We present a stable, inexpensive wavelength reference, based on a white-light interferometer for the use on current and future (arrays of) diffraction-limited radial velocity (RV) spectrographs. The primary aim of using an interferometer is to obtain a dense sinusoidal wavelength reference with spectral coverage between 450-650 nm. Its basic setup consists of an unbalanced fiber Mach-Zehnder interferometer (FMZI) that creates an interference pattern in the spectral domain due to superposition of phase delayed light, set by a fixed optical path-length difference (OPD). To achieve long-term stability, the interferometer is actively locked to a stable atomic line. The system operates in closed-loop using a thermo-optic modulator as the phase feedback component. We conducted stability measurements by superimposing the wavelength reference with thorium-argon (ThAr) emission lines
and found the differential RMS shift to be ~5 m s-1 within 30 minute bins in an experiment lasting 5 hours.
We present the opto-mechanical design and the characterization of the Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph. RHEA is an ultra-compact fiber-fed echelle spectrograph designed to be used at 0.2-0.4 m class robotic telescopes where long term dedicated projects are possible. The instrument will be primarily used for radial velocity (RV) studies of low to intermediate-mass giant stars for the purpose of searching for hot Jupiters and using asteroseismology to simultaneously measure the host star parameters and de-correlate stellar pulsations. The optical design comprises a double-pass (i.e. near Littrow) configuration with
prism cross-disperser and single-mode fiber (SMF) input. The spectrograph has a resolving power of R>70,000 and operates at 430–670 nm with minimum order separation of ~180 μm. This separation allows a 1x6 photonic
lantern integration at a later stage which is currently under development. The current design is built with the aim of creating an inexpensive and replicable unit. The spectrograph is optimised for long-baseline RV observations through careful temperature stabilisation and simultaneous wavelength calibration. As a further improvement the echelle grating is housed in a vacuum chamber to maintain pressure stability. The performance of the current prototype is currently being tested on a 0.4 m telescope at the Macquarie University Observatory.
The Echelle spectrograph FOCES1 is currently located at the laboratories of Munich University Observatories
under pressure and temperature stabilized conditions. It is being used as a test bed for a number of different
stability issues related to high precision radial velocity spectroscopy.
We utilize FOCES to study spectrograph stability, illumination stability and fiber transport stability. With
this work we continue the series of papers that present our efforts to obtain temperature and pressure stabilization
in the spectrograph environment. In particular we present first optical measurement results and compare them
to simulations previously published. We show the movement of the image on the CCD with changes of pressure
and temperature and the stability of the spot positions in the stabilized system using measurements done by a
ThAr gas discharge source.
To improve our understanding of fiber scrambling properties a test bed where fiber near-field and far-field can be measured simultaneously is described. A variety of measurements has been conducted with a selection of fibers from different vendors, including state-of-the-art octagonal and hexagonal fibers. After characterization of the test bench with respect to stability and resolution, scrambling measurements have been conducted using LEDs with central wavelengths ranging between 465-635 nm. The dependence on wavelength regarding to photometrical scrambling has been initially demonstrated. Moreover, two mechanical combined fiber cables have been analyzed that were made from octagonal-circular and hexagonal-octagonal fiber sections. In this context an apparatus for focal ratio degradation (FRD) measurements was assembled to compare different shaped fibers and fiber combinations. Finally, all these preliminary investigations will help in choosing a fiber with good radial scrambling performance for the next generation fiber-link of the fiber optic coupled Cassegrain echelle spectrograph FOCES intended to be operated at the 2.0m Fraunhofer Telescope at the Wendelstein Observatory.