IEO
IEO
Boom
M0
M2
Lyot stop IO
Polarimeter
UV
detector
VL detector
M1
world news
January 2015
www.laserfocusworld.com Laser Focus World 20
ASTRONOMY
In-space UV calibration of the METIS solar coronagraph
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
In-flight calibration
For the METIS UV channel, in-flight pro-
cedures must calibrate the solar corona
UV brightness, monitor intensity changes
throughout the
mission, and verify
prior radiomet-
ric calibrations per-
formed in the lab-
oratory 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 contin-
uum 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-hydro-
gen (HI) Lyman-
alpha (Ly-α) 121. 6
nm spectral line.
Assuming that this
UV stellar emission
is temporally stable,
radiometric calibra-
tion is performed
using the spectra of
these known stars
based on previous
data obtained by
other in-orbit coro-
nagraph instru-
ments including
SOLSTICE, SPICAM, and the Interna-
tional UV Explorer (IUE).
Coronal-radiance values from the
The METIS coronagraph uses two main
ultraviolet (UV)-enhanced coated mirrors (M1
and M2), a UV detector, an inverted external
occulter (IEO), a sun disk light rejection
mirror (M0), and an internal occulter (IO); VL
= visible light. (Courtesy of INAF)
