The science instrument on-board the PicSat satellite has been specifically designed for the observation of the transit β Pictoris. It is capable of achieving high precision photometric measurements on bright targets, in the visible band (up to 100 ppm/hour on β Pictoris, which has a magnitude of 3.86 in the visible band). To do so, it uses a 5 cm optical telescope, coupled to a state-of-the-art single-pixel avalanche photodiode by a single-mode optical fiber. The photodiode provides extremely precise measurements of the number of photons hitting the telescope at any given time, and the optical fiber, with its very small core, helps to get rid of all the background noises (light coming from the Moon or the Earth, and scattered by the telescope structure, for example).

But because the optical fiber is so small (about 3 μm in the focal plane of the telescope, which is equivalent to about 2 arcsec on the sky), a very fine pointing system is required, to ensure that the fiber will always stay centered on the science target. This is achieved by a dedicated algorithm driving a two-axis piezoelectric actuator on which the fiber is mounted.

Principle of the PicSat instrument.