This description is mostly based on the information contained in the papers
"Calibration of the XMM Flight Modules - Image quality", and
"Calibration of the XMM Flight Modules - Effective area", by Gondoin, Ph. et al,
in SPIE 3444 (1998),
"In-orbit performance of the XMM-Newton telescopes: images and spectra", by Aschenbach, B., in SPIE 4492 (2001),
and
"XMM-Newton Observatory: The Spacecraft and Operations", by Jansen, F. et al., A&A 365, L1 (2001)
Further information can be obtained from the
XMM-Newton Users' Handbook
Introduction
Each of the three X-ray telescopes on board XMM-Newton consists of 58
Wolter I grazing-incidence mirrors which are nested in a coaxial and
cofocal configuration. The design of the optics was driven by the
requirement of obtaining the highest possible effective area over a
wide range of energies, with particular emphasis in the region around
7 keV. Thus, the mirror system had to utilize a very shallow grazing
angle of 30' in order to provide sufficient reflectivity at high
energies. The telescopes focal length is 7.5 meters and the diameter
of the largest mirrors is 70 cm, to be compatible with the shroud of
the launcher. Each telescope consists includes, apart from the mirror
modules, baffles for visible and X-ray stray-light suppression and an
electron deflector for diverting soft electrons. Two of the telescopes
carry a Reflection Grating Array (RGA).
Telescope Configuration
Each of the XMM-Newton telescopes consists of:
- the mirror assembly door, which protected the optics during
integration, launch and early orbit phase,
- the entrance baffle, which provides visible straylight
suppression at angles larger than 47°,
- the X-ray baffle,
- the Mirror Module,
- an electron deflector, which produces a circumferential magnetic
field which prevents low energy electrons reflected by the mirrors
reaching the focal plane detectors,
- in two of the telescopes, the Reflection Grating Array, and
- the exit baffle, which provides an appropriate thermal environment.
The 58 Wolter I mirrors of each telescope are bonded on their entrance
aperture to the 16 spokes of a single spider made out of Inconel. The
spider is connected to the support platform via an aluminium interface
structure (the MIS: Mirror Interface Structure) consisting of an outer
cylinder and an interface ring. On two of the modules, the ring
interfaces the mirror module to a Reflection Grating Assembly
(RGA). To minimise the mechanical deformation of the mirrors and
therefore the optical degradation, the flatness of the interface
between the spider and the MIS had to be better than 5 micron
X-ray baffles are located in front of the mirror systems. They act as
collimators and reduce considerably the amount of straylight in the
field of view of the focal plane cameras.
The XMM-Newton X-ray baffle was constructed as two
sieve-plates made out of circular strips. The plates were mounted
coaxial to and coaligned with the front aperture cross section of the
58 mirror shells, such that they block single-reflection rays, but do
not eclipse two-reflection rays. Each sieve plate is a disk 1 mm thick
with 59 circular strips and 16 radial spokes. The offset of the two
sieve plates from the front of the mirror system is 385 mm and 439 mm,
respectively. All the baffle surfaces facing the mirrors are
blackened.
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Light path in the XMM-Newton telescope with only an EPIC camera in its
primary focus (left), and in the two telescopes in which a RGA is
mounted into the optical path (right)
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Optical Design
Each Mirror Module is a grazing-incidence Wolter I telescope,
consisting of 58 gold-coated nested mirrors. Each mirror shell
consists of a paraboloid and an associated hyperboloid which were
replicated together in one piece to facilitate alignment and
integration.
In grazing incidence optics the effective area is increased by nesting
a number of mirrors and thus filling the front aperture as far as
possible. The nesting efficiency is determined by the mirror shell
thickness and, in case of very low grazing angles, by the minimum
radial mirror separation which is required for integration and
alignment. The thinner the mirror shells are and the narrower the
shells are spaced, the larger is the collecting area.
The thickness of the smallest mirror (diameter=306 mm) is 0.47 mm, and
it increases linearly with shell diameter in order to guarantee
sufficient stiffness. The thickness of the 700 mm diameter mirror is
1.07 mm. The minimum radial separation between adjacent shells is 1mm.
Adding more shells is rather inefficient in building up more
collecting area because of the mass penalty involved and the low gain
in effective area.
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Telescope Specifications |
| Telescope focal length | 7500 mm |
| Number of mirrors per telescope | 58 |
| Outer mirror radius | 350 mm |
| Inner mirror radius | 153 mm |
| Axial mirror length | 600 mm |
| Outer mirror thickness | 1.07 mm |
| Inner mirror thickness | 0.47 mm |
| Minimum packing distance | 1 mm |
| Mirror substrate material | Nickel |
| Reflective Coating | Gold |
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The performance of the X-ray telescopes can be characterized by:
Image Quality
The point spread functions and effective areas of the three telescopes
were first characterized on-ground during an extensive calibration
campaign. A comprehensive numerical model of the mirror system was
used to generate an initial calibration database by extrapolating
on-ground tests to in-orbit operation conditions and by interpolating
between the finite number of measurement points.
On January 19 2000 the X-ray telescope FM2 saw "First Light", followed
by FM3 and FM4. After "First Light" a number of observations were
made during the commissioning phase in order to characterize the
imaging performance of the telescopes. Analysis of the results
indicated that the telescopes point responses measured in-orbit were
basically the same as derived from on-ground calibration measurements
out to 30". In particular, extended sources in the center of the
telescope field of view can be studied with a 5" spatial
resolution.
For on-axis sources, high energy photons are focused predominantly by
the inner shells of the telescope. These inner shells apparently give
better focus that the average hence the fractional encircled energy
increases with increasing photon energy.
The figure on the left displays the contours of the first X-ray image
of Castor in which the two components of the binary system are
spatially resolved. Both stars are separated by 3.9". The image was
taken with MOS1 (from Gudel et al. 2001).
In-flight on-axis PSF of the XMM-Newton telescopes |
|---|
| Mirror Module Number |
2 (pn) |
3 (MOS1+RGS1) |
4 (MOS2+RGS2) |
| 1.5 keV |
8 keV |
1.5 keV |
8 keV |
1.5 keV |
8 keV |
| FWHM (arcsec) |
6.6 |
6.6 |
6.0 |
5.1 |
4.5 |
4.2 |
| HEW (arcsec) |
15.1 |
14.8 |
13.6 |
12.5 |
12.8 |
12.2 |
Effective Area
The design driver for the XMM-Newton telescopes was to achieve maximal
area at low energies (2 keV) without sacrificing area at high energies
(7 keV). XMM mirrors are most efficient in the energy range from 0.1 to
10 keV, with a maximum around 1.5 keV and a pronounced edge near 2 keV
(the Au M edge). The design goal was to achieve a collecting area of
1900 cm2 for energies up to 150 eV, 1500 cm2 at
2 keV, 900 cm2 at 7 keV, and 350 cm2 at 10 keV,
for each of the telescopes.
The effective area for each telescope was measured in the PANTER X-ray
test facility illuminating the full aperture with line radiation
between 0.28 and 10 keV, and using a copy of the ROSAT PSPC as focal
plane detector. The figure shows the on-axis effective area of the
XMM-Newton telescopes without (solid line) and with (dot-dashed line)
RGA (from Jansen et al. 2001).
Measurements of the in-orbit vignetting function (i.e. the decrease of
the effective area with increasing field angle), match simulation
results extrapolated from on-ground calibration.
X-Ray Straylight Rejection
X-rays from outside the field of view can reach the sensitive area of
the focal plane detectors by single reflection from the rear end of
the hyperbola, if the source is at an off-axis angle between 20' and
80'. Rays reflected just once from any one the parabolas cannot leave
the mirror assembly because of the close packing of the mirror shells.
The efficiency of the sieve plate system was ray traced and
demonstrated to reduce the straylight level by a factor of 5 to 10
depending on the position in the focal plane. Pointings in the
vicinity of the Crab Nebula confirmed the high efficiency of the
baffles. The straylight collecting area of the EPIC detectors as a
function of off-axis angle is about 3 cm2 for sources
located between 20' and 1.4° from the optical axis, and completely
negligible at higher angles.
The XMM-Newton telescopes have been built by
Carl Zeiss (Oberkochen, Germany),
Kayser-Threde (Munich, Germany) and
Media Lario (Bosisio Parini, Italy).
X-rays tests were performed in the
PANTER test facility of the
Max-Plank-Institut fur extraterrestrische Physik.
The qualification of the mirror modules was performed in EUV light at the Focal X-ray
facility of the
Centre Spatial de Liege (Belgium)
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