Abstract
We present the design and testing of a prototype in-plane echelle spectrograph based on an actively controlled fiber-fed double-pass design. This system aims to be small and efficient with the minimum number of optical surfaces—currently a collimator/camera lens, cross-dispersing prism, grating and a reflector to send light to the detector. It is built from catalog optical components and has dimensions of approximately 20 × 30 cm. It works in the optical regime with a resolution of >70,000. The spectrograph is fed by a bifurcated fiber with one fiber to a telescope and the other used to provide simultaneous Thorium Argon light illumination for wavelength calibration. The positions of the arc lines on the detector are processed in real time and commercial auto-guiding software is used to treat the positions of the arc lines as guide stars. The guiding software sends any required adjustments to mechanical piezo-electric actuators which move the mirror sending light to the camera removing any drift in the position of the arc lines. The current configuration using an sCMOS detector provides a precision of 3.5 milli-pixels equivalent to 4 m s−1 in a standard laboratory environment.
Highlights
The achievement of meter-per-second radial velocity precision is one of the major technological breakthroughs of recent decades
The path from the suggestion that stellar velocities might be accurately calibrated (Struve 1952), to solar studies (Becker 1976; Koch & Woehl 1984) to pioneering stellar observations using Hydrogen Fluoride gas cells (Campbell & Walker 1979; Campbell, Walker & Yang 1988) to sub-m s−1with ESPRESSO (e.g., Pepe et al 2020), long-term several m s−1 RMS (and short-term sub-m s−1 measurements with ESO 3.6m/HARPS (e.g., Pepe et al 2000), Keck/HIRES, e.g., Vogt et al 1994, Butler et al, 1996), Magellan/PFS (e.g. Crane et al 2006), HET/HRS(e.g., Tull 1998), TNG/HARPS-N (Sosnowska et al 2012) and VLT-UVES (e.g., Butler et al 2019) and is one that leads through several generations of similar spectrographs, detectors, and calibration methodologies
The need for mosaic gratings and bespoke large optics continues to contribute to the ongoing high cost of such instruments, e.g., 23.7 MEuros1 for ESPRESSO and might further escalate as such instruments are built for larger telescopes
Summary
The achievement of meter-per-second radial velocity precision is one of the major technological breakthroughs of recent decades. The need for mosaic gratings and bespoke large optics continues to contribute to the ongoing high cost of such instruments, e.g., 23.7 MEuros for ESPRESSO and might further escalate as such instruments are built for larger telescopes. We contribute to this series of work by investigating some alternative approaches for the configuration of a high-resolution spectrograph. Our currently preferred methodology for achieving efficient focal reduction is through efficient tapering of the input fibre from the telescope as presented by Choochalerm et al (2020) Alongside these efforts we have developed a data reduction pipeline (Errmann et al 2020) which can be used for EXOhSPEC but is built with the flexibility to examine data from our laboratory prototypes.
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