Use of solid fused silica etalon with broad-band metallic coatings for calibration of high-resolution optical spectrograph

Wavelength calibration is a key factor for high-resolution spectroscopic measurements for precision radial velocities. Hollow-cathode lamps (e.g. ThAr), absorption cells (e.g. iodine cell), dielectric coated Fabry-Pérot etalons, and laser frequency combs have been implemented over the years for precise wavelength calibration and wavelength drift measurements. However, due to their various impediments as wavelength calibrators, investigations of alternative methods remain of prime interest. In this paper, we examined the feasibility of low-cost ( $1000) commercially available solid fused silica etalon with a broad-band metallic coating as a calibrator. We studied the behaviour for two cavity spacings (free spectral range of 1 and 0.5 cm−1) with temperature from theoretical derivation and experimental data. Our setup had a temperature stability of 0.8 mK for a calibrator system using an off-the-shelf dewar flask with active stabilization. Our result from radial velocity drift measurements demonstrated that such a calibration system is capable of providing higher signal-to-noise calibration and better nightly drift measurement relative to ThAr in the wavelength range between 470 and 780 nm. A similar result has been previously found for Fabry-Pérot etalons, and although the metalon solution lacks the efficiency of an etalon, it does offers a cost-effective broad-band solution, which should be less prone to aging relative to complex dielectric mirror coatings. None the less, long-term monitoring is required to understand the metalon behaviour in detail.