One of the main task devolved to the payload board is the real-time fine tracking of the science target. The core of the optical fiber measuring only 3 μm, it has about the same size as the airy disk (diffraction pattern) generated by the telescope. Thus, in order to make sure that no flux is lost in the focal plane, the fiber has to be kept centered on the star.
If the star moves in the focal plane (because of the satellite jitter, for example), then the flux injected in the fiber rapidly decreases. But because the acquisition system only has one pixel, it does not have any proper “field-of-view”, and it cannot easily see where the star is going.
The main idea behind the tracking system of the PicSat instrument is to avoid staying centered on the star, and to deliberately introduce an offset between the known position of the star and the position of the fiber. When this offsets is modulated at high speed (100 Hz, typically), the fiber moves around the star, in a known pattern. If the star position does not match the center of the pattern, 100 Hz modulations start to appear in the observed photometry, which can be used to correct the position of the fiber.
In practice, this general idea takes the form of a dedicated Kalman algorithm, which uses the data coming from the piezoelectric stage position sensors and from the photodiode (and possibly even from the embedded three-axis gyroscope), to estimate the position of the star in real-time. The algorithm itself runs at 1 Khz, and is able to correct for the jitter of the star up to about 30 Hz.
Some of the other tasks devolved to the payload board are:
- Gathering and monitoring house-keeping data
- Regulating the photodiode temperature (thanks to its buit-in thermo-electric cooler)
- Sending science data and various reports to the main computer
The whole software embedded on the STM32 microchip is designed and implemented within the GERICOS (GEneRIC Onboard Software) framework, developed at LESIA.