  Mini-EUSO is a telescope observing the Earth in the ultraviolet band (290-430
nm) since 2019, through a nadir-facing UV-transparent window in the Russian
Zvezda module of the International Space Station. The main camera has an
optical system composed of two 25 cm diameter Fresnel lenses and a focal
surface consisting of 36 multi-anode photomultiplier tubes, 64 pixels each, for
a total of 2304 channels. The instrument has a square field of view with a side
of 44 degrees, a spatial resolution of about 6.3 km on the Earth surface and a
sampling time of 2.5 microseconds. Mini- EUSO has also two cameras in the near
infrared and visible ranges and silicon photomultiplier sensors to complement
the UV observations. Mini-EUSO has been designed as a small-size version of the
original JEM-EUSO space telescope to demonstrate its observational principle.
Mini-EUSO is in fact potentially capable of observing extensive air showers
generated by ultra-high-energy cosmic rays with an energy above 10 21 eV and of
detecting artificial showers generated with lasers from the ground. Other main
scientific objectives of the mission are the study of atmospheric phenomena
(transient luminous events such as ELVES and sprites), the observation of
meteors and among them the search for interstellar meteors and nuclearites such
as strange quark matter. Moreover, Mini-EUSO can map night-time UV Earth
emissions, both anthropogenic and natural. In this work, we will discuss
results and performance of the telescope during its first four years of
activity.
