| Authors : | Swapnil Singh, Bhavesh Jaiswal, Anand Jain, Reenu Palawat, Smrati Verma, Brajpal Singh, Ravishankar BT, Bijoy Raha, Bhavesh Mendhekar, Sathyanarayana Raju K, Srinivasa Rao Kondapi V, Yogesh Prasad K R, Priyanka Das, Supratik Bose, Supriya Verma, Waghmare Rahul Gautam, Mukund Kumar Thakur, Sumit Kumar, Sankarasubramanian K and Anuj Nandi |
| Abstract : | Earth is the only known habitable planet, and it serves as a test bed to benchmark the observations of temperate and more Earth-like exoplanets. It is required to observe the disc-integrated signatures of Earth for a range of phase angles, resembling the observations of an exoplanet. We present the instrument configuration of an Acousto-Optic Tunable Filter (AOTF) based Near-Infrared (NIR: 1.0 – 1.7 μm) spectro-polarimeter to observe the spectro-polarimetric signatures of Earth. The AOTF is driven by a radio frequency signal generated with the in-house developed radio frequency system. On application of the RF signal, the AOTF filters the incident light and produces two diffracted narrow-band beams, which are linearly polarized in mutually perpendicular directions. The instrument optics with a FOV of ~ 2.6 degrees is configured to focus two output beams onto two Indium-Gallium-Arsenide (InGaAs) detectors. In-house front-end, processing and power electronics have been developed to read out the data. Preliminary results of the radiometric, spectroscopic and polarimetric calibration of the instrument will be discussed. These include Frequency-Wavelength relation, spectral bandpass, the Mueller matrix of the AOTF and field variation in the desired wavelength range. Based on the instrument characteristics, simulations are carried out to optimize the design for the expected signal levels in various observing conditions. The usefulness of an AOTF-based spectro-polarimeter is established from this study and it is found that, in the present configuration, the instrument can achieve a polarimetric accuracy of <0.3% for linear polarization for an integration time of 100 ms or larger. The plans for observing Earth as an exoplanet from the Lunar orbit will also be discussed. |
