Characterization of the dark signal of the Solar Orbiter/Metis detectors

Metis, one of the instruments of the ESA mission Solar Orbiter (launched on February 10th, 2020, from Cape Canaveral), is a coronagraph with 2 channels, capable of performing broadband polarization imaging in the visible range (580-640 nm), and narrow-band imaging in UV (HI Lyman-alpha 121.6 nm). It is equipped with two detectors based on CMOS APS sensors: the visible channel includes a custom CMOS sensor with direct illumination, while the UV channel is provided with an intensified camera, based on a Star-1000 rad-hard CMOS APS coupled via a 2:1 fiber optic taper to a single-stage Microchannel Plate intensifier coated with an opaque KBr photocathode and sealed with an entrance MgF2 window. Dark subtraction is a crucial step in the data reduction pipeline, thus requiring careful in-flight monitoring and characterization of the dark signal. Since it is not possible to directly acquire dark images with the visible detector, as the door of the instrument is not light-tight, an ad hoc procedure has been designed to estimate the correction to be applied. In the case of the UV detector, however, it is possible to acquire dark frames by turning off the intensifier. Due to small fluctuations occurring on the bias signal level even on short timescales, an algorithm has been developed to correct the dark matrix frame by frame.