High precision radial velocity studies on VBT echelle spectrograph

Abstract

The Echelle spectrograph operating at Vainu Bappu Telescope (VBT) is a general-purpose instrument designed for high-resolution spectroscopy. It was considered for precision Radial Velocity (RV) measurements without altering the existing design and primary usage. The design level limitations and environmental perturbations at the spectrograph constitute a significant source error for precise RV studies. Initially, the inherent stability of the spectrograph was estimated. From the results of the stability analysis, a few upgrade activities at the spectrograph were taken up. As part of the upgrade, an autoguider was installed at the telescope prime focus. Along with this, the wavelength calibration unit was upgraded; the alignment errors of the instrument were also corrected. With these enhancements, the stability of the spectrograph was improved from ± 1 pixel (1000 ms−1 ) to ± 0.4 pixel (400 ms−1 ). The next major limitation that was addressed is the lack of repeatability of moving components in the spectrograph. For this purpose, the Zemax model of the spectrograph was developed as a reference. ThAr spectral lines obtained from observations were evaluated with respect to the Zemax reference positions, to estimate the drifts in the components. Apart from this, to eliminate the instrumental drifts during observations, an iodine absorption cell was integrated with the spectrograph. A generic algorithm was developed that uses iodine exposures to correct for the instrumental drifts from RV estimates. Finally, with all the upgrades mentioned above, the RV precision limits of the spectrograph were pushed to a few 10-100 ms−1 regime. The technique was validated on a well-studied exoplanet-hosting star. Later, efforts were made towards the implementation of the iodine cell forward modelling technique to push the RV precision limits to a few ms−1 level. The procedure was validated using synthetic stellar spectra. The algorithm was developed on an open-source platform in Python to increase the accessibility of the approach.