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We describe the evolution and the analysis of the design that led to the development of the Flat-field and wavelength Calibration Unit (FCU) for the Multi-AO Imaging CAmera for Deep Observations (MICADO) instrument. MICADO will be one of the first light instruments of the Extremely Large Telescope. The FCU challenge in terms of calibration is related to the large size of the MICADO entrance and final focal plane, ∼200 mm × 200 mm. Such a focal plane scale and its segmentation in 3 × 3 detectors, require significant design modifications with respect to the calibration units of the current and past generation of instruments. The design analysis and ray tracing calculations are complemented with the test and verification of lab prototypes to assess the reliability of the FCU architecture in terms of flat-field illumination uniformity and signal to noise, spectral calibration line coverage and radial velocity stability of the wavelength solution provided to the instrument. [ABSTRACT FROM AUTHOR]

The ASTRI Mini-Array is an international collaboration led by the Italian National Institute for Astrophysics (INAF) that will operate nine telescopes to perform Cherenkov and optical stellar intensity interferometry (SII) observations. At the focal plane of these telescopes, we are planning to install a stellar intensity interferometry instrument. Here we present the selected design, based on Silicon Photomultiplier (SiPM) detectors matching the telescope point spread function together with dedicated front-end electronics. [ABSTRACT FROM AUTHOR]

Highly reddened type Ia supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, RV, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths (λmax ≲ 0.4 μm). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similar RV values. We investigate polarization profiles of a sample of Galactic stars with low RV values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, used archival data for 3 additional stars, and ran dust extinction and polarization simulations (by adopting the picket-fence alignment model) to infer a simple dust model (size distribution, alignment) that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with low RV values and anomalous extinction sightlines displays normal polarization profiles with an average λmax ~ 0.53 μm, and is consistent within 3σ to a larger coherent sample of Galactic stars from the literature. Despite the low RV values of dust toward the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed toward SNe Ia with low RV values. There is a correlation between the best-fit Serkowski parameters K and λmax, but we did not find any significant correlation between RV and λmax. Our simulations show that the K–λmax relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal λmax and low RV values, a population of large (a ≥ 0.1μm) interstellar silicate grains must be contained in the dust composition. [ABSTRACT FROM AUTHOR]

Cherenkov Telescopes are equipped with optical dishes of large diameter – in general based on segmented mirrors – with typical angular resolution of a few arc-minutes. To evaluate the mirror׳s quality specific metrological systems are required that possibly take into account the environmental conditions in which typically these telescopes operate (in open air without dome protection). For this purpose a new facility for the characterization of mirrors has been developed at the labs of the Osservatorio Astronomico di Brera of the Italian National Institute of Astrophysics. The facility allows the precise measurement of the radius of curvature and the distribution of the concentred light in terms of focused and scattered components and it works in open air. In this paper we describe the facility and report some examples of its measuring capabilities. [ABSTRACT FROM AUTHOR]