catalogues is complete to fainter than the Gaia isolation limit of $G$ =13.7 mag.

Once the master list was completed with the compilation of all the catalogues, we estimated the red $R_F$, blue $B_J$, Gaia $G$ and Gaia $G_{\mathrm{RVS}}$ using the relations and priorities listed in Smart and Nicastro (2014) with the photometry from the contributing catalogues. We then dropped any objects fainter than $G$ =13.7 mag. These compilation and selection criteria result in 15 million objects. We assume all objects are stellar and then examine each object one-by-one and indicate for each object the number of neighbours within 40${}^{\mathrm{\prime \prime }}$. From this list, we drop any star (i) with a neighbour, (ii) with $G$  mag or $G$ $>13.4$ mag, or (iii) present in the Washington Double Star or Tycho Double Star catalogues.

The Attitude Star Catalogue was made by combining seven all-sky catalogues and selecting entries based on magnitude, isolation, and astrometric precision criteria. The catalogue has 8 173 331 entries with estimates of the positions at J2000, proper motions, and magnitudes (Gaia $G$, Gaia $G_{\mathrm{RVS}}$, red $R_F$, and blue $B_J$) in the magnitude range $G$  mag.

From the commissioning phase through the period used for Gaia DR2 the ASC was used in the SOC daily pipeline as the astrometric reference in the first on-ground attitude reconstruction, OGA1 (see Section 3.4.5).

On 2016, December 1${}^{st}$, the ASC as described above was replaced in the SOC daily pipeline by a new version built from the results of the second DPAC data reduction cycle using similar criteria than the ground-based version.

For the preparation of Gaia DR3, the cyclic pipeline used a similar subset based on the Gaia DR2 catalogue for the refinement of the on-ground attitude reconstruction in IDU, SDMOGA (see Section 3.4.5). This refined attitude solution (similar to OGA1 in method and quality) is needed only for the purposes of the observations-to-sources matching process in IDU (see Section 3.4.5).