The Pilot scientific instrument (developed and operated under IRAP responsibility with support from IAS, CEA and CNES, with contributions from Roma and Cardiff universities) is composed of:
- the mechanical frame with its baffle and mechanism for adjusting the primary mirror
- the 830-mm diameter primary mirror
- the photometer
- the on-detectors electronics
- the house-keeping electronics (UGTI), the instrument computer and the DC-DC converters (28V to ± 15V and +5V)
- the inclinometer.
The pointed payload mechanical structure is under the responsibility of IRAP. It is composed of the following sub-systems:
- the bearing structure
The bearing structure holds the photometer, the primary mirror and the ESTADIUS stellar sensor, whose relative alignment has been verified during ground and in-flight calibration tests. It also houses the detector and house-keeping electronics. This structure, which provides the frame for the pointed payload (PILOT Instrument + Estadius), is located in the corridor between the walls of the gondola. It is supported by the elevation rotation axis and a hydraulic jack (supplied and operated by CNES).
- the primary mirror adjustment system
This system is used to adjust the translation and rotation of the primary mirror with six degrees of freedom. The option selected for PILOT is a manually adjusted hexapod. A motorized version was abandoned for the PILOT project due to cost, but can be easily implemented within such a system.
- the instrument baffle
The baffle protects the instrument from stray light. It is designed to minimize the entry of stray light rays inside the photometer. It also protects the primary mirror from reflections of light rays coming from the balloon and the Earth (even if the reflection does not enter directly the photometer).
The primary mirror is an off-axis parabolic mirror. In combination with the photometer's secondary mirror, it forms an off-axis Gregorian telescope. The primary mirror is 830 mm in diameter and is mounted on a support that connects to the mirror adjustment system (a manually adjusted hexapod). The mirror support structure includes reference balls and cubes that are used to identify the position of the mirror’s focal point, as well as the main incident axis and the main reflection axis. The temperature of the primary mirror is precisely monitored by several sensors that are read during the flight by the temperature monitoring electronics.
The photometer (developed and operated under IAS responsibility) collects, filters, and analyzes the polarization direction of photons coming from the primary mirror. The photometer is constituted of a cryostat containing all the optics cooled by a tank of liquid helium (4He). The detectors themselves are cooled to 300 mK by a helium (3He) cooler. Inside the photometer, the detected signal is conditioned so that it can be processed by the electronics outside the photometer. Reference balls and cubes inside the photometer are used to identify the output axis and the focal point of the optics. This focal point must be coincident with the focal point of the primary mirror.
BolC (developed and operated under CEA responsibility) controls the detectors. They are grouped in eight matrices of 16x16 bolometers (for a total of 2048 detectors) that are situated at the focal plane of the instrument. BolC also performs some house-keeping functions such as the monitoring the temperature of the detector blocks, as well as recycling of the 3He cooler.
The instrument's electronics (developed and operated under IRAP responsibility) perform the following functions:
- monitor the temperature of cryogenic (2 K to 77 K) and non-cryogenic (-70 °C and 60 °C) components of the photometer,
- read and control the internal calibration source (ICS) of the detectors,
- control the instrument's other house-keeping measurements (e.g. current tensions),
- interface with commands sent to the half-wave plate mechanism (which enables the selection of the polarisation desired for the measurements),
- interface with commands sent to the cryogenic valve that open or close the helium tank,
- recycling of the redundant Helium-3 cooler if there are two coolers (the primary cooler is managed by the BolC),
- control the on/off switches of the UGTI electronics cards, the instrument computer (OBC) and the CEA electronics (BolC),
- communication with the instrument computer (OBC) installed in one of the gondola walls.