The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of similar to 100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 mu m with the goal of extending it to 0.35-2.4 mu m with the addition of an U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allows ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coude room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.

Astronomical spectrographs require calibration of their dispersion relation, for which external sources like hollow-cathode lamps or absorption-gas cells are useful. Laser frequency combs (LFCs) are often regarded as ideal calibrators because they provide the highest accuracy and dense sampling, but LFCs are facing operational challenges such as generating blue visual light or tunable offset frequencies. As an example of an external source, we aim to provide a precise and accurate frequency solution for the spectrum of molecular iodine absorption by referencing to an LFC that does not cover the same frequency range. We used a Fourier Transform Spectrometer (FTS) to produce a consistent frequency scale for the combined spectrum from an iodine absorption cell at 5200- 6200 angstrom and an LFC at 8200 & Aring;. We used 17 807 comb lines to determine the FTS frequency offset and compared the calibrated iodine spectrum to a synthetic spectrum computed from a molecular potential model. In a single scan, the frequency offset was determined from the comb spectrum with an uncertainty of similar to 1 cms(-1). The distribution of comb line frequencies is consistent with no deviation from linearity. The iodine observation matches the model with an offset of smaller than the model uncertainties of similar to 1 m s(-1), which confirms that the FTS zero point is valid outside the range covered by the LFC, and that the frequencies of the iodine absorption model are accurate. We also report small systematic effects regarding the iodine model's energy scale. We conclude that Fourier Transform Spectrometry can transfer LFC accuracy into frequency ranges not originally covered by the comb. This allows us to assign accurate frequency scales to the spectra of customized wavelength calibrators. The calibrators can be optimized for individual spectrograph designs regarding resolution and spectral bandwidth, and requirements on their long-term stability are relaxed because FTS monitoring can be performed during operation. This provides flexibility for the design and operation of calibration sources for high-precision Doppler experiments.

The Solar observatory at the Institute for Astrophysics and Geophysics Göttingen makes use of the ultra-high resolving power (R < 900, 000 at 600 nm) of a Fourier transform spectrograph (FTS) to obtain spectra of the resolved and integrated Sun. To improve the radial velocity (RV) stability of the FTS measurements we develop a new calibration unit based on a passively stabilized Fabry-Pérot Etalon (FP) (FSR= 3.6 GHz and F ≈ 7) for simultaneous calibration in the near-infrared. The FP is illuminated by two LEDs, covering the wavelength range from 800 to 1000 nm. To mitigate environmental effects, the FP is placed in a temperature and pressure controlled vessel. We explore the impact of the choice of input fiber as well as fiber coupler focal length on the calibration spectrum. In 150 laser frequency comb calibrated measurements over 8 hours we achieve an an RMS of the FP-RV of 0.58 m s−1 .

The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 µm with the goal of extending it to 0.35-2.4 µm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states.

Light of a stabilized laser frequency comb is coupled into a multimode fiber and recorded with a Fourier-Transform spectrometer. The interferograms consist of equidistant, burst-like interference signals, which decorrelate due to modal dispersion and distribution.

We have used a laser frequency comb with a repetition frequency of nu(rep) approximate to 1 GHz to measure the drift and dispersion of a Fourier-transform spectrometer (FTS). We used the electronic measurements of nu(rep) and nu(CEO )to create a reference line list. We measured 28 interferograms and computed the phase and power spectra. The analysis of the interferograms and phase spectra allowed for compensation of several spectroscopic artifacts. In the computed power spectra, we detected similar to 64.000 suitable lines in the near-infrared bandwidth Delta nu = 308.79-374.74 THz. The residual dispersion of the FTS can be described by two factors, a linear dispersion and a constant offset. Both are highly correlated and need to be computed simultaneously. The factors were computed from the comparison of a reference with measured line lists. The linear dispersion factor is found to be varying on the order of 10(-8) Hz/Hz, while the constant offset is of the order of 10(7) Hz. Using two factors for calibration, the difference between the reference and the measured line list can be removed completely with an uncertainty of similar to 65 kHz corresponding to a precision of 0.5 . 10(-9) Hz/Hz. (C) 2019 Optical Society of America