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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.
At the end of 2021, the ESO council approved the start of the construction phase for a High Resolution Spectrograph for the ELT, formerly known as ELT-HIRES, renamed recently as ANDES (ArmazoNes high Dispersion Echelle Spectrograph). The current initial schedule foresees a 9-years development aimed to bring the instrument on-sky soon after the first-generation ELT instruments. ANDES combines high spectral resolution (up to 100,000), wide spectral range (0.4 µm to 1.8 µm with a goal from 0.35 µm to 2.4 µm) and extreme stability in wavelength calibration accuracy (better than 0.02 m/s rms over a 10-year period in a selected wavelength range) with massive optical collecting power of the ELT thus enabling to achieve possible breakthrough groundbreaking scientific discoveries. The main science cases cover a possible detection of life signatures in exoplanets, the study of the stability of Nature’s physical constants along the universe lifetime and a first direct measurement of the cosmic acceleration. The reference design of this instrument in its extended version (with goals included) foresees 4 spectrographic modules fed by fibers, operating in seeing and diffraction limited (adaptive optics assisted) mode carried out by an international consortium composed by 24 institutes from 13 countries which poses big challenges in several areas. In this paper we will describe the approach we intend to pursue to master management and system engineering aspects of this challenging instrument focused mainly on the preliminary design phase, but looking also ahead towards its final construction.
We establish a baseline signal-to-noise ratio (SNR) requirement for the European Space Agency (ESA)-funded Solar Coronagraph for OPErations (SCOPE) instrument in its field of view of 2.5–30 solar radii based on existing observations by the Solar and Heliospheric Observatory (SOHO). Using automatic detection of coronal mass ejections (CMEs), we anaylse the impacts when SNR deviates significantly from our previously established baseline. For our analysis, SNR values are estimated from observations made by the C3 coronagraph on the Solar and Heliospheric Observatory (SOHO) spacecraft for a number of different CMEs. Additionally, we generate a series of artificial coronagraph images, each consisting of a modelled coronal background and a CME, the latter simulated using the graduated cylindrical shell (GCS) model together with the SCRaytrace code available in the Interactive Data Language (IDL) SolarSoft library. Images are created with CME SNR levels between 0.5 and 10 at the outer edge of the field of view (FOV), generated by adding Poisson noise, and velocities between 700 km s−1 and 2800 km s−1. The images are analysed for the detectability of the CME above the noise with the automatic CME detection tool CACTus. We find in the analysed C3 images that CMEs near the outer edge of the field of view are typically 2% of the total brightness and have an SNR between 1 and 4 at their leading edge. An SNR of 4 is defined as the baseline SNR for SCOPE. The automated detection of CMEs in our simulated images by CACTus succeeded well down to SNR = 1 and for CME velocities up to 1400 km s−1. At lower SNR and higher velocity of ≥ 2100 km s−1 the detection started to break down. For SCOPE, the results from the two approaches confirm that the initial design goal of SNR = 4 would, if achieved, deliver a comparable performance to established data used in operations today, with a more compact instrument design, and a margin in SNR before existing automatic detection produces significant false positives.
Direct metal deposition (DMD) can be used for the cladding of surfaces as well as repairing and additive manufacturing of parts and features. Process monitoring and control methods ensure a consistent quality during manufacturing. Monitoring by optical emission spectroscopy of the process radiation can provide information on process conditions and the deposition layer. The object of this work is to measure optical emissions from the process using a spectrometer and identify element lines within the spectra. Single spectra have been recorded from the process. Single tracks of Co-based powder (MetcoClad21) were clad on an S235 base material. The influence of varying process parameters on the incidence and intensity of element lines has been investigated. Moreover, the interactions between the laser beam, powder jet, and substrate with regard to spectral emissions have been examined individually. The results showed that element lines do not occur regularly. Therefore, single spectra are sorted into spectra including element lines (type A) and those not including element lines (type B). Furthermore, only non-ionised elements could be detected, with chromium appearing frequently. It was shown that increasing the laser power increases the incidence of type A spectra and the intensity of specific Cr I lines. Moreover, element lines only occurred frequently during the interaction of the laser beam with the melt pool of the deposition layer.
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 .
Absorption cells filled with diatomic iodine are frequently employed as wavelength reference
for high-precision stellar radial velocity determination due to their long-term stability and
low cost. Despite their wide-spread usage in the community, there is little documentation on
how to determine the ideal operating temperature of an individual cell. We have developed a
new approach to measuring the effective molecular temperature inside a gas absorption cell
and searching for effects detrimental to a high-precision wavelength reference, utilizing the
Boltzmann distribution of relative line depths within absorption bands of single vibrational
transitions. With a high-resolution Fourier transform spectrometer, we took a series of 632
spectra at temperatures between 23 and 66◦C. These spectra provide a sufficient basis to test the
algorithm and demonstrate the stability and repeatability of the temperature determination via
molecular lines on a single iodine absorption cell. The achievable radial velocity precision σ RV
is found to be independent of the cell temperature and a detailed analysis shows a wavelength
dependence, which originates in the resolving power of the spectrometer in use and the signal-
to-noise ratio. Two effects were found to cause apparent absolute shifts in radial velocity, a
temperature-induced shift of the order of ∼1 m s−1 K−1 and a more significant effect resulting
in abrupt jumps of ≥50 m s−1 is determined to be caused by the temperature crossing the dew
point of the molecular iodine
Fourier-transform spectrographs (FTS) are among the most important tools for high-resolution spectroscopy over a broad spectral bandwidth. Usually, the frequency axis of an FTS is calibrated with relatively few atomic lines and an absolute wavelength reference, which is often a stabilized He–Ne laser. Normally, the phase-spectrum is measured using a continuous light source to enable phase correction. Laser frequency combs (LFC) provide a much higher stability. Their spectrum consists of closely spaced narrow lines, which are very well suited for the characterization and calibration of an FTS. Due to the pulsed nature of the LFC, however, the phase spectrum cannot be measured in the same way as for continuous light sources. We show how a proper phase spectrum from an FTS measurement of an LFC can be obtained and how the strongly varying phase spectrum noise can be filtered. We analyzed a narrow spectral band 10.200–12.500 cm−1 in which we detected ∼60.000 lines with sufficient intensity. Only with an accurate truncation of the interferogram and a proper shifting, the complex structure of the phase spectrum is revealed. For phase filtering, we adapted Mertz’s algorithm and show how the instrumental line shape is significantly improved.
Characterization and calibration of a Fourier-transform spectrometer using a laser frequency comb
(2019)
We have used a laser frequency comb with a repetition frequency of 𝜈rep≈1 GHz to measure the drift and dispersion of a Fourier-transform spectrometer (FTS). We used the electronic measurements of 𝜈rep and 𝜈CEO 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 ∼64.000 suitable lines in the near-infrared bandwidth Δ𝜈=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 107 Hz. Using two factors for calibration, the difference between the reference and the measured line list can be removed completely with an uncertainty of ∼65 kHz corresponding to a precision of 0.5·10−9 Hz/Hz.
Fourier Transform Spectrographs (FTS) are versatile tools for measuring accurate, high resolution spectra. They are internally calibrated by a reference laser that runs in parallel to the science light. Therefore it is crucial to properly align these two beams with respect to each other. We show how this can be achieved by feeding a part of the reference light into the optical path of the science beam. For astronomical applications it’s often useful to use optical fibers. We present a coupling setup for our Bruker Optics IFS 125 FTS, consisting of (1) two hexagonal input fibers, (2) dichroic beam-combining for measuring two light sources simultaneously and (3) optimized optics to match the original Bruker design. The hexagonal shape of the fiber cores secures sufficient mode scrambling inside the fibers, resulting in constant beam parameters and a more homogeneous illumination of the entrance aperture of the FTS.
Laser frequency combs have properties which make them promising spectrograph calibration light sources. One drawback for this application is the high dynamic range in the supercontinuum spectra of some frequency combs. We aim to flatten the spectrum of a Ti:sapphire laser frequency comb to improve the calibration performance for a Fourier transform spectrograph. For this, we develop a compact Fourier transform optical pulse shaping setup, which enables control of the spectral envelope via dispersion of the light onto a spatial light modulator. We demonstrate, that this setup allows us to flatten the comb spectrum from a dynamic range of 20 dB to less than 6 dB in the wavelength range 739-939 nm. For 86 % of the wavelength range, the dynamic range is below 1 dB.