Mini-EUSO is a telescope observing the Earth in the ultraviolet band from the International Space Station. It is a part of the JEM-EUSO program, paving the way to future larger missions, such as K-EUSO and POEMMA, devoted primarily to the observation of ultrahigh-energy cosmic rays from space. Mini-EUSO is capable of observing extensive air showers generated by ultrahigh-energy cosmic rays with an energy above 1021 eV and to detect artificial showers generated with lasers from the ground. Other main scientific objectives of the mission are the search for nuclearites and strange quark matter, the study of atmospheric phenomena such as transient luminous events, meteors, and meteoroids, the observation of sea bioluminescence and of artificial satellites and man-made space debris. Mini-EUSO will map the nighttime Earth in the UV range (290–430 nm), with a spatial resolution of about 6.3 km and a temporal resolution of 2.5 μs, through a nadir-facing UV-transparent window in the Russian Zvezda module. The instrument, launched on 2019 August 22, from the Baikonur Cosmodrome, is based on an optical system employing two Fresnel lenses and a focal surface composed of 36 multianode photomultiplier tubes, 64 channels each, for a total of 2304 channels with single-photon counting sensitivity and an overall field of view of 44°. Mini-EUSO also contains two ancillary cameras to complement measurements in the near-infrared and visible ranges. In this paper, we describe the detector and present the various phenomena observed in the first months of operations.
Mini-EUSO ("UV atmosphere" in Russian Space Program) is a science mission to observe UV phenomena in the night Earth atmosphere from a UV transparent porthole of the International Space Station and will serve as a pathfinder for future ultra-high energy cosmic ray missions such as KLYPVE-EUSO (K-EUSO) and JEM-EUSO. The Mini-EUSO optical design has two 250 mm double-sided Fresnel lenses and a flat focal surface. The overall length of the optical system is 300 mm. Its field of view is about ±22 •. The RMS spot sizes between 0 • and 22 • field angle are 2.0 mm ∼ 3.5 mm in diameter, and conform to the pixel size of the M64 multi-anode photomultiplier (HAMAMATSU Photonics). The material of lenses is UV transparent PMMA (Poly Methyl Methacrylate, PMMA-000, MITSUBISHI RAYON) and the lens thickness is 11 mm. The lenses were manufactured using the diamond turning machine by the Materials Fabrication Laboratory of RIKEN. We confirmed that the surface roughness of lenses is less than 20 nm (RMS). In this paper, we will describe the details of the optical system and test results.
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