5.8.2 Reasoning behind choice of this field
A number of fields were investigated for their suitability for this survey. These included the Andromeda galaxy, the Ursa Minor dwarf Spheroidal, NGC 2516, the Kepler field and the two PLATO long duration fields.
Due to the behaviour of the Gaia scanning law (see Section 1.1.4), the Ursa Minor dwarf Spheroidal and NGC 2516 fields, being close to the ecliptic poles and being observed with the Ecliptic Pole Scanning Law at the start of the mission, are excluded since they would not be typical of the the Gaia sampling for the majority of sources.
The PLATO and Kepler fields have locations where the scanning law has lower numbers of measurements and was thus also excluded.
However, the Andromeda galaxy (M31) has a location where a number of scanning law behaviours are present and, as a consequence, sources have a range of FoV transit numbers from approximately 10 to 80. Additionally, this field has both crowded and normal density areas and thus the user can assess some of the spurious effects on variability that is caused by crowding. Also, due to the wide field chosen (5.5 centred on M31), a significant number of foreground stars are present from the Milky Way which can be studied in addition to those from M31.