Design and development of Fabry-Pérot based wavelength calibration system for PARAS-2 spectrograph

Precise wavelength calibration is essential for high-precision radial velocity spectrographs, necessitating a stable calibrator that provides a dense grid of uniformly spaced lines to accurately determine stellar line positions and monitor instrumental drifts. In this work, we present the development of a cost-effective Fabry–Pérot (FP) etalon-based wavelength calibrator designed to overcome the limitations of conventional sources such as hollow cathode lamps (HCLs) and iodine cells. This FP calibrator, combined with a xenon (Xe) arc lamp assembly, has been integrated with the PRL Advanced Radial-velocity Abu-sky Search-2 spectrograph on the Physical Research Laboratory 2.5 m telescope at Mount Abu Observatory. Operated under controlled temperature and pressure conditions, the system generates a dense, comb-like spectrum covering 62 echelle orders with more than 10,000 well-defined and stable spectral lines, enabling precise measurement of instrumental drift. Initial results show that the free spectral range varies from 0.16 Å near 4000 Å to 0.49 Å near 7000 Å, with a value of 0.3 Å around the central wavelength of 5500 Å. The estimated finesse ranges from 9 near 4000 Å to 19 near 6900 Å, with an approximate value of 17 at 5500 Å. The temperature and pressure stability tests demonstrate RMS variations of 0.002°C and 5×10−4mbar, respectively. Based on these values, the theoretical stability of the FP wavelength calibrator is estimated to be within 10cm/s, establishing it as a reliable alternative to laser frequency combs for high-resolution spectroscopic calibration. We present an initial assessment of the RV stability of the Fabry–Pérot calibrator, yielding 40 to 70cm/s of relative drift, which are up for further investigations. The observed excess over the theoretically estimated limit is likely attributable to instabilities arising from arc wandering in the Xe arc lamp