2. The MECS components
The Medium Energy Concentrator Spectrometer (operating in the energy band 1.3-10 keV)
is composed by three nearly identical
units (named M1, M2, M3) sharing a common
Electronic Unit (EU).
Each MECS unit (Field of View of 28' radius and angular resolution at the
arcmin level) is composed by two major parts:
Unfortunately, since 6 May 1997, unit M1 is no longer operating due to a
failure in the electronic equipment (power supply).
The MU is connected to the DU by a carbon fiber envelope
(there are actually two envelopes, one containing M2 and M3,
and the other one M1 and the LECS. The carbon fiber envelopes are mounted
on the spacecraft optical bench, to which also the other SAX instruments
are connected.
The misalignments of the MU optical axes
w.r.t. the spacecraft axes have been calibrated in flight using some raster scan
observations.
2.1.1 The mirrors
Each MU is composed by 30 nested coaxial and confocal grazing incidence mirrors
in a double cone configuration (the exploded view
shows only 10 shells for clarity). A fourth identical MU is used in front of the
LECS detector.
The mirrors have a double cone geometry
to approximate the Wolter I configuration (paraboloid-hyperboloid), with characteristics listed
here below.
The MU design was optimized to have the best response at 6 keV. A
replica technique by nickel electroforming from super-polished mandrels was
used to build up the mirrors. A 1000 Å thick gold layer provides the
X-ray reflecting surface.
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MU characteristics
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Geometry double cone
(paraboloid-hyperboloid)
Number of nested mirrors 30 (coaxial, confocal)
Grazing incidence angles:
- outermost mirror 0.62°
- innermost mirror 0.25°
Mirror diameter from 68 to 162 mm
Mirror overall lenght 300 mm
Mirror thickness:
- 10 inner mirrors 0.2 mm
- 10 medium mirrors 0.3 mm
- 10 outer mirrors 0.4 mm
Mirror material nickel (electrodeposited)
Reflecting surface:
- material gold (evaporated)
- roughness < 10 Å
Focal length 1850 mm
Geometric collecting area:
- per MU unit 123.9 cm2
Case and spiders material stainless steel
Mirror Unit weight 13 Kg
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The mirror units calibration parameters are the
effective area and the
Point Spread Function.
The MU effective area has been calibrated at the Panter facility
and it constitutes the only on-ground effective area measurement.
The mirror shells are held together by two eight-arm spiders. Their shadow is
clearly visible when observing very bright sources.
The ~9% reduction in geometric area due to the spider obscuration is implicitly
taken into account in the mirror effective area
An Au-coated Tungsten grid located behind the Mirror Units and kept at +28 V shields
the
Be entrance window
of the detector from (charged) plasma particles.
Its calibration parameter is the relevant
transmission
A (passive) filter, placed in front of the detector entrance window,
is used both to stop high velocity charged particles passing through
the plasma grid, and to stop
UV photons. The three MECS units carry different filters :
- a Kapton + Al filter in front of M1
- a Lexan + Al filter in front of M2 and M3
Their calibration parameters are the relevant
transmissions
The detector is a
GSPC (Gas Scintillation Proportional Counter), constituted
by a 96 mm diameter ceramic chamber filled with Xenon. One can subdivide it in the following
components :
The
principles of operation of a GSPC,
together with a close view of the MECS Detector Unit (and its
characteristics)
are recalled elsewhere.
The relevant calibration files are also listed
there,
as well as below under individual sub-components.
The voltage setting of the Be window and drift/scintillation grid and of the PMT may affect the
gain and resolution parameters. Note however that these values are kept fixed at the nominal values (to which all ground and flight calibrations refer).
The PMT nominal HV setting used during ground calibration and first light was changed
in flight before the begin of the Performance Verification phase.
Two radioactive iron (55Fe) calibration sources are located in containers on the top
cover of the gas cell, on two
diagonally opposite positions
on the edge of the sensitive part of the Field of View.
They supply the primary source for the
gain monitoring and their energy (5.894 keV)
is assumed as primary gain normalization
reference.
The 50 micron Beryllium window is supported by a thicker
strongback (shown in figure below)
in form of a circular ring and four ribs. It delimits the sensitive part at the centre
of the Field of View.
![[strongback figure]](strongback.gif)
Fig. 2.2.2.2-I : geometrical characteristics of the Be window and strongback
Its calibration parameter is the relevant
transmission, which is function both of energy
(being the
prime responsible of the low energy cutoff in the effective area), and position
(because of the presence of the strongback). The effect of the strongback is
its geometrical shadow convolved through the
detector PSF.
The drift region is the primary region where the X-ray photons interact with the gas and liberate
an electron cloud which then drifts towards the -7000 V grid.
The calibration parameters due to this region are
In this region the electron clouds scintillate giving rise to bursts of
UV light via interaction with the Xenon ions. This region contribute also
to the total efficiency especially for hard X-ray photons. However, the energy
of events interacting there cannot be reconstructed. They are then rejected
by Burst Length selection.
The calibration parameters due to this region are
the relevant quantum efficiency and the
Burst Length related corrections.
The Hamamatsu position sensitive PMT, with 15 dynodes and a crossed-wire anode,
is operated at about 1000 V. For each burst of light corresponding to a detected photon
it gives rise to the signals processed by the
EU
to measure arrival time, energy and position of photons.
The PMT setting and behaviour affects several calibration parameters :
- its voltage setting is the main contribution to the
absolute gain setting. It has been changed
during the Commissioning Phase (with respect the on ground calibration
settings) to optimize the coverage of the 1-10 keV
range over the entire Field of View
- disuniformities in the entrance window and in the electrode arrangement are
responsible of the
positional gain dependency
- the sensitivity of the PMT to ambient temperature is responsible of the
variations of
gain with time
- disuniformities in the electrodes are responsible of the
geometrical distortion
The Electronics Unit consists of :
- (2.3.1) the Front End Electronics receives the signals from the PMT (a trigger, the digitized
energy, or PHA, and four raw position signals). It reconstructs the event X,Y position
and generates the
Burst Length
and arrival time information, and places everything in
a FIFO register. It also performs some event qualification (taking account of valid
and rejected events in
ratemeters named trigger,
valemin, valblmin, rejemax and rejblmax.
- (2.3.2) the Event Processor reads the events from the FIFO, and performs some (software)
rejection based on programmable E, BL and XY thresholds. In case of high rates
some events may be lost if the microprocessor is busy. All these losses are counted
and are available in
ratemeters named rejmpbus,
reje, rejbl, and rejxy,.
- ((2.3.3) the Communication Processor is responsible of assembling event data into scientific
telemetry packets,
and ratemeter data into engineering
telemetry packets. In case of
high rates it is possible that the buffer space for scientific packets fills up : the
information on the total number of lost events and the non-transmission interval is
available in the header of the first packet transmitted afterwards.