How is it possible to calibrate the ultraviolet (UV) channel of an imaging solar coronagraph as it flies through space? Just look to the stars. Researchers from the National Institute for Astrophysics (INAF) and the University of Florence (both in Florence, Italy) are developing procedures to use the UV emission of stars to calibrate the METIS coronagraph. It is an external-occulted novel coronagraph that will obtain simultaneous UV and polarized-visible-light images of the solar corona aboard the European Space Agency’s (ESA’s) Solar Orbiter satellite that will be launched in 2017.1
For the METIS UV channel, in-flight procedures must calibrate the solar corona UV brightness, monitor intensity changes throughout the mission, and verify prior radiometric calibrations performed in the laboratory before launch.
To calibrate the UV channel of METIS, a set of well-known UV-emitting early-type stars that produce bright and stable far-UV continuum spectra are selected. METIS, thanks to its UV interference filter, will select only the part of these spectra lying within a narrow (±10 nm) wavelength band around the neutral-hydrogen (HI) Lymanalpha (Ly-α) 121.6 nm spectral line. Assuming that this UV stellar emission is temporally stable, radiometric calibration is performed using the spectra of these known stars based on previous data obtained by other in-orbit coronagraph instruments including SOLSTICE, SPICAM, and the International UV Explorer (IUE).
Coronal-radiance values from the intensified active-pixel sensor (IAPS) of the UV-imaging detector are compared to previously imaged radiance values from the selected UV-emitting stars, and associated conversion factors are computed. The UV transmission must also be calibrated against the vignetting function of the telescope, the reflectivity of its mirrors, the interference-filter transmissivity, the detector quantum efficiency, and the telescope point-spread function as produced by optics on the UV focal plane.
These in-flight observations are then compared to predicted/previously measured data to perform radiometric calibration, allowing the instrument to monitor system throughput and track intensity variations that could be caused by optical contamination or degradation due to outgassing or cosmic radiation (mainly protons) that characterize the harsh space environment in which Solar Orbiter shall operate. In addition to using UV-emitting stars, UV emission from other suitable sources (such as planets) could also be used if they fall in the field of view of METIS during its coronal observations.
“Solar Orbiter will be a very exciting mission, leading to a deep knowledge of our Star and its atmosphere—responsible for our daily life,” says Mauro Focardi, a Young Scientist fellow at the INAF-OAA Arcetri Astrophysical Observatory. “At present, our research group is also working on the development of new technologies based on nanostructures to perform UV-imaging polarimetry in the HI Lyman-alpha line. This would provide, for the first time, the capabilities to perform diagnostics on the hot coronal plasma, measuring the magnetic fields defining the thin coronal structures and following the Hanle effect rules.”
1. Mauro Focardi, et al., SPIE Newsroom; doi:10.1117/2.1201411.005684 (December 1, 2014) or see