MISTRAL is a new generation receiver installed on the Sardinia Radio Telescope (SRT) and built by the Sapienza University of Rome for the National Institute for Astrophysics (INAF) as part of the upgrade of the radio telescope for the study of the Universe at high frequencies, funded by a PON (National Operational Program) project, concluded in 2023 and now providing its first significant scientific results. MISTRAL stands for "MIllimetric Sardinia radio Telescope Receiver based on Array of Lumped elements kids".
MISTRAL is an innovative receiver in many ways. Radio astronomy receivers are typically "mono-pixel", i.e. sensitive to radiation coming from a single direction. Creating panoramic images of the area of the sky of interest requires long scans with the telescope. One way to overcome this limitation is to build "multi-pixel" receivers, i.e. sensitive to radiation coming from multiple directions simultaneously. MISTRAL takes this concept to the extreme. It contains an ultra-cold core composed of a matrix of 415 Kinetic Inductance Detectors (KIDs), developed in collaboration with CNR-IFN in Rome, and cooled to just a fraction of a degree above the temperature of absolute zero, or -273.15 degrees Celsius. "It is precisely this high number of detectors, combined with a specifically developed optical system, that makes MISTRAL an extremely effective and fast instrument for wide-field imaging of weak and extended sources", comments Paolo de Bernardis, Scientific Coordinator of the receiver for Sapienza University of Rome. MISTRAL was installed in May 2023 in the Gregorian focus, located at the center of the large 64-meter diameter SRT dish. Commissioning of the receiver began soon after and consisted of an intensive series of technical and observational tests aimed at integrating the receiver into the telescope system. A team of researchers from INAF and Sapienza have been working side by side with the aim of bringing MISTRAL to its maximum performance, and making it available to the scientific community for regular observations. "Commissioning", explains Matteo Murgia, Scientific Manager of the receiver for INAF, "is normally a routine phase in the installation of new instrumentation. However, it becomes a real challenge in the case of a millimeter-wave receiver like MISTRAL, which requires the telescope's performance to be pushed to the limit in every respect".
"Initially, we faced and overcame several obstacles related to the truly exceptional cryogenics of the receiver, finally obtaining the temperature necessary for the activation of the KIDs, that is, just 0.2 degrees above absolute zero", says Elia Battistelli, Project Manager of the receiver for Sapienza University of Rome.
Starting in September 2024, the improvement in the performance of the SRT active surface allowed us to reach the sensitivity required to calibrate the instrument. It was then possible to proceed with the optimization of the alignment between the MISTRAL optics and those of SRT.
The commissioning team also worked tirelessly to develop the procedures and software needed for pointing and focusing. At the same time, INAF and Sapienza developed the calibration and imaging procedures. MISTRAL was finally ready for "first light" observations of extended radio sources. Three iconic celestial objects were observed in succession: the Orion Nebula, the radio galaxy M87, and the supernova remnant Cassiopeia A. These observations highlighted MISTRAL's remarkable versatility and confirmed its ability to produce highly detailed images of celestial objects in extremely diverse astrophysical contexts.
"The milestone achieved with the first light images of SRT at 90 GHz," commented Isabella Pagano, Scientific Director of INAF, "marks an important step in broadening the scientific horizons of this radio telescope, thus demonstrating its ability to operate successfully at the high radio frequencies for which it was designed." With the "first light" obtained by observing these fascinating cosmic objects, this first phase of technical tests is concluded and a no less important phase of scientific validation begins, aimed at verifying the performance of MISTRAL with increasingly weak sources, to ensure that it is ready for the numerous scientific challenges for which it was designed. MISTRAL will address a wide range of scientific questions, from cosmology and the physics of galaxy clusters, to the study of active galactic nuclei, the structure of molecular clouds and their relationship with star formation in nearby galaxies and the Milky Way, and the study of celestial bodies in our Solar System. The commissioning team's activities therefore continue, with the aim of verifying MISTRAL's performance in each of these scientific cases and making the receiver available to the scientific community as soon as possible.
The first images acquired by MISTRAL
In December 2024, MISTRAL was pointed at the famous Orion Nebula (also known as M42) in the center of the Orion constellation. Located about 1350 light-years from Earth, M42 is one of the closest active star-forming regions and is characterized by ionized hydrogen excited by a group of massive stars known as the Trapezium. M42 is part of a vast complex of molecular clouds that extends over 30 degrees across the sky, and MISTRAL observed its central part at an angular resolution of 12 arcseconds. The Orion Bar is clearly visible in the image to the south, marking a sharp boundary between the region of ionized hydrogen and the molecular cloud below. Emission peaks can also be seen near the stars of the Trapezium and the Kleinmann–Low Nebula, a dense star-forming molecular cloud that hosts a star cluster which underwent an explosive event in the past. The emission from M42 visible at 90 GHz is an almost equal mixture of radiation from ionized hydrogen and that from cold dust contained in the underlying molecular cloud complex.
In February 2025, MISTRAL observed the radio galaxy M87 in the constellation Virgo, whose active nucleus contains a now famous supermassive black hole, directly imaged thanks to the historic observation of the Event Horizon Telescope in 2019. The radio source surrounding M87 has a complex structure, made up of internal lobes measuring about thirty thousand light years (just over the distance that separates us from the center of the Milky Way) surrounded by an external plasma bubble on a larger scale. These structures are the result of the activity of the central black hole over the past several million years. The internal radio lobes are visible in MISTRAL's image – the most recent structures still powered by a pair of relativistic radio jets propagating from the central black hole. Observing these structures at such high frequencies provides new and valuable insights into the physical mechanisms powering the radio-emitting particles inside the source.
Finally, in the April 2025 session, MISTRAL observed – through two cross-scans of about half an hour each – the supernova remnant Cassiopeia A (Cas-A), one of the most intense radio sources in the sky, with an angular size of about 5 arcminutes (about one-sixth the apparent diameter of the full Moon). The expanding gas shell is visible in its entirety and, thanks to the angular resolution of SRT at these wavelengths, it is possible to appreciate the details and brightness variations of the filamentary structure.
