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).

2.1 Mirror Unit

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.

	     MU characteristics
Geometry                   double cone                
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                      

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.

2.1.2 The spider

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

2.1.3 The Plasma suppression grid

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

2.2 Detector Unit

2.2.1 The ion shield and UV filter

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 : Their calibration parameters are the relevant transmissions

2.2.2 The gas cell

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. The calibration sources

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 Be window

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]

Fig. : 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

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 The scintillation region

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.

2.2.3 The Photo Multiplier Tube

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 :

2.3 Electronics Unit

The Electronics Unit consists of :

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