We report progress in the design of the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). This instrument
consists of two modules; a Wolter I soft X-ray telescope based on radially packed microchannel plate
optics (MIXS-T) and a profiled collimator which uses a square pore square packed microchannel plate array to
restrict its field of view (MIXS-C). Both instrument modules have identical focal planes (DEPFET macropixel
array) providing an energy resolution of better than 200 eV FWHM throughout the mission.
The primary science goal of MIXS is to perform X-ray fluorescence spectroscopy of the Hermean surface with
unprecedented spatial and energy resolution. This allows discrimination between different regolith types, and
by combining with data from other instruments, between competing models of crustal evolution and planetary
formation. MIXS will also probe the complex coupling between the planet's surface, exosphere and magnetosphere
by observing Particle Induced X-ray Emission (PIXE).
We report progress in the design, theoretical modeling and experimental characterisation of microchannel plate
(MCP) X-ray optics for the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). We show that MCP
optics technology allows the design of a highly capable imaging telescope with 1 m focal length, a 1° field of
view and approximately 50 cm<sup>2</sup> of on-axis effective area at 1 keV. Of a total instrument mass budget 7.3 kg, less
than 2.3 kg is allocated to the optics assemblies, telescope tubes, support structures and the electron diverters
(used to deflect electrons from the focal plane). The instrument science goals require an imaging resolution of 9
arcminutes, with a design goal of 2 arcminutes. Recent experimental data, taken from individual optic elements
is presented to show that MCP quality is in good agreement with the error budgets assumed in theoretical
calculations of performance.
CCD detectors in the focal plane cameras of grazing incidence X-ray telescopes on the XMM-Newton and SWIFT
satellites have encountered damage which has been attributed to impacts by external particles. The apparent mechanism
is one whereby interplanetary micrometeoroid particles or space debris have been ingested by the grazing incidence
mirrors and scattered down the telescope tube on to the CCD detectors in the focal plane.
At the time of writing, there have been 5 such events detected in total by the three XMM telescopes during five years of
operations and one event detected by the SWIFT X-ray Telescope (XRT) during one year in orbit. Significantly, no
events of this type have been reported for Chandra.
Modelling and analysis of scattering of small particles from grazing incidence mirrors allows us to explain the different
impact rates seen by these three satellites. Furthermore, using the ESA MASTER2005 micrometeoroid and space debris
impacts flux model, impact rates have been derived from consideration of Swift's orbit, pointing history and the dust and
debris particle environment. This modelling can be used to determine whether risk mitigation strategies are required for
the continuing operation of SWIFT and other operating observatories, and also provides a basis for predicting particle
impact rates for grazing incidence telescopes on future missions such as XEUS, Constellation-X and others.