Instrumental Design

Principaux sous-systèmes de PHEBUS

The instrument is composed of several subsystems, all mounted on the main structure. The front panel consists of a parasitic light deflector and a parabolic stalker mirror allowing the exosphere of mercury to be scanned through a rotating mechanism. This mobile mirror captures the light of the Exosphere and focuses it on the input slot.

Pointing System


The baffle is conceived to trap photons that do not come from the observed area (located in an 8 ° cone) and that disrupt the measurement. Thus, the amount of light that does not come from this cone is divided by 1 000 000 thanks to the different stages of the baffle and its black internal cover which absorbs 99% of the light. Learn more

Primary mirror

This parabolic mirror deflects the light entering through the baffle towards the inside of the instrument. It is composed of silicon carbide which gives it good reflectivity in UV as well as good thermo-mechanical properties.


The scanner is provided by the Russian Space Agency (IKI). This mechanism allows the baffle and the primary mirror to be rotated at 360 ° with a precision lower than the degree to adjust the line of sight of the instrument. The scanner must be locked to withstand the vibrations that have occurred during the launch. The release mechanism can only be activated once, just after the launch.


FUV detector (145 nm - 315 nm)

The FUV detector is provided by the University of Tokyo.
It is used to convert the intensity of wavelength lines between 145 nm and 315 nm in numerical value. This detector is extremely sensitive: it is able to count one-by-one the photons that arrive at its surface. These photons are "sorted" by wavelength, left (145 nm) to the right (315 nm) by the associated diffraction lattice. This breakdown in wavelengths forms a spectrum.

EUV detector (55 nm - 155 nm)

The EUV detector, similar to the FUV, covers the range 55 nm-155 nm. Its photosensitive surface deteriorates when exposed to ambient air. So the detector casing is an enclosure for storing the sensor in vaccum. Since no material is transparent at wavelengths less than 110nm, this enclosure is equipped with a window that will be opened in flight thanks to a dedicated mechanism.

NUV detector (405 nm et 423 nm)

These detectors measure two specific emission lines (at 405 nm and 423 nm) that inform us about the presence and quantity of Calcium and Potassium in the Mercury Exosphere. They also work in photon counting.

Photon Sorting System

Slit mechanism

Depending on the observation, the slit mechanism inserts or removes the slit of the spectrometer into the optical beam of the instrument, by switching to 0 or 90 °. The slit is a determining factor in the definition of the field of view and the spectral resolution of the instrument.

Diffraction gratings

The light breaks down into different "colors" characterized by wavelength. For example, the green has a wavelength of about 530 nm. Like a rainbow, the gratings disperse the light into its different wavelengths. One grating is dedicated to the EUV channel and another to the FUV and NUV channels combined.

Processing unit

DPU (Data Processing Unit)

The Data Processing unit (DPU) is the digital processing unit of the instrument. It ensures the interfaces with the satellite and is in charge of the control of the instrument, the sequential activation of the different subsystems, as well as their monitoring.