A new generation of optical intensity interferometers are emerging in recent years taking advantage of the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC SII (Stellar Intensity Interferometer) in La Palma, Spain, has been operating since its first successful measurements in 2019 and its current design allows it to operate regularly. The current setup is ready to follow up on bright optical transients, as changing from regular gamma-ray observations to SII mode can be done in a matter of minutes. A paper studying the system performance, first measurements and future upgrades has been recently published. MAGIC SII’s first scientific results are the measurement of the angular size of 22 stars, 13 of which with no previous measurements in the B band. More recently the Large Sized Telescope prototype from the Cherenkov Telescope Array Observatory (CTAOLST1) has been upgraded to operate together with MAGIC as a SII, leading to its first correlation measurements at the beginning of 2024. MAGIC+CTAO-LST1 SII will be further upgraded by adding the remaining CTAOLSTs at the north site to the system (which are foreseen to be built by the end of 2025). MAGIC+CTAO-LST1 SII shows a feasible technical solution to extend SII to the whole CTAO.
The ASTRI Stellar Intensity Interferometry Instrument (SI3) is a fast single photon counting instrument for performing intensity interferometry observations of bright stars with the ASTRI Mini-Array. SI3 is designed to perform accurate measurements of single photon arrival times (1ns) in a narrow optical bandwidth (1-8nm) centered at a wavelength in the range 420-500nm. The instrument will exploit the 36 simultaneous baselines over distances between 100m and 700m of the ASTRI Mini-Array to achieve angular resolutions below 100 microarcsec. At this level of resolution it turns out to be possible to reveal details on the surface and of the environment surrounding bright stars on the sky. During 2023 SI3 underwent a significant redesign, with an optical fiber positioned on the focal plane to feed the detectors and electronics. Here we present this new baseline design of SI3, and the motivations behind this choice, including the possibility of future upgrades of the instrument with dedicated front-end electronics and channel multiplexing. We will also show the first results of the target selection procedure based on simulations. Stars with angular diameters of less than 500- 600 microarcseconds up to about magnitude 4.5 will be observable. Thanks to the 36 simultaneous baselines, accurate (up to ∼1%) angular measurements can be obtained with 10-30 hours of observations. This accuracy can rival with that obtained with other arrays of Cherenkov telescopes, despite the smaller collecting area of a single ASTRI telescope.
Stellar intensity interferometry (SII) is based on the correlation of the light intensity fluctuations of a star detected at two or more telescopes, with no need to combine the collected photons directly. A measurement of the correlation in full ”photon-counting mode” was experimented with fast photon counters in Italy (2016-2020) and is currently being adapted to the ASTRI Mini-Array. Performing image synthesis with ”photon-counting” SII requires a series of preparatory activities that involve the optimization of the pipelines for the treatment of time series acquired at extremely high photon rates, the development of efficient and innovative algorithms for the cross-correlation of the arrival times in large time series and the development of a preliminary version of a dedicated pipeline for the synthesis of images starting from interferometric data. Here we present the project and the present status of the activities.
The ASTRI Mini-Array is an international project led by INAF to construct and operate nine Imaging Atmospheric Cherenkov Telescopes with the scientific goals of studying several classes of objects possibly emitting at energies higher than some TeVs and of performing stellar intensity interferometry observations. The telescopes array will be installed at the Teide Observatory (Tenerife, Spain). A Supervisory Control And Data Acquisition (SCADA) software system will be developed to manage the ASTRI Mini-Array allowing its control remotely, from several locations. One of the most important components of the SCADA system is the Telescope Control System (TCS), i.e. the system responsible for the control and supervision of each telescope. The TCS includes several supervisor components, that interface with the telescope local control systems, the hardware and software that control the telescopes hardware devices such as the telescope mount drive systems and the Cherenkov camera, via the Open Platform Communications - Unified Architecture (OPC-UA) standard. These supervisors are then controlled by a telescope manager component responsible for the execution of the single telescope scientific and technical operations requested, orchestrated and synchronized centrally by the SCADA array central controller. This contribution describes the TCS architecture, design and development approach in the context of the general SCADA architecture and of the ALMA Common Software, the framework chosen for the development of all SCADA software of the ASTRI Mini-Array.
KEYWORDS: Atmospheric Cherenkov telescopes, Data acquisition, Cameras, Control systems, Telescopes, Interferometry, Data centers, Software development, Computer architecture, Quality systems
The ASTRI Mini-Array is an international collaboration led by the Italian National Institute for Astrophysics. This project aims to construct and operate an array of nine Imaging Atmospheric Cherenkov Telescopes to study gamma-ray sources at very high energy (TeV) and perform stellar intensity interferometry observations. We describe the software architecture and the technologies used to implement the Online Observation Quality System (OOQS) for the ASTRI Mini-Array project. The OOQS aims to execute data quality checks on the data acquired in real-time by the Cherenkov cameras and intensity interferometry instruments, and provides feedback to both the Central Control System and the Operator about abnormal conditions detected. The OOQS can notify other sub-systems, triggering their reaction to promptly correct anomalies. The results from the data quality analyses (e.g. camera plots, histograms, tables, and more) are stored in the Quality Archive for further investigation and they are summarised in reports available to the Operator. Once the OOQS results are stored, the operator can visualize them using the Human Machine Interface. The OOQS is designed to manage the high data rate generated by the instruments (up to 4.5 GB/s) and received from the Array Data Acquisition System through the Kafka service. The data are serialized and deserialized during the transmission using the Avro framework. The Slurm workload scheduler executes the analyses exploiting key features such as parallel analyses and scalability.
The two MAGIC 17-m diameter Imaging Atmospheric Cherenkov Telescopes have been equipped to work also as an intensity interferometer with a deadtime-free, 4-channel, GPU-based, real-time correlator. Operating with baselines between ∼40 and 90 m the MAGIC interferometer is able to measure stellar diameters of 0.5 - 1 mas in the 400-440 nm wavelength range with a sensitivity roughly 10 times better than that achieved in the 1970’s by the Narrabri Stellar Intensity Interferometer. Besides, active mirror control allows to split the primary mirrors into sub-mirrors. This allows to make simultaneous calibration measurements of the zero-baseline correlation or to simultaneously collect six baselines below 17 m with almost arbitrary orientation, corresponding to angular scales of ∼1-50 mas. We plan to perform test observations adding the nearby Cherenkov Telescope Array (CTA) LST-1 23 m diameter telescope by next year. All three telescope pairs will be correlated simultaneously. Adding LST-1 is expected to increase the sensitivity by at least 1 mag and significantly improve the u-v plane coverage. If successful, the proposed correlator setup is scalable enough to be implemented to the full CTA arrays.
The ASTRI Mini-Array is an International collaboration, led by the Italian National Institute for Astrophysics, that is constructing and operating an array of nine Imaging Atmospheric Cherenkov Telescopes to study gamma-ray sources at very high energy and perform optical stellar intensity interferometry (SII) observations. Angular resolutions below 100 microarcsec are achievable with stellar intensity interferometry, using telescopes separated by hundreds to thousands of meters baselines. At this level of resolution it turns out to be possible to reveal details on the surface and of the environment surrounding bright stars on the sky. The ASTRI Mini-Array will provide a suitable infrastructure for performing these measurements thanks to the capabilities offered by its 9 telescopes, which provide 36 simultaneous baselines over distances between 100 m and 700 m. After providing an overview of the scientific context and motivations for performing SII science with the ASTRI Mini-Array telescopes, we present the baseline design for the ASTRI Stellar Intensity Interferometry Instrument, a fast single photon counting instrument that will be mounted on the ASTRI telescopes and dedicated to performing SII observations of bright stars.
This paper illustrates the results of an experiment performed in the frame of the Asiago Observatory Stellar Intensity Interferometry program, aimed to exploit the quantum properties of the photon stream emitted from celestial objects. Data are acquired over two telescopes separated by approximately 4 km in photon-counting mode and analyzed in post-processing. The temporal and spatial correlation function g(2) on the bright star Vega has been successfully measured at zero baseline and at a ~2 km baseline. The result is fully consistent with that expected for a source with the angular diameter of Vega (approximately 3.3 mas).
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