Development of a stabilized Fabry-Perot etalon-based calibrator for Hanle echelle spectrograph

Abstract

Accurate wavelength calibration is an important factor for any measurement with high-resolution spectrographs. The stellar spectrum comprises discrete absorption or emission lines whose position is precisely determined by calibrating the spectrograph using known reference lines generated from laboratory sources. For the spectrograph to measure small variations in Doppler shift, the wavelength calibration must be sufficiently stable during observation time. Instrument instability, mainly due to environmental factors like temperature and pressure variations and limitations of traditional calibration methods, for example, Th-Ar lamps, is the main challenge with high-precision spectroscopy. Through proper environmental control, by maintaining pressure at few mbar and temperature fluctuations within +/- 0.05 degrees C, Fabry-Perot etalon (FP) can yield a velocity precision of 1-10 m/s when used for wavelength calibration. We have developed a passively stabilized FP-based wavelength calibrator for a Hanle echelle spectrograph (HESP) installed on the Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory (IAO), Hank, India. The etalon has been characterized using a Fourier transform spectrograph (FTS), and initial test runs have been performed with HESP. In this paper, we present the design and construction of the instrument along with preliminary test results obtained from HESP. (C) 2020 Optical Society of America