Instrumental concept

Payload:
The current instrumental concept of PLATO, resulting from an internal assessement study at the ESTEC Concurrent Design Facility (CDF), is evolved from the "staring concept", described in the PLATO proposal. A set of 28 telescopes observe simultaneously the same very wide stellar field. The 28 telescopes are identical in terms of optics, structure and thermal control. Two of them may include in addition a broadband filter. Each telescope illuminates its own focal plane, made of 4 large size CCDs. Twenty-six out of the 28 focal planes work in the same mode (full frame, 25sec exposure, 1.89sec readout), while the remaining 2 focal planes work at higher cadence (1sec), in frame transfer mode.


Schematic view of the PLATO payload (click to enlarge)


A front-end electronics (FEE) box is associated to each focal plane. It includes CCD control electronics, as well as a digital processing unit (DPU) devoted to some basic data treatment : aperture photometry, star centroiding, dark window monitoring, local housekeeping, local light curve corrections, ... The 2 high cadence focal planes have a special software running on their DPU, in particular to deliver high cadence depointing information to the AOCS loop.

All data processed by the telescope DPU are transmitted to the central  instrument control unit (ICU) or central onboard calculator, where they are post-processed : « intelligent » averaging of light curves and centroids, compression, transmission.


Service module:
The PLATO three-axis stabilized service module (SVM) will be custom made for the mission. The 0.2 arcsec pointing stability required for limiting jitter-induced photometric noise (see here) will be achieved by the use of reaction wheels and fine gyris, and also by introducing in the AOCS loop high cadence very accurate measurements of star centroids by the payload instrument.

The power will be delivered by three solar array panels totalling 6.4 m2, mounted on three faces of the hexagonal SVM. In order to keep the solar arrays oriented toward the sun throughout the year without moving the line of sight, the whole spacecraft will be rotated about the lien of sight by 90° every 3 months.


Schematic view of the PLATO service module (click to enlarge)


Observation strategy:
In the main phase of the mission, two successive fields will be monitored continuously with a duty cycle ≥90-95%, one for 3 years, the other one for 2 (goal 3) years. The field of view continuously surveyed for each one of these two major pointings will be ≥ 550 deg2, therefore totalling more than 1100 deg2. After completion of these two initial monitorings, a one-year step&stare phase will be performed, in which up to 6 additional 550 deg2 fields will be surveyed for  up to 3 months each, and also during which the instrument may return episodically to the two fields already monitored in the main phase of the mission.  This step&stare phase will be used to extend the sample of stars surveyed for short period planets and for stellar structure studies, as well as for revisiting targets of the first two pointings in an optimized way, to confirm longer period exoplanets.