tokens: their faintness ( V > 13.5, J > 11.5). This is somewhat expected since the two catalogues are based on different methodologies and input catalogues reflecting different scientific priorities. While K2-TESS is more focused on cool SpTs (late K and M dwarfs) and mainly based on the 2MASS infrared photometry, UCAC4-RPM is focused on solar-type FGK targets, implying different selection algorithms and thresholds. In other words, the two approaches will be highly synergic in selecting the optimal targets for TESS (Stassun et al. 2014 ). 5.2 PLATO The unique optical design of PLATO (Rauer et al. 2014 ) is driven by the requirement to cover a significant fraction of the sky with a good image quality in order to achieve an unprecedented photometric precision: 34 ppm h −1 (parts per million measured over a 1 h time-scale) for the main scientific sample limited at V 11, and even better for the brightest stars of the sample at V ≃ 6 (dominated by systematic errors). The present design of PLATO implies that the array of 12 cm telescopes is subdivided into four groups, each group pointing towards a direction which is displaced by an angle θ = 9.2 deg from the centre of the overall FOV. Such strategy provides a non-uniform instantaneous coverage of a 2124 deg 2 field. The observing strategy of PLATO consists of a two-step approach in order to maximize the scientific return of the mission. PLATO will image two ‘long-duration’ (LD) fields for more than two years each, and some supplementary fields for a few months each, during the ‘step and stare’ (SS) phase. The LD phase will fulfil the main scientific objective of the mission, being sensitive to planets down to the Earth's size orbiting within the habitable zone of solar-type (FGK) stars. The SS fields will expand the accessible sky area by about one order of magnitude, giving us access to a larger number of bright nearby stars at the expense of probing a smaller range of orbital periods. A simple calculation shows that, at the end of a nominal 6.5 year mission with 2 LD and 10 SS fields, PLATO will have surveyed about two thirds of the whole sky. The PLATO Science Requirements Document 10 defines five complementary stellar samples to be surveyed, listed in decreasing priority and summarized below. P1: DSLF5 stars brighter than V = 11, monitored with a photometric noise level σ ≤ 34 ppm h −1 , to be observed during the LD phase. P2, P3: DSLF5 stars brighter than V = 8, monitored at σ ≤ 34 ppm h −1 , to be observed during the LD and SS phase, respectively. P4: cool M dwarfs (M0 or later), to be monitored during the LD phase of the mission (at V 16), or during the SS phase (at V 15). The photometric noise level for both subsamples must be below 800 ppm h −1 . P5: DSLF5 stars brighter than V = 13, to be observed during the LD phase. Samples P1, P2, P3, P5 are therefore all made of ‘solar-like’ stars brighter than V = 13. Due to telemetry limitations, PLATO will not be able to download the full images, collected
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