Figure 3.1.3/1: Composite image of the XMM QM mirror module 'best focus' point spread functions at 1.5 keV. The measurements were made at off-axis angles of 7 arcmin and 14 arcmin over a range of azimuthal positions.
XMM homepage http://astro.estec.esa.nl/XMM/xmm.html
XMM is an X-ray astrophysics observatory scheduled for launch in 1999 and, with a projected lifetime of 10 years, it will enable astronomers to conduct sensitive X-ray spectroscopic observations of a wide variety of cosmic sources.
X-ray astronomy primarily involves the study of plasmas with temperatures of 106- 108K. Such plasmas radiate the bulk of their energy at X-ray wavelengths of 1-50 Å (250 eV 12 keV). Apart from X-ray continuum emission, produced through such processes as thermal bremsstrahlung, a significant fraction of the total emissivity can arise from line emission. At these high temperatures, cosmic abundant elements such as hydrogen and helium are stripped of all their electrons. Only heavier elements can, depending on the temperature, retain their K or L shell electrons. The study of transitions from these elements, which are primarily in a hydrogenic or helium-like state, represents an important diagnostic tool for an understanding of the physics of cosmic X-ray sources.
XMM is designed specifically to investigate in detail the spectra of cosmic X-ray sources down to a limiting flux of 10-15 ergs/cm²/s. XMM's principal characteristics can be summarised as
These characteristics are achieved by a suite of complementary instruments aboard XMM.
The European Photon Imaging Camera (EPIC) provides a focal plane detector for each of XMM's three telescope modules. The detectors are based on cooled CCDs, operating in a photon- counting mode to provide simultaneous imaging and non-dispersive spectroscopy for every field that XMM observes. These powerful diagnostics will have a significant impact on every branch of X- ray astrophysics.
The mission's aim of providing a medium-resolution spectroscopic capability is achieved by two reflection grating experiments (RGS), which intercept about 60% of the converging beams for the prime focus. The grating arrays are placed directly behind the mirror modules and in front of the EPIC MOS cameras. Position and energy sensitive readout at the secondary focus is performed by strip arrays of nine MOS CCDs each.
The Optical Monitor (OM) enables XMM to provide simultaneous coverage of the telescope field at 1700-6000 Å. The 30 cm Cassegrain telescope will cover a 23 arcmin field with a spatial resolution of 2 arcsec through the use of two redundant photon- counting detectors.
The XMM Science Operations Centre (XMM-SOC) team at the Astrophysics Division has been building up over the last 2 years. The main focus of the team has been the definition of requirements on all the software subsystems, the design of, support for and execution of instrument calibration campaigns, as well as the design of a software simulator (SciSIM) which, in detail, models the scientific performance of the spacecraft, mirror module and experiments.
The XMM-SOC software architecture comprises eight major subsystems. The detailed scientific definition of all these subsystems has been circulated widely in the XMM users' community, and most of the comments received have been incorporated. Bundled together with added system requirements, these documents have been approved by a review panel to serve as the formal input to the software contractor, under ESOC's responsibility, for the start of their work on the SOC software subsystems.
An important part of the XMM archive will be the results obtained through the pipeline processing system (PPS), data processing software that systematically analyses every XMM observation and produces a number of standard, high level, data products for every XMM observation. The PPS will be run by an AO- selected consortium called the Survey Science Centre (SSC). The SSC is a Leicester University (UK)-led consortium consisting of a large number of European astronomical institutes. The PPS will be developed jointly by the XMM-SOC and the SSC.
The SOC has also been heavily involved in the XMM overall system calibration definition and in the XMM science simulator development. The first of these involves the analysis of the way in which the XMM instruments are to be calibrated both at unit level and at the integrated instrument level. This exercise is crucial for the SOC to gain understanding of the detailed performance of the instruments, knowledge that is extremely important when operating the instruments in orbit. The second point involves development of the XMM science simulator, a tool that is fully integrated in the XMM calibration philosophy and that will be used heavily both in the interpretation of ground and in-flight calibration data as well as in the generation of the actual XMM calibration files, since the in-flight conditions (especially the source distance which is infinite in-flight, but finite during ground calibrations) cannot be accurately represented during gound calibrations.
Recent work has also involved the SOC in various aspects of mission planning, such as the study of the visibility of X-ray and visible light sources on the sky, to be used for either the XMM in-flight calibration phase or the XMM performance verification phase. These two phases will be performed in the first 3 months of XMM in orbit. The calibration work will, of course, also have to be carried on partly on a repetitive basis in order to maintain an accurate instrument calibration knowledge. This work has been started because the currently envisaged XMM orbit, with an apogee in the southern hemisphere, limits the percentage of time many sources in the northern sky are visible.
As an example of the ground calibration work in which the SOC has been involved, Fig. 3.1.3/1 shows a composite image of XMM's Qualification Model mirror module 'best focus' point spread functions measured at 1.5 keV. The measurements were made at off- axis angles of 7 arcmin and 14 arcmin, and over a range of azimuthal positions. The XMM-SOC team in the Astrophysics Division has been supporting these measurement activities at the Panter facility in Garching, Germany, and has been performing detailed simulations (using SciSIM) to match the measured response, by accounting for minute mirror shell distortions detected in metrology data. The Half Energy Width (HEW) of these point spread functions does not degrade by more than ~25% in going from the centre to the edge of the field of view.
Figure 3.1.3/1: Composite image of the XMM QM mirror module
'best focus' point spread functions at 1.5 keV. The measurements
were made at off-axis angles of 7 arcmin and 14 arcmin over a
range of azimuthal positions.
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