Shaping SHAPE - A spectro-polarimeter onboard Chandrayaan-3 to observe Earth as an Exoplanet

SHAPE (Spectro-polarimetry of HAbitable Planet Earth) is an experimental instrument onboard the Propulsion Module (Orbiter) of the Chandrayaan-3 mission. It is designed to perform disc-integrated spectro-polarimetric observations of Earth from lunar and highly elliptical Earth orbits, using our planet as a stand-in for Earth-like exoplanets.

The compact, lightweight instrument comprises three subsystems: EODS-Optics, EODS-Electronics, and a Radio Frequency Source (RFS). An Acousto-Optic Tunable Filter (AOTF), driven by an in-house-developed 80–135 MHz RF source, provides spectral filtering across the near-infrared (1.0–1.7 μm) and produces two narrow-band beams with mutually perpendicular linear polarization states. With a field of view of about 2.6°, the optics focus both beams onto InGaAs detectors, and a spectral resolution of 2–4 nm is achieved through in-house low-noise front-end electronics. The instrument also includes processing and power electronics for signal processing, detector biasing, and subsystem control.

The paper presents the overall instrument design, results from pre-launch ground-based testing, and in-orbit operational performance. The current configuration enables SHAPE to measure disc-integrated Earth signatures over a range of phase angles, providing a test bed for characterizing Earth-like exoplanets and benchmarking future exoplanet observations.