Marco Riva

We present the design, implementation, and initial performance of the ultraviolet-visible (UV-VIS) arm of the Son Of X-Shooter (SOXS) spectrograph, developed for medium-resolution time-domain spectroscopy on the ESO 3.6 m New Technology Telescope. SOXS is a dual-arm spectrograph optimized for rapid spectroscopic follow-up of faint and evolving transient sources. The UV-VIS arm covers the wavelength range 350 to 850 nm at a resolving power of R∼4500 for a 1-arcsec slit. It employs a multi-band first-order dispersion concept in which the optical bandpass is divided into four spectral intervals and dispersed by dedicated high-efficiency gratings, with all spectra recorded simultaneously on a single charge-coupled device (CCD) detector. This architecture enables broad wavelength coverage with high throughput and operational simplicity. We describe the optical and mechanical design of the UV-VIS spectrograph; summarize the assembly, integration, and testing campaign carried out across multiple sites; and present initial laboratory and on-sky performance, including image quality, resolving power, throughput, and flexure characterization. We also discuss practical lessons learned during integration, particularly alignment strategies and distributed collaboration. The experience gained during the development and commissioning of SOXS provides guidance for future medium-resolution spectrograph projects designed, among others, for time-domain astronomy.

HRMOS is a very high-resolution (R~80,000), multi-object (50-80) spectrograph, proposed as a new instrument for the VLT. HRMOS will address a broad range of transformative science questions. To achieve its science goals, one of the key technical challenges is that of atmospheric dispersion correction over the 25' field of view of the VLT. One of the concepts being explored is to achieve this correction in the focal plane fibre positioning system, reducing the need for very large ADC opto-mechanics. In this paper, we present a novel concept for a traditional pick-and-place system (named QUACKS) that utilises configurable pick-off mirror units that are oriented prior to placement (named DUCKS). The ADC is done using optical receiver units placed around the edge of the telescope field (named ROCKS). The current opto-mechanical design of the developing concept, which can also be used by other future facilities, will be presented together with an overview of its benefits and challenges.

ANDES (ArmazoNes high Dispersion Echelle Spectrograph) is the high-resolution spectrograph currently under development for the European Extremely Large Telescope (ELT). The instrument covers a spectral range from 380 nm to 1800 nm (with K-band as a goal), and supports two main observing modes: a seeing-limited mode for all spectral bands, and a diffraction-limited Integral Field Unit (IFU) mode for the YJH band (950-1800 nm). The IFU module is located within the Front-End structure at the Nasmyth focal plane and operates in conjunction with the Single Conjugate Adaptive Optics (SCAO) system. It provides four selectable spatial scales (8, 16, 35, and 100 mas/spaxel) over a 61-spaxel hexagonal field, feeding the YJH spectrograph via a dedicated fiber bundle. This paper presents the design status of the IFU module as it progresses toward the Final Design Review (FDR). We describe the general concept and science observing strategy, the full optical architecture — including the Offner-relay derotator, the Atmospheric Dispersion Corrector (ADC), the tiptilt steering mirror, the four-scale zoom re-imaging system, the micro-lens array (MLA), and the Align Check System (ACS) for long-term PSF stability monitoring. The mechanical design, encompassing finite element analyses for gravitational, seismic, and thermal loads, is also discussed. The control electronics architecture, based on Beckhoff PLC and EtherCAT communication, is presented together with the actuator and detector selection strategy. Finally, the Manufacturing, Assembly, Integration and Testing (MAIT) plan is summarized, covering prototyping activities, integration at INAF-OAA (Arcetri), provisional acceptance in Europe, and final commissioning at the ELT telescope site.

MORFEO (Multi-conjugate adaptive Optics Relay For ELT Observations) is the post-focal adaptive optics module for the ESO Extremely Large Telescope. The instrument provides MCAO correction for two instruments: MICADO and HARMONI. MORFEO will make use of laser guide stars (LGS) as reference sources to extend the sky coverage of the MCAO system. The light beams proceeding from the Post-Focal Relay Optics (PFRO) are split by a dichroic that allows the “Sodium” LGS light (∼589 nm) to pass to the LGS Objective (LGSO), which reshapes the beams to match the optical interface of the LGS-WFS. We present the Mini-Test-and-Alignment-Camera (MiniTAC), a compact support equipment (SEq) designed to perform key optical checks after the LGSO to validate the MORFEO internal interface toward the LGS-WFS. MiniTAC integrates three channels: a focal plane PSF imager, a pupil imaging system and a SH-WFS, mounted on motorized stages for field-of-view and axial scanning. Using the Calibration Unit (CU) LGS mask as a source, MiniTAC measures: F-number, telecentricity, pupil distortion and asterism delivery position. Preliminary simulations and tolerance analyses show that MiniTAC can test the requirements on LGSO without introducing a relevant metrological bias. This tool will play a pivotal role in de-risking the MORFEO integration process and ensuring compliance with ESO specifications before final deployment at the telescope. We will present here the full SEq design and performance analysis.

MORFEO (Multi-conjugate adaptive Optics Relay For ELT Observations) is the post-focal adaptive optics module for the ESO Extremely Large Telescope (ELT). The instrument provides MCAO correction for two instruments: MICADO and HARMONI. The TAC (Test and Alignment Camera) is a piece of equipment that will be installed inside the MES (MICADO Emulating Structure), under the LOR (Low Order and Reference Wavefront Sensor): its main function is to simulate and verify all the requirements regarding the interface between MORFEO and MICADO in the Bologna Integration Hall, before the whole system is sent to the ELT in Chile. The TAC is subdivided into a Stage system, equipped with three linear stages – one for each axis – to precisely control the position of the assembly in space, and a Camera system, an Aluminium plate housing sensors and the necessary opto-mechanics. This paper focuses on the optomechanical design of the TAC, presenting how challenges were overcome and the choice of equipment.

Son Of X-Shooter (SOXS) is a spectrograph for the European Southern Observatory (ESO), recently installed at the New Technology Telescope at the La Silla Observatory, Chile. The main instrument goal consists of the characterization of transient sources, based on alerts. It covers from (partially) ultraviolet to visible and near-infrared bands, with a spectral resolution of R∼4500, using two separate, wavelength-optimized spectrographs. A scientific-grade visible camera, primarily intended for target acquisition, also provides a “light imaging” mode. We present the design of the SOXS Instrument Control Software, which is in charge of controlling all motors, calibration lamps, and detectors; monitoring sensors and components’ status; coordinating the execution of exposures; and implementing all observation, calibration, and maintenance procedures. Given the extensive experience of the SOXS consortium in the development of instruments for the ESO Very Large Telescope, we decided to base the design of the control system on the same standards, both for hardware and software control. We illustrate the control network, the instrument functions and detectors to be controlled, the overall design of the SOXS Instrument Software and its main components. Then, we provide details about the control software for the most SOXS-specific components and peculiar features: the piezoelectric tip–tilt corrector used for active compensation of mechanical flexures of the instrument, the cryogenic piezoelectric slit exchanger for the near-infrared spectrograph, the co-rotator monitoring system, and the control of the commercial-off-the-shelf-based imaging camera.

The next generation of Extremely Large Telescopes (ELTs), with their wide apertures and advanced multi-conjugate adaptive optics (MCAO) systems, will provide unprecedented sharp and deep observations, even surpassing the capabilities of the James Webb Space Telescope. SHARP, a near-infrared (0.95 to 2.45 μm) spectrograph, is designed to optimally exploit the collecting area and angular resolution of these forthcoming ELTs and specifically optimized for the MCAO unit MORFEO at the ELT. SHARP includes two main units: NEXUS, a multi-object spectrograph, and VESPER, a multi-integral field unit. We outline the opto-mechanical design of SHARP based on the scientific requirements of the project. The optical design is engineered to meet project specifications, featuring a compact mechanical structure that minimizes the required cryogenic power while ensuring ease of access for maintenance and straightforward assembly procedures.

The EXtreme PREcision Spectrograph (EXPRES) is a new Doppler spectrograph designed to reach a radial-velocity measurement precision sufficient to detect Earth-like exoplanets orbiting nearby, bright stars. We report on extensive laboratory testing and on-sky observations to quantitatively assess the instrumental radial-velocity measurement precision of EXPRES, with a focused discussion of individual terms in the instrument error budget. We find that EXPRES can reach a single-measurement instrument calibration precision better than 10 cm s⁻¹, not including photon noise from stellar observations. We also report on the performance of the various environmental, mechanical, and optical subsystems of EXPRES, assessing any contributions to radial-velocity error. For atmospheric and telescope related effects, this includes the fast tip-tilt guiding system, atmospheric dispersion compensation, and the chromatic exposure meter. For instrument calibration, this includes the laser fRequency comb (LFC), flat-field light source, CCD detector, and effects in the optical fibers. Modal noise is mitigated to a negligible level via a chaotic fiber agitator, which is especially important for wavelength calibration with the LFC. Regarding detector effects, we empirically assess the impact on the radial-velocity precision due to pixel-position nonuniformities and charge transfer inefficiency (CTI). EXPRES has begun its science survey to discover exoplanets orbiting G-dwarf and K-dwarf stars, in addition to transit spectroscopy and measurements of the Rossiter–McLaughlin effect.

The EXtreme PREcision Spectrograph (EXPRES) is a new Doppler spectrograph designed to reach a radial velocity measurement precision sufficient to detect Earth-like exoplanets orbiting nearby, bright stars. We report on extensive laboratory testing and on-sky observations to quantitatively assess the instrumental radial velocity measurement precision of EXPRES, with a focused discussion of individual terms in the instrument error budget. We find that EXPRES can reach a single-measurement instrument calibration precision better than 10 cm/s, not including photon noise from stellar observations. We also report on the performance of the various environmental, mechanical, and optical subsystems of EXPRES, assessing any contributions to radial velocity error. For atmospheric and telescope related effects, this includes the fast tip-tilt guiding system, atmospheric dispersion compensation, and the chromatic exposure meter. For instrument calibration, this includes the laser frequency comb (LFC), flat-field light source, CCD detector, and effects in the optical fibers. Modal noise is mitigated to a negligible level via a chaotic fiber agitator, which is especially important for wavelength calibration with the LFC. Regarding detector effects, we empirically assess the impact on radial velocity precision due to pixel-position non-uniformities (PPNU) and charge transfer inefficiency (CTI). EXPRES has begun its science survey to discover exoplanets orbiting G-dwarf and K-dwarf stars, in addition to transit spectroscopy and measurements of the Rossiter-McLaughlin effect.

ESPRESSO is a fibre-fed, cross-dispersed, high-resolution, echelle spectrograph developed to fully exploit the European Southern Observatory VLT (Very Large Telescope), and it was open to the astronomical community at the end of 2018. This spectrograph was installed at the Combined Coudé Laboratory (CCL) of the VLT, fed by four Coudé Trains, which bring the light to the CCL from the Nasmyth platforms of the four 8.2-metre Unit Telescopes. With all four Telescopes combining their light-collecting power to feed a single instrument, the ESPRESSO Coudé Train effectively transforms the VLT into the largest optical telescope in the world in terms of collecting area. The Coudé Train is composed of a set of prisms and lenses delivering a pupil and an image to the CCL, up to 70 m away, including an Atmospheric Dispersion Compensator. The use of only refractive optics, namely Total Internal Reflection prisms, has the advantage of the inherent higher throughput, especially in the blue region of the spectrum. With these complex optics, ESPRESSO can either collect the light from up to all four Unit Telescopes together, or alternatively receive light from any one of the telescopes independently, allowing for more flexible usage of observing time. In this paper, we present the ESPRESSO Coudé Train concept, the design and the implementation on the VLT.