Feasibility Study for Adaptive Optics Implementation on the 2m Himalayan Chandra Telescope

Abstract

Atmospheric turbulence in the Earth’s atmosphere significantly impacts the performance of ground-based telescopes. Adaptive Optics (AO) has emerged as an indispensable technology in ground-based astronomy, revolutionizing our ability to observe celestial objects with unprecedented clarity. Recognizing the increasing importance of AO in enhancing the capabilities of telescopes, it is imperative for the Indian Institute of Astrophysics (IIA) to explore the potential integration of AO into its 2m Himalayan Chandra Telescope (HCT) located in hanle, where the median seeing condition is 1 arcsec. This project focuses on the comprehensive study of implementing AO on the HCT and is based on end-to-end AO simulations. Our study identifies that the most suitable AO system for HCT is a Dual Adaptive Optics system, incorporating both Natural Guide Star (NGS) and Rayleigh Laser Guide Star (LGS) techniques. Deformable mirror and wavefront sensor are identified as two crucial components in any AO system, and this study includes a meticulous quantification process to inform the selection of these components. Our AO simulations indicate that a Shack-Hartmann Wavefront Sensor (SHWS) with an 11 x 11 lenslet array and a Micro Electro-Mechanical System (MEMS) deformable mirror with 12 x 12 actuators provide optimal Strehl performance in the R, J, H, and K bands. For the LGS AO system, we selected a 10 W UV pulsed laser to generate a bright laser beacon, defining a range gate length of 800m at a 10 km altitude above hanle. We find that the limiting magnitude for a bright guide star is 9th magnitude, and a faint star upto 15th magnitude is sufficient for tip-tilt sensing. Sky coverage, which is the probability of finding a suitable guide star near the science target, is a critical performance metric. It depends on guide star magnitude, guide star density, and the separation between the guide star and the science target. Using the Besancon Model for guide star density in optical wavelengths, our calculations show that sky coverage for NGS AO is less than 1 %, whereas it is approximately 100 % for LGS AO, including tip-tilt sensing. Due to the inadequate sky coverage with NGS AO alone, the implementation of a dual adaptive optics system is necessary. This integration aims to significantly enhance the observational capabilities of the HCT, providing valuable insights for advancing ground-based astronomy at the challenging hanle site.