High resolution fiber fed echelle spectrograph: calibration and characterisation for precise radial velocities and chemical abundances

Abstract

Modern astronomical spectrographs have been in use for several hundred years now, contributing to the study of the physical, chemical and kinematical properties of stars and other celestial bodies. In the recent years, there has been new developement in the design choices, calibration sources and detec- tors for spectrograph that led to large improvements in resolution, efficiency

and stability, which have been a key to the discoveries of low mass exoplanets, cosmic dynamics and variability of physical constants. Even with these major improvements in the instrumentation, a little has changed in the way data is calibrated. The physics used in the designing and building of the instrument is seldom put to use for instrument calibrations. In the current work, we have developed a computing scheme whereby an astronomical spectrograph can be modelled and controlled without recourse to a ray-tracing program. This is achieved by using paraxial ray tracing, exact corrections for certain surface types and the aberration coefficients of Buch- dahl for more complex modules. We have shown that the resultant chain of paraxial ray trace matrices and correction matrices can predict the location of any spectral line on the detector under all normal operating conditions with a high degree of certainty. This kind of model will allow a semi-autonomous control via simple in-house, program modules. The theory and formulation in developing a model for HESP (Hanle Echelle Spectrograph), comparison with commercial ray tracing software and validation the model with calibra- tion exposures taken in the lab are described in detail. We have demonstrated that the wavelength calibration using the dispersion solution from model per- forms superior to the emipirical solution that is used in standard spectroscopic reduction software (e.g. IRAF), especially in the regime of sparse and low signal calibration lines. We also discuss the potential use of model in si- multaneous calibration using double fiber in tracking the instrument drifts and issues related to positional and wavelength dependancy of instrumental drift. We conclude by listing several science areas that would benefit using a model based calibration. Apart from calibration of science data, the model along with an optimization routine can be used in the instrument mainte- nance pipeline to study the real time behaviour of the instrument, trending and long-term stability. We also present the work related to the development of autoguiding alogrithm and fiber scrambling for HESP and implication to radial velocity shift.