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.
Fiber modal noise is a performance limiting factor in high-precision radial velocity measurements with multi-mode fiber fed high-resolution spectrographs. Traditionally, modal noise is mitigated by agitating the fiber, this way redistributing the light that propagates in the fiber over many different modes. However, in case of fibers with only a limited number of modes, e.g. at near-infrared wavelengths or in adaptive-optics assisted systems, this method becomes very inefficient. The strong agitation that would be needed stresses the fiber and can lead to focal ratio degradation. As an alternative approach, we propose to use a classic optical double scrambler and to rotate the scrambler’s first fiber end during each exposure. Because of the rotating illumination pattern of the scrambler’s second fiber, the modes that are excited vary continuously. This leads to very efficient averaging of the modal pattern at the fiber exit and to a strong reduction of modal noise. In this contribution, we present a prototype design and first laboratory results of the rotating double scrambler.
Precise wavelength calibration is a persistent problem for highest precision Doppler spectroscopy. The ideal calibrator provides an extremely stable spectrum of equidistant, narrow lines over a wide bandwidth, is reliable over timescales of years, and is simple to operate. Unlike traditional hollow cathode lamps, etalons provide an engineered spectrum with adjustable line distance and width and can cover a very broad spectral bandwidth. We have shown that laser locked etalons provide the necessary stability with an ideal spectral format for calibrating precision Echelle spectrographs, in a cost-effective and robust package. Anchoring the etalon spectrum to a very precisely known hyperfine transition of rubidium delivers cm/s-level stability over timescales of years. We have engineered a fieldable system which is currently being constructed as calibrator for the MAROON-X, HERMES, KPF, FIES and iLocater spectrographs.