Credit: INAF

Achieving a very large FoV in common with a relatively arge aperture lead to fast focal ratio optics. How fast depends upon the available detector format. With a pixel size of the order of ten micrometers, as nowadays state of the art scientific grade – space qualified – CCDs are available (Endicott et al. 2012), this translates into extremely short focal length in order to covere a few ten of degrees of FoV on a reasonable detector surface encompassing order of magnitude of one hundred million of pixels. As this would leads to an impossibly short focal ratio (much lower than unity) the only solution would be to segment the optics in some manner.
Segmentation is a well known approach that can be used at several levels (for istance in the FoV or in the pupil plane). In PLATO the natural choice is to segment the telescope into several small telescopes with the same large FoV, mimicing a larger aperture by the numerical coaddition of the collected frames. This approach has also other advantages, namely:

• the equivalent full well, and hence the dynamic response of the ensemble of the CCDs, is proportionally augmented to the number of individual telescopes;
• the inherent multiplexing allows for a much robust payload, as the failure of one detector or of one telescope only lower the overall performances;
• an additional degree of freedom can be introduced, namely the pointing of the telescopes, individually or as groups, into slightly different regions with some partial overlap in order to encompass – as an ensemble – a larger FoV, although at the expense of an equivalent smaller telescope aperture, leading to a larger chance to detect transits around particularly bright and nearby stars.

Except from Ragazzoni et al.,  2015, A one meter class eye for the PLAnetary Transit and Oscillation spacecraft,  Acta Astronautica 115 p.18