Second-Generation Fabry-Pérot Unit for CARMENES

Abstract

CARMENES consists of two high-resolution spectrographs located at the Calar Alto 3.5m telescope, covering the wavelength range from 520nm to 1710 nm. The original Fabry-Perot (FP) units have been essential to track and remove instrumental drift while also improving the wavelength solution provided by the hollow cathode lamps. In this paper we present the second-generation FP unit that has begun operation at Calar Alto in April 2024. It features drastically improved temperature stabilization, additional temperature monitoring, and enhanced optomechanics. Temperature stabilization is improved through better vacuum vessel design, thermal shielding and improved thermal control. The temperature is now monitored with Pt-100 sensors inside the vacuum vessel, one of which is used to drive the temperature control. The new purpose-built optomechanics allow for an improved alignment procedure leading to higher line contrast. The etalon coatings now cover a broader wavelength range, the optics have been upgraded, and the fibers have been updated to an octagonal shape. In laboratory measurements with a Fourier transform spectrometer we compare the FP radial velocity (RV) drift to that of an iodine cell. In a two hour binning we achieve an RMS of the differential RVs of 6cm/s. The new FP unit has already been successfully integrated at CARMENES, with first calibration data indicating improved RV precision.