The three flight MECS units have been extensively calibrated ( millions photons per unit) at the 130-meter long X-ray PANTER facility of the Max-Planck-Institut für Extraterrestrische Physik in Munich, during a period of 7 weeks in October-November 1994. The limited size of the PANTER beam did not allow a simultaneous calibration of all units; therefore two MECS units, hereafter named ME2 and ME3, were calibrated together during the first run of measurements while the third MECS unit, hereafter named ME1, was calibrated during the second run together with the LECS instrument (Parmar et al. 1996).
A detailed description of the experimental set-up used for on-ground calibrations can be found in Boella et al. (1995). Briefly, the MUs and the detectors were mounted on two separate optical benches, remotely controlled by manipulators (one linear and two rotational for the MUs, and three linear for the detectors); moreover, the assembly MUs-detectors was mounted on a table that could be rotated and tilted for off-axis measurements. In particular, the linear manipulator of the MUs bench could position, in front of the detectors, alternatively the two MUs, or two 40 mm diameter holes for direct beam measurements (the so-called flat fields), or two multipinhole masks with a square matrix of 123 holes ( mm diameter) with 4 mm pitch. The MUs and detectors alignment was performed in two steps: a first rough alignment with a divergent He-Ne laser beam that simulates the X-ray PANTER beam, and a final adjustment at 1.5 keV by using those photons that, as a result of the divergence of the beam, are reflected only by the first cone of the MUs.
Two interchangeable X-ray sources are available at the PANTER facility: the first one is directly installed in vacuum for energy lines up to 3 keV; the second one, separated from the vacuum by a Be window, is used for higher energies. Both systems are equipped with a set of filters of various material and thickness. The high voltage supply, the emission current of the X-ray source, and the position of the filter wheel can be adjusted to cut the source continuum bremsstrahlung and to obtain the desired counting rate and the more convenient energy spectrum. An independent proportional counter (Monitor Counter) is placed at the entrance of the test chamber and it is used to monitor the X-ray beam intensity. All the information concerning the basic experimental conditions (time, X-ray source voltage and current, manipulator encoder outputs, monitor counter rate) was continuously stored in housekeeping files useful for the off-line calibration data analysis.
The instrument Electronic Unit, common to all the three MECS units, was connected to a Test Equipment including a bus probe emulating the satellite OnBoard Data Handling bus. Test Equipment control, instrument control, and data acquisition (in the form of block transfer bus data packets) were performed via a VAX-station connected on a Local Area Network with a second VAX-station for intermediate storage, with two PCs for Quick Look, and with one more PC for data archiving onto magneto-optical disks for off-line analysis. The instrument Electronic Unit performed very well and supported source rates up to 4000 cts/s beyond the specification of 2000 cts/s.
The principal X-ray energy lines used to calibrate the MECS units are reported in Table 3 (click here). For almost all the energy lines, three main kind of measures were performed: mirror units (MU) on-axis and off-axis, flat fields (FF) of the detectors alone, and multipinhole scans of the detector units alone. All these measurements were performed at the nominal instrument setting. The statistical quality of the data was very high: a typical MU acquisition contained about half a million events. Additional measurements of the detector performance were performed changing the high voltage of the cell and of the PMT respect to their nominal values; moreover, the behaviour of the detector has been checked in function of the count rate.
Table 3: PANTER calibration lines ( ME1 only; ME2 and ME3 only)
Figure 3: Relative gain maps (gain as percentage of the value at detector centre). Thick lines indicate a gain of 1.00. Contours are spaced by 0.01. Contours lower than 1.00 are in lighter shade