We propose a method to transform a Cherenkov telescope, primarily designed for gamma-ray astronomy, into an imaging telescope at the diffraction limit of an equivalent astronomical telescope. The method can be applied onto existing and planned Cherenkov telescopes, both taken as a single dish or a set of such telescopes configured for aperture synthesis operating as intensity interferometers. We examine the sensitivity of our method by performing extensive numerical simulations including two and three point correlations, for amplitude and closure phase quantities permitting image reconstruction of stellar surfaces with subtle structures in the range of milli-arc-second resolutions. As a case of study we apply our method to a single 20m class Cherenkov telescope with the perspective of its generalization to the diluted Cherenkov Telescope Array (CTA) under construction in North and South hemispheres.
We present the current status of the I2C stellar intensity interferometer used towards high angular resolution observations of stars in visible wavelengths. In these proceedings, we present recent technical improvements to the instrument, and share results from ongoing campaigns using arrays of small diameter optical telescopes. A tip-tilt adaptive optics unit was integrated into the optical system to stabilize light injection into an optical fiber. The setup was successfully tested with several facilities on the Calern Plateau site of the Observatoire de la Côte d’Azur. These include one of the 1m diameter telescopes of the C2PU observatory, a portable 1m diameter telescope, and also the 1.5m MéO telescope. To better constrain on-sky measurements, the spectral transmission of instrument was characterized in the laboratory using a high resolution spectrograph. The system was also tested with two of the auxiliary telescopes of the VLTI resulting in successful temporal and spatial correlation measurements of three stars.
Imaging air Cherenkov telescope (IACT) arrays have long been viewed as potential observatories for performing stellar intensity interferometry (SII). We present results and the current status of an ongoing SII campaign using the four 12 m diameter VERITAS telescopes located at the Fred Lawrence Whipple Observatory. Each of the telescopes have been equipped with SII instrumentation, consisting of a removable SII camera and a continuous high speed data acquisition system. The correlation of digitized starlight intensities is performed off-line using FPGA hardware to calculate the squared visibilities. The system enables high angular resolution measurements with baselines ranging from approximately 81.5 to 173 m in the B photometric band. Stellar angular diameter measurements were successfully performed using all four VERITAS telescopes. These results show that IACTs can operate as SII observatories, demonstrating a technological pathway for a similar system with future IACT observatories with a hundred fold increase in the number of simultaneous baselines.
Over recent years, several independent groups have pursued the realization of a modern stellar intensity interferometry (SII) system to perform high angular resolution observations at optical wavelengths. Here, we present a general purpose SII observation planner (ASIIP) that can be used to aid in SII observational efforts. ASIIP can be used to coordinate and prioritize SII observations based on observational and instrumental parameters. ASIIP constructs a master catalog by gathering information from several stellar catalogs, and targets within the master catalog are ranked based on the ability to make stellar diameter estimates using a Monte Carlo analysis. The Monte Carlo analysis takes into account the estimated angular diameter, apparent brightness, a target’s uv-plane baseline coverage for a given observation, and instrumental sensitivity.
A modern implementation of a stellar intensity interferometry (SII) system on an array of large optical telescopes would be a highly valuable complement to the current generation of optical amplitude interferometers. The SII technique allows for observations at short optical wavelengths (U/B/V bands) with potentially dense (u,v) plane coverage. We describe a complete SII system that is used to measure the spatial coherence of a laboratory source which exhibits signal to noise ratios comparable to actual stellar sources. A novel analysis method, based on the correlation measurements between orthogonal polarization states, was developed to remove unwanted effects of spurious correlations. Our system is currently being tested in night sky observations at the StarBase Observatory (Grantsville, Utah) and will soon be ported to the VERITAS (Amado, AZ) telescopes. The system can readily be integrated with current optical telescopes at minimal cost. The work here serves as a technological pathfinder for implementing SII on the future Cherenkov Telescope Array.
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