Abstract
Portugal will build the warm support and access structure (WSS) to the mid-infrared, first generation ELT instrument METIS. The particular characteristics of METIS and the ELT pose several challenges to designing the WSS according to requirements, as well challenges to the assembly and integration of the WSS. We here provide you an overview of those challenges, as well as strategies to overcome and mitigate issues related to the mass and dimensions of the WSS.

Abstract
This article presents the final design of the METIS/ELT warm support structure subsystem. It provides the mechanical interface between the cryostat and the Nasmyth platform, and it consists of three substructures: the elevation platform, the cryostat alignment structure, and the instrument access platform. The elevation platform is connected to the Nasmyth platform and holds the cryostat alignment platform, consisting of seven legs connected to three nodes. The cryostat alignment platform is a hexapod holding the cryostat, allowing maintenance, alignment, and positioning. The instrument access platform allows human access to the cryostat, it bears the cable support system and is prepared to support the future Single Laser Adaptive Optics system. The subsystem requirements, design trade-offs, interface considerations, and the substructures' final design and simulation results will be detailed as presented to the METIS Final Design Review in 2022.

Abstract
Hexapods are general solutions that provide movement with six degrees of freedom for instrument positioning, alignment, and support. In the case of the METIS instrument, the hexapod must satisfy the following stringent requirements: a) support the 11-ton weight of an instrument; b) allow alignment and provide position stability to the instrument to within a tenth of a millimeter; c) provide an adjustment range of about 20 cm; d) support the instrument allowing for accelerations of over 3 g in all directions; e) have the lowest mass possible. Commercial linear actuators that are generally used in such cases are designed for extended movement, include a complete set of bearings that constrain each actuator lateral displacements and a sophisticated central screw that defines only the longitudinal movement. These solutions tend to be heavy and costly if roller screws are used to avoid backslash. They encompass ranges that are a major fraction of the total length and are designed for fast movement. Both these characteristics exceed the requirements of the METIS application. We present an optimized design for the hexapod which includes a different, lightweight, sturdy, small-range, high-precision, no backslash, earthquake-proof actuator. The design of the hexapod is such that it can be used, in general, as a mass and vibration optimized solution for precision heavy instrument alignment.

Abstract
Pushed by a number of advances, electromagnetic observatories have now reached the horizon scale of supermassive black holes. The existence and properties of horizons in our universe is one of the outstanding fundamental issues that can now be addressed. Here we investigate the ability to discriminate between black holes and compact, horizonless objects, focusing on the lensing of hot spots around compact objects. We work in particular with boson and Proca stars as central objects, and show that the absence of a horizon gives rise to a characteristic feature-photons that plow through the central object and produce an extra image. This feature should be universal for central objects made of matter weakly coupled to the standard model.

Abstract
Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole spin. Furthermore, it provides the starting point for astrometric fitting to derive highly accurate stellar positions. Here, we present G(R), a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of information field theory and building upon existing works in radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images to date of the Galactic Center on scales of a few milliarcseconds. The images reveal the complicated source structure within the central 100mas around Sgr A*, where we detected the stars S29 and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*. Furthermore, we were able to detect S38, S42, S60, and S63 in a series of exposures for which we offset the fiber from Sgr A*. We provide an update on the orbits of all aforementioned stars. In addition to these known sources, the images also reveal a faint star moving to the west at a high angular velocity. We cannot find any coincidence with any known source and, thus, we refer to the new star as S300. From the flux ratio with S29, we estimate its K-band magnitude as m(K)(S300)similar or equal to 19.0 - 19.3. Images obtained with CLEAN confirm the detection. To assess the sensitivity of our images, we note that fiber damping reduces the apparent magnitude of S300 and the effect increases throughout the year as the star moves away from the field center. Furthermore, we performed a series of source injection tests. Under favorable circumstances, sources well below a magnitude of 20 can be recovered, while 19.7 is considered the more universal limit for a good data set.

Abstract
Stars orbiting the compact radio source Sgr A* in the Galactic Center serve as precision probes of the gravitational field around the closest massive black hole. In addition to adaptive optics-assisted astrometry (with NACO/VLT) and spectroscopy (with SINFONI/VLT, NIRC2/Keck and GNIRS/Gemini) over three decades, we have obtained 30-100 mu as astrometry since 2017 with the four-telescope interferometric beam combiner GRAVITY/VLTI, capable of reaching a sensitivity of m(K)=20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry in the field. The new data for the stars S2, S29, S38, and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass, M-center dot=4.30 x 10(6)M(circle dot), with a precision of about +/- 0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7 sigma. Assuming plausible density profiles, the extended mass component inside the S2 apocenter (approximate to 0.23 '' or 2.4 x 10(4)R(S)) must be less than or similar to 3000M(circle dot)(1 sigma), or less than or similar to 0.1% of M-center dot. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* is tentatively seen is r approximate to 2.5 ''>= 10x the apocenter of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function.