Developement of a stabilized Fabry-Perot etalon based calibrator for Hanle Echelle Spectrograph (HESP)

TL;DR

This paper presents the design and initial testing of a passively stabilized Fabry-Perot etalon for wavelength calibration of the Hanle Echelle Spectrograph, aiming to improve stability and precision in high-resolution spectroscopy.

Contribution

It introduces a passively stabilized Fabry-Perot etalon specifically developed for HESP, addressing environmental stability challenges for precise wavelength calibration.

Findings

Achieved velocity precision of 1-10 m/s with the etalon.

Maintained temperature fluctuations within ±0.05°C and pressure at a few mbar.

Initial tests demonstrate promising calibration stability.

Abstract

Accurate wavelength calibration is an important factor for any measurement with high resolution spectrographs. Stellar spectrum comprises of 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, limitations of traditional calibration methods, for example Th-Ar lamps, are the main challenges which high precision spectroscopy. Through proper environmental control, by maintaining pressure at few mbar and temperature fluctuations within $\pm$0.05$^{\circ}$C, Fabry P\'erot 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 Hanle Echelle Spectrograph (HESP) installed on Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory (IAO), Hanle, India. The etalon has been characterized using 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.