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The PSPC as part of the focal plane instrumentation of the XRT\ has been thoroughly calibrated using ground based as well as in-flight measurements. These results are used during the SASS processing (cf. module CT of the TEL processing in Section 8.2.2) to correct the recorded raw event information (RAWX, RAWY, and PHA = RAW_AMPL) for temporal and positional variations, saturation effects, and non-linearities of the PSPC and its following electronics yielding the corrected event (XDET, YDET, and PI = AMPL). A complete record of an event delivered by the standard processing is then labeled (RDF format) together with a corrected event time (see Section 3.8) and sky coordinates determined from the attitude solution (see Chap. 2.2) as (TIME, XDET, YDET, AMPL, XPIX, YPIX).
|Shortcut||CT task||description and dependencies||name of calibration file|
|BAL||CTA||bin of Al K line (Prescott fit)||steering database (INGRES )|
|NLC||CADC||correction for ADC non-linearity||p$manpar:adc_bins.dat|
|GSC||CGS||correction for gain saturation||(none)|
|TGC||CTG||correction for temporal gain variations||(none)|
|EPC||DCORE||electronic position correction||p$manpar:para_c.dat|
|SGC||DCORG||correction for spatial gain variations||p$manpar:gain_kor3_c.dat|
|here||SASS CT||RDF format||short description|
|A||RAW_AMPL||ADC pulse height channel of event (integer in [1,256])|
|AN||pulse height of event, corrected for ADC non-linearity|
|ANG||pulse height of event, corrected for ADC non-linearity and gain saturation|
|ANGT||pulse height of event, corrected for ADC non-linearity, gain saturation, and temporal variations|
|ANGG||AMPL||pulse height of event, corrected for ADC non-linearity, gain saturation, temporal and spatial variations (PI channel)|
|IX_EV||RAWX||raw detector x coordinate (integer number in [1,8192])|
|IY_EV||RAWY||raw detector y coordinate (integer number in [1,8192])|
|XE, YE||detector coordinates (electronic position correction applied)|
|XW, YW||detector coordinates (electronic position correction and window correction applied)|
|XC, YC||XDET, YDET||detector coordinates (electronic position correction, window correction and correction for XRT field distortion applied)|
|FAK||PHA channel of the Al K line (BAL ) (from Prescott fit)|
|GAIN||correction factor for temporal gain variations (close to 1; gain saturation is taken into account)|
|r||RAN||random number (real number in [0.0,1.0])|
|SASS convention||short description|
|x'||corresponds to a simple transformation of x (`modified x')|
The raw event is actually triggered by an incoming X-ray photon scattered off the XRT mirror is passing through the PSPC window framed by the support structure. This process is described by the effective area of the XRT and the window transmission of the PSPC. Then the photon is absorbed by the counter gas of the PSPC\ (cf. Fig. 3.1) and yields a (primary) electron which is thermalized and causes secondary ionization of the counter gas. The electron cloud drifts through the K1 cathode grid (25 strips) towards the A1 anode grid. When close enough to an anode wire the charge cloud can be amplified by a (gain) factor of because the electric field strength becomes sufficiently high. This avalanche of charge onto the anode leads to a charge pulse at the anode A1 and an induced signal is detected at the cathode K1 and K2 which are aligned perpendicular to each other to determine the position of the photon inside the detector. Typically two to five wires are excited by a signal which is digitized by means of pulse shapers, peak detectors, and ADCs. After validity checking the event position is calculated by a centre of gravity determination. The signal caused by the photon inside the detector is described by the quantum efficiency of the counter gas and the gain, which depends on the gas state and the HV of the anode.
SASS processing stores the raw event information in the file TRGEV.TEL and module CT converts this into the files CTEV.TEL (for target events), CCEV.TEL (for calibration events), NOSEV.TEL (for non-scientific events like filter-wheel-closed position), RDAT5.TEL (for screened events due to housekeeping status).
The following sections summarize comprehensively the present state of the correction, whose individual steps are listed in Table F.1. Parameters and conventions often used in these sections are collected for a better overview in Table F.2.