Dionne M. Haynes

The 4MOST instrument concept overview

The 4MOST[1] instrument is a concept for a wide-field, fibre-fed high multiplex spectroscopic instrument facility on the ESO VISTA telescope designed to perform a massive (initially >25x106 spectra in 5 years) combined all-sky public survey. The main science drivers are: Gaia follow up of chemo-dynamical structure of the Milky Way, stellar radial velocities, parameters and abundances, chemical tagging; eROSITA follow up of cosmology with x-ray clusters of galaxies, X-ray AGN/galaxy evolution to z~5, Galactic X-ray sources and resolving the Galactic edge; Euclid/LSST/SKA and other survey follow up of Dark Energy, Galaxy evolution and transients. The surveys will be undertaken simultaneously requiring: highly advanced targeting and scheduling software, also comprehensive data reduction and analysis tools to produce high-level data products. The instrument will allow simultaneous observations of ~1600 targets at R~5,000 from 390-900nm and ~800 targets at R<18,000 in three channels between ~395-675nm (channel bandwidth: 45nm blue, 57nm green and 69nm red) over a hexagonal field of view of ~ 4.1 degrees. The initial 5-year 4MOST survey is currently expect to start in 2020. We provide and overview of the 4MOST systems: optomechanical, control, data management and operations concepts; and initial performance estimates.

4MOST-4-metre Multi-Object Spectroscopic Telescope

4MOST is a wide-field, high-multiplex spectroscopic survey facility under development for the VISTA telescope of the European Southern Observatory (ESO). Its main science drivers are in the fields of galactic archeology, high-energy physics, galaxy evolution and cosmology. 4MOST will in particular provide the spectroscopic complements to the large area surveys coming from space missions like Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like VISTA, VST, DES, LSST and SKA. The 4MOST baseline concept features a 2.5 degree diameter field-of-view with ~2400 fibres in the focal surface that are configured by a fibre positioner based on the tilting spine principle. The fibres feed two types of spectrographs; ~1600 fibres go to two spectrographs with resolution R<5000 (λ~390-930 nm) and ~800 fibres to a spectrograph with R>18,000 (λ~392-437 nm and 515-572 nm and 605-675 nm). Both types of spectrographs are fixed-configuration, three-channel spectrographs. 4MOST will have an unique operations concept in which 5 year public surveys from both the consortium and the ESO community will be combined and observed in parallel during each exposure, resulting in more than 25 million spectra of targets spread over a large fraction of the southern sky. The 4MOST Facility Simulator (4FS) was developed to demonstrate the feasibility of this observing concept. 4MOST has been accepted for implementation by ESO with operations expected to start by the end of 2020. This paper provides a top-level overview of the 4MOST facility, while other papers in these proceedings provide more detailed descriptions of the instrument concept[1], the instrument requirements development[2], the systems engineering implementation[3], the instrument model[4], the fibre positioner concepts[5], the fibre feed[6], and the spectrographs[7].

HERMES: revisions in the design for a high-resolution multi-element spectrograph for the AAT

The AAO is building an optical high resolution multi-object spectrograph for the AAT for Galactic Archaeology. The instrument has undergone significant design revision over that presented at the 2008 Marseilles SPIE meeting. The current design is a 4-channel VPH-grating based spectrograph providing a nominal spectral resolving power of 28,000 and a high-resolution mode of 45,000 with the use of a slit mask. The total spectral coverage is about 1000 Angstroms for up to 392 simultaneous targets within the 2 degree field of view. Major challenges in the design include the mechanical stability, grating and dichroic efficiencies, and fibre slit relay implementation. An overview of the current design and discussion of these challenges is presented.

VIRUS: production and deployment of a massively replicated fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope

The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical spectrographs (arrayed as 75 unit pairs) fed by 33,600 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed starting at the end of 2014 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large area surveys of the emission line universe for the first time. VIRUS is in full production, and we are about half way through. We review the production design, lessons learned in reaching volume production, and preparation for deployment of this massive instrument. We also discuss the application of the replicated spectrograph concept to next generation instrumentation on ELTs.

VIRUS: production of a massively replicated 33k fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope

The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter, deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 82 units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. This approach, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers, offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed by early 2014 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope. VIRUS and HET will open up wide-field surveys of the emission-line universe for the first time. We present the production design and current status of VIRUS.

Focal ratio degradation: a new perspective

We have developed an alternative FRD empirical model for the parallel laser beam technique which can accommodate contributions from both scattering and modal diffusion. It is consistent with scattering inducing a Lorentzian contribution and modal diffusion inducing a Gaussian contribution. The convolution of these two functions produces a Voigt function which is shown to better simulate the observed behavior of the FRD distribution and provides a greatly improved fit over the standard Gaussian fitting approach. The Voigt model can also be used to quantify the amount of energy displaced by FRD, therefore allowing astronomical instrument scientists to identify, quantify and potentially minimize the various sources of FRD, and optimise the fiber and instrument performance.

New multicore low mode noise scrambling fiber for applications in high-resolution spectroscopy

We present a new type of multicore fiber (MCF) and photonic lantern that consists of 511 individual cores designed to operate over a broadband visible wavelength range (380-860nm). It combines the coupling efficiency of a multimode fiber with modal stability intrinsic to a single mode fibre. It is designed to provide phase and amplitude scrambling to achieve a stable near field and far field illumination pattern during input coupling variations; it also has low modal noise for increased photometric stability. Preliminary results are presented for the new MCF as well as current state of the art octagonal fiber for comparison.

The construction, alignment, and installation of the VIRUS spectrograph

VIRUS is the massively replicated fiber-fed spectrograph being built for the Hobby-Eberly Telescope to support HETDEX (the Hobby-Eberly Telescope Dark Energy Experiment). The instrument consists of 156 identical channels, fed by 34,944 fibers contained in 78 integral field units, deployed in the 22 arcminute field of the upgraded HET. VIRUS covers 350-550nm at R ≈ 700 and is built to target Lyman α emitters at 1.9 < z < 3.5 to measure the evolution of dark energy. Here we present the assembly line construction of the VIRUS spectrographs, including their alignment and plans for characterization. We briefly discuss plans for installation on the telescope. The spectrographs are being installed on the HET in several stages, and the instrument is due for completion by the end of 2014.

TOAD: a numerical model for the 4MOST instrument

TOAD, the “Top Of the Atmosphere to Detector” simulator, is a primary engineering tool that accompanies the development of the 4MOST instrument. The ultimate goal is to provide a detailed, end-to-end performance model of 4MOST by providing the detector image for an artificial target field with less then 5% error. TOAD will be able to create a realistic output for any reasonable input. The input can be anything, from point sources through extended sources, calibration lamps or stray-light, entering the system at virtually any point in a optical path. During the development of the 4MOST facility, the TOAD simulator will give invaluable insight into the interaction of various parts of the instrument and the impact of engineering design decisions on the system performance.

Multimode to single-mode converters: new results on 1-to-61 photonic lanterns

Photonic Lanterns are a fibre-based component performing the adiabatic conversion from a multimode fibre to a series of single-mode fibres. This conversion is required for combining fibre-based instruments used in astronomy with complex photonic functions. As any fibre-based system, the optical properties of the Photonic Lanterns need to be fully evaluated. In this paper, we present results on the performance of a 1-to-61 Photonic Lantern in terms of spectral transmission and modal noise characteristics. Firstly, we compare the spectra obtained at the output of two photonic lanterns spliced together in multimode-to-multimode configuration with spectra obtained when transmitting light through step-index single-mode and multimode fibres. We then show that the photonic lantern is generating less modal noise than a step-index multimode fibre of same core diameter, when it is submitted to bending and stretching, and we propose an interpretation of this result based on static mode scrambling performance and single-mode behavior.