The SCAO subsystem is crucial to enable METIS to achieve its scientific goals, delivering a Strehl ratio of 0.93 in the N band at the ELT. The ELT itself will only be available after integration of METIS at the facility, clearly too late in the development life cycle for subsystem testing. Therefore, we are building a hardware ELT simulator for use in the laboratory, capable of applying atmospheric distortions and receiving and applying realtime corrections from SCAO, closing the loop in the laboratory. In this contribution we present the design of the telescope simulator, MITESI, highlighting which functionality of the ELT it can and cannot replicate. We also detail what aspects of SCAO MITESI will allow us to validate. Finally, we present tests of components of MITESI and a timeline for developing it further.

MIRC-X and MYSTIC are six-telescope near-infrared beam (1.08-2.38 mu m) combiners at the CHARA Array on Mt Wilson CA, USA. Ever since the commissioning of MIRC-X (J and H bands) in 2018 and MYSTIC (K bands) in 2021, they have been the most popular and over-subscribed instruments at the array. Observers have been able to image stellar objects with sensitivity down to 8.1 mag in H and 7.8 mag in K-band under the very best conditions. In 2022 MYSTIC was upgraded with a new ABCD mode using the VLTI/GRAVITY 4-beam integrated optics chip, with the goal of improving the sensitivity and calibration. The ABCD mode has been used to observe more than 20 T Tauri stars; however, the data pipeline is still being developed. Alongside software upgrades, we detail planned upgrades to both instruments in this paper. The main upgrades are: 1) Adding a motorized filter wheel to MIRC-X along with new high spectral resolution modes 2) Updating MIRC-X optics to allow for simultaneous 6T J+H observations 3) Removing the warm window between the spectrograph and the warm optics in MYSTIC 4) Adding a 6T ABCD mode to MIRC-X in collaboration with CHARA/SPICA 5) Updating the MIRC-X CRED-ONE camera funded by Prof. Kraus from U. Exeter 6) Carrying out science verification of the MIRC-X polarization mode 7) Developing new software for ABCD-mode data reduction and more efficient calibration routines. We expect these upgrades to not only improve the observing experience, but also increase the sensitivity by 0.4 mag in J+H-bands, and 1 mag in K-band.

BIFROST, an upcoming instrument for the VLTI, is part of the Asgard Suite, a VLTI visitor instrument. It comprises two spectrograph arms that are optimised for wavelength range of 1- 1.75 mu m (fed by a fiber that is placed on-axis for fringe tracking/monitoring) and 1-1.3 mu m (fed by a fiber that can positioned either onaxis or off-axis to observe a faint target) wavelength range, respectively. Volume phase holographic gratings (VPHGs) are employed to achieve high spectral resolving power up to 25,000 and a throughput above 75% for all dispersing elements. In this contribution, we discuss the optical and optomechanical design of the spectrographs, as well as a new relay optics design that minimizes the thermal background, with a factor 4 reduction in thermal background compared to the non-relay optics design. We will also present the first lab results obtained with the YJH spectrograph.

METIS, the Mid-Infrared ELT Imager and Spectrograph is a first-generation ELT instrument scheduled to see first light in 2029. Its two main science modules are supported by an adaptive optics system featuring a pyramid sensor with 90x90 sub-apertures working in H- and K-band. The wavefront control concept for METIS' singleconjugate adaptive optics relies on a synthetic calibration that uses a model of the telescope and instrument to generate the interaction and control matrices, as well as the final projection on a modal command vector. This concept is enforced owing to the absence of a calibration source in front of the ELT's main deformable mirror. The core of the synthetic calibration functionality is the Command Matrix Optimiser module, which consists of several components providing models for various parts and aspects of the instrument, as well as the entire reconstructor. Many are present in the simulation environment used during the design phases, but need to be re-written and/or adapted for real-life use. In this paper, we present the design of the full command matrix optimisation module, the status of these efforts and the overall final concept of METIS' soft real-time system.

BIFROST is the short-wavelength, high-spectral resolution instrument in the Asgard Suite of VLTI visitor instruments. It will be optimized for spectral line studies in the Y, J, and H bands (1.05-1.75 μm) that include many strong lines & molecular features. In this presentation, we outline the BIFROST science drivers that have guided our design choices and map them against the operational modes that are being implemented. We give an overview about the status of the project and the milestones from the ongoing integration & testing phase in Exeter to shipping & commissioning on Paranal, scheduled for 2025 and 2026. We review the BIFROST subsystems and discuss how they interface with the broader Asgard Suite. Finally, we outline other BIFROST-related activities pursued by our group that are intended for implementation in BIFROST as part of future upgrades.

BIFROST is a new Y+J and H band beam combiner for the VLTI, and part of the Asgard suite of visitor instruments. BIFROST will unlock a new parameter space at the VLTI by including the astronomical J band and high spectral resolution (up to R approximate to 25,000). It will also have the ability to simultaneously observe on- and off-axis targets. BIFROST's beam combiner will be an integrated optics chip, fed by single mode optical fibers. BIFROST therefore requires a light injection module to couple the starlight from free space in the VLTI laboratory into the single mode fibers. The light injection module of BIFROST is also responsible for redirecting the starlight towards the fiber couplers; removing the optical path difference between the beams; co-phasing BIFROST with the rest of the Asgard suite; splitting the light off for the off-axis field; selecting the pointing of the off-axis field; optimizing the injection into the fibers; co-phasing the on- and off-axis light; supporting the passage of the full two arcsecond diameter field of view and providing sufficient space for additional BIFROST pre-injection optics. In this contribution we detail the novel design of the BIFROST light injection module, highlighting how it achieves this functionality using as few optics as possible. We also present Zemax Opticstudio tolerancing analysis, demonstrating the feasibility of building this design in the laboratory.

The BIFROST instrument is poised to revolutionize high-spectral resolution interferometry at the VLTI by extending the accessible wavelength range down to 1.0 mu m, encompassing the Y, J, and H bands. In this paper, we discuss the optical design for BIFROST's pre-injection optics which correct for birefringence effects and longitudinal dispersion between the different beam lines. We present the optomechanical design for the light injection module that compresses the beams and injects them into single-mode fibres, while maintaining the full field-of-view of the VLTI auxiliary or unit telescopes. Our fibre switching module will allow for the injection of light into photonic devices optimised for different wavebands or applications. Finally, we outline our integration & alignment strategy and present the first characterisation results obtained in the optics lab at the University of Exeter.

ESO's Very Large Telescope Interferometer has a history of record-breaking discoveries in astrophysics and significant advances in instrumentation. The next leap forward is its new visitor instrument, called Asgard. It comprises four natively collaborating instruments: HEIMDALLR, an instrument performing both fringe tracking and stellar interferometry simultaneously with the same optics, operating in the K band; Baldr, a Strehl optimizer in the H band; BIFROST, a spectroscopic combiner to study the formation processes and properties of stellar and planetary systems in the Y-J-H bands; and NOTT, a nulling interferometer dedicated to imaging nearby young planetary systems in the L band. The suite is in its integration phase in Europe and should be shipped to Paranal in 2025. In this article, we present details of the alignment and calibration unit, the observing modes, the integration plan, the software architecture, and the roadmap to completion of the project.

We report progress on Project Prime (PRecision Interferometry with MIRC for Exoplanets) to detect exoplanets using precision closures using MIRC-X and MYSTIC at CHARA. Our investigations include modeling systematics caused by OPD drifts, differential dispersion, beamtrain birefringence, and flatfielding errors. Injection tests suggest we can recover hot Jupiter companions as faint at 1/5000 of the host star brightness with 4 nights of observing and we will present some results of our recent searches for the hot Jupiters. Our upper limits are starting to constrain current-generation Global Circulation Models (GCMs). We propose the addition of modest nulling (10:1) to today's interferometers in order to vastly increase the ease of this work and to open up many more targets for detections.

The Michigan Young Star Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the U.S. National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA Array in July 2021, with baselines up to 331 m, MYSTIC provides a maximum angular resolution of lambda/2B similar to 0.7 mas. The instrument injects phase-corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC uses a high frame rate, ultra-low read noise SAPHIRA detector and implements two beam combiners: a six-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a four-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J + H band) instrument for simultaneous fringe-tracking and imaging and shares its software suite with the latter to allow a single observer to operate both instruments. We present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future. (c) The Authors.

BIFROST will be a short-wavelength (λ = 1.0 - 1.7 μm) beam combiner for the VLT Interferometer, combining both high spatial (λ/2B = 0.8 mas) and spectral (up to R = 25,000) resolution. It will be part of the Asgard Suite of visitor instruments. The new window of high spectral resolution, short wavelength observations brings with it new challenges. Here we outline the instrumental design of BIFROST, highlighting which beam combiner subsystems are required and why. This is followed by a comparison All-In-One (AIO) beam combination scheme and an Integrated Optics (IO) scheme with ABCD modulation both in terms of expected sensitivity and the practical implementation of each system.

The BIFROST instrument will be the first VLTI instrument optimised for high spectral resolution up to R=25,000 and operate between 1.05 and 1.7 μm. A key component of the instrument will be the spectrograph, where we require a high throughput over a broad bandwidth. In this contribution, we discuss the four planned spectral modes (R=50, R=1000, R=5000, and R=25,000), the key spectral windows that we need to cover, and the technology choices that we have considered. We present our plan to use Volume Phase Holographic Gratings (VPHGs) to achieve a high efficiency > 85%. We present our preliminary optical design and our strategies for wavelength calibration.

We present science cases and instrument design considerations for the BIFROST instrument that will open the short-wavelength (Y/J/H-band), high spectral dispersion (up to R=25,000) window for the VLT Interferometer. BIFROST will be part of the Asgard Suite of instruments and unlock powerful venues for studying accretion & mass-loss processes at the early/late stages of stellar evolution, for detecting accreting protoplanets around young stars, and for probing the spin-orbit alignment in directly-imaged planetary systems and multiple star systems. Our survey on GAIA binaries aims to provide masses and precision ages for a thousand stars, providing a legacy data set for improving stellar evolutionary models as well as for Galactic Archaeology. BIFROST will enable off-axis spectroscopy of exoplanets in the 0.025-1" separation range, enabling high-SNR, high spectral resolution follow-up of exoplanets detected with ELT and JWST. We give an update on the status of the project, outline our key technology choices, and discuss synergies with other instruments in the proposed Asgard Suite of instruments.

ABSTRACT Image plane beam combination in optical interferometers multiplexes the interference fringes from multiple baselines on to a single detector. The beams of starlight are arranged in a non-redundant pattern at the entrance of the combiner so that the signal from each baseline can be separated from one another in the frequency domain. If the signals from different baselines overlap in the frequency domain, this can give rise to a systematic error in the fringe measurements known as baseline crosstalk. In this paper, we quantify crosstalk arising from the combination of atmospheric seeing and beam propagation over distances of the order of hundreds of metres. We find that in idealized conditions atmospheric wavefront errors and beam propagation do not contribute to crosstalk. However, when aperture stops are included in the optical beam train we observe that wavefront errors can result in squared visibility errors arising from crosstalk as high as ΔV2 = 6.6 × 10−3 under realistic observing conditions.

FOURIER is the first-generation science beam combiner for the MROI. It is a three-way, J, H and K band image plane combiner. The FOURIER design emphasises low visibility losses and high optical throughput and is designed around a low-noise SAPHIRA detector. Based on laboratory measurements of its throughput and visibility losses, FOURIER is expected to reach limiting magnitudes of 12.3, 13.2 and 11.7 in the J, H and K bands, respectively, within 5 minutes of incoherent integration assuming 0.7′′ seeing and a detector read noise of 0.3 electrons. As FOURIER observes as red as the K band, the detector and most of its optics are placed within a liquid nitrogen cryostat. We present the design of FOURIER’s cryostat, as well as laboratory tests of the instrument’s cryogenic performance. We also report room temperature characterisation of the optics. Finally, we discuss the path forward from the current status of the instrument to first fringes in 2023.

We present the design and testing of FOURIER, the first generation science beam combiner for the MROI. FOURIER is a three-way, J, H and K band image plane combiner which is designed primarily for observations at faint limiting magnitudes. We outline the main science requirements and discuss how the design of FOURIER contributes to meeting these requirements. We present the first laboratory characterisation of the instrument including validation of the PSF profile, demonstration of high contrast fringes, and the spectral resolution of the instrument, all of which show promising results. We conclude by discussing the path to deployment of FOURIER at the MROI ahead of the first science observation at the array.