for these follow-up studies [@two_meter; @existing_spectrograph]. This can save the telescope time on the large telescopes and adapt the existing instruments for precision RV measurements. The first step in utilizing the existing spectrographs is to critically evaluate the technical and operational level limitations of the instrument and develop a mechanism/model to address them.

Here we have estimated the limitations of a general purpose echelle spectrograph operating on the 2.3 $\textrm{m}$ Vainu Bappu Telescope (VBT) in Kavalur, India for precision RV capabilities. The telescope is equipped with a fibre-fed high resolution (R = 60,000), Echelle spectrograph [@VBTechelle]. Since its commissioning in 2003, the instrument is used in a variety of spectroscopic observations of solar system objects, chemical evolution, age and abundance determination of stars and galaxies. However, the spectrograph was not designed originally for the precision RV measurements [@stabilityVBT]. For such a general purpose spectrograph, it is essential that the zero-point variations must be accurately tracked and corrected. In this paper, we have investigated the instability of the spectrograph subjected to perturbations of optical components arising from its current functionality. From the analysis, we propose a methodology for the correction of the limitations from the spectrograph for RV measurements.

A Zemax 