A dual-functional calibration source for astronomical spectrograph

The current precision of radial-velocity spectrographs is limited by the wavelength calibration and used calibration sources. New types of calibration sources are needed to enable better wavelength calibration. The goal of my thesis work was to test several Fabry-Perots, which are systems that produce a spectrum of nearly-equidistant emission lines over a very large wavelength range. As such, and provided that the lines remain stable in wavelength over time, this source can be used as a wavelength reference. During the the second part of my thesis work, I investigated furthermore possible developments of our Fabry-Perot system as an alternative to current calibration sources. The declared objective was to improve the wavelength calibration accuracy of the HARPS spectrograph by combining the absolute spectral reference provided by the emission lines of a thorium-argon hollow-cathode lamp (HCL) with the spectrally rich and precise information delivered by the Fabry-Perot-based calibration source.

Precise wavelength calibration is a critical issue for high-resolution spectroscopic observations. The ideal calibration source should be able to provide a very stable and dense grid of evenly distributed spectral lines of constant intensity. A new method which satisfies all mentioned conditions has been developed by our group. The approach is to actively measure the exact position of a single spectral line of a Fabry-Perot etalon with very high precision with a wavelength-tuneable laser and compare it to an extremely stable wavelength standard. The ideal choice of standard is the D2 absorption line of Rubidium, which has been used as an optical frequency standard for decades. With this technique, the problem of stable wavelength calibration of spectrographs becomes a problem of how reliably we can measure and anchor one etalon line to the Rb transition. In this work we present our self-built module for Rb saturated absorption spectroscopy and discuss its stability.

The European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF), and the US National Institute of Standards and Technology (NIST) are collaborating to study Th-Ar hollow cathode lamps as used for the calibration of VLT (Very Large Telescope) spectrographs. In the near IR only a limited number of wavelength standards are available. The density and distribution of lines in Ne or Kr lamps, for example, are inadequate for high-resolution spectroscopy. Th-Ar hollow cathode lamps provide a rich spectrum in the UV-visible region and have been used in astronomy for a long time; current examples at ESO include the spectrographs UVES and FLAMES. The Th spectrum from 278 nm to about 1000 nm was studied at high resolution about 20 years ago (Palmer and Engleman, 1983). Two studies of the Th-Ar spectrum in the near IR have recently been published, but neither work is directly applicable to the calibration of IR astronomical spectrographs at ESO. We report new measurements using the 2-m UV/visible/IR Fourier transform spectrometer (FTS) at NIST that establish more than 2000 lines as wavelength standards in the range 900 nm to 4500 nm. This line list is used as input for a physical model that provides the wavelength calibration for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), ESO's new high resolution (R~100,000) IR spectrograph at the VLT. We also present first calibration results from laboratory testing of CRIRES. The newly established wavelength standards will also be available for use by X-shooter and other spectrographs in the future. Measurements of the variation of the spectrum of Th-Ar lamps as a function of operating current allow us to optimise the spectral output in terms of relative intensity and line density for operation on the telescope. Since Th and Ar line intensities show a different response with respect to operating current, such measurements can be used as a diagnostic tool for distinguishing the gas and metal lines. Our findings show that Th-Ar lamps hold the promise of becoming a standard source for wavelength calibration in near IR astronomy.

Laser frequency combs are an ideal calibration source for precision astronomical spectrographs. We report on the demonstrated long term operation of a laser frequency comb that we designed and built as the primary calibrator for the Habitable Zone Planet Finder (HPF). The core technology of the comb is based on robust, polarization maintaining fiber coupled electro-optic modulators and broadband supercontinuum generation spanning 700-1600 nm in an efficient silicon nitride waveguide. The comb is continuously maintained on and ready to use, and since May 2018 the laser frequency comb has had a total uptime of 97%.