pcsasscor -- Corrects PI channel in Events file
pcsasscor infile outfile
This tool operates on Rev0, or RDF PSPC events file. The tool also operates on any German PSPC events file converted to RDF format using the 'gevt2rdf' ftool. The PI column in all events files are currently incorrect, in the sense that there is an error in all versions of the SASS spatial gain correction.
This tool recalculates the PI column from the PHA and position information, a new events file is written out with the same format as the original, but with the PI column replaced by the new corrected values of PI. This tool allows correction of the events files without the need for reprocessing of the data with SASS.
(Later versions of this program will eventually correct the PI data for temporal deviations).
EVENT CORRECTION :
A detailed description of the calibration corrections applied to individual PSPC events is available in the HEASARC memo
/caldb/docs/rosat/cal_ros_95_010/cal_ros_95_010.tex
available in latex and post script format from the anonymous ftp account at heasarc.gsfc.nasa.gov
The aforementioned memo is essentially a copy of the original MPE memo TN-ROS-ME-ZA00/027 (Hasinger \& Snowden, 1990), but updated and expanded to refer to the FITS versions of the calibration files available from the HEASARC and to detail the corrected algorithms as performed by PCSASSCOR.
In summary, the calibration corrections performed on PSPC events in the SASS processing include corrections for electronic variations (detailed characteristics of the anode amplifier chains), electric-field variations due to the (slight) variable spacing of the anode wires, and long-term temporal gain variation. The corrections are performed using event arrival position, time, and pulse height. The PHA to PI corrections are performed in SASS within the CT module.
THE ORIGIN OF THE ERROR :
Detailed analysis of the PSPC in-flight aluminum K (1487 eV) calibration-source data showed that the corrections, as they stand in SASS, have an error in the spatial gain correction. The error produces spurious fluctuations in PI channels across the image. For more detail on the problem, see
http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/summary/ cal_ros_95_003_summary.html
The spatial gain correction should remove the gain variations due to the variable distance of the anode wires and the relative position of the events with respect to the anode wires. The SASS DCORG subroutine applies the spatial gain correction to the events data. That subroutine uses corrections based on ground calibration data from the gain_kor3_b SASS calibration file. An error in the implementation of the original ground calibration files within SASS has lead to a mis-correction which produces spurious variations in PI channel across the detector. To correct this error, PCSASSCOR recalculates the PI column from the PHA column, using the Y-based spatial correction instead of the X-based correction.
The electronically corrected coordinate information is required to convert PHA channels to PI. As the electronically corrected coordinates are an intermediate coordinate system, they are not stored in the Rev0 or RDF events file. This version of PCSASSCOR calculates electronically corrected positions starting from DETX and DETY and undoing the field and window corrections. In the RDF case the RAWX and RAWY coordinates are available, and one can avoid the minor approximations that undoing the field and window correction entails, by simply applying the electronic position correction to RAWY. This enhancement will be available in the next release of this tool.
THE CALIBRATION FILES REQUIRED FOR THE TASK
The default setting for the input calibration files is 'CALDB', this facility can only be used by beta-test sites of the calibration database access software. Most users will need to obtain a set of calibration files via anonymous ftp from heasarc.gsfc.nasa.gov. The datasets are located in the directory:
/caldb/data/rosat/pspc/bcf/sass_ct
and have the names/descriptions as follows:
Some files are valid for both PSPC instruments, and hence have 'PSPC' as the instrument keyword.
Note: Correct use of this task requires that the ymapfile used should be either taby_093_b.fits or taby_093_c.fits, use of the tabx files will result in a reproduction of the error currently occurring in SASS.
EXAMPLE:
An example of the pcsasscor parameter file might be:
REMINDER:
If your observation was made before Jan 25 1991 then you should use PSPCC calibration files, after that date, use PSPCB calibration files (the observation date is stored as the DATE-OBS keyword.
Note: Correct use of this task requires that the ymapfile used should be either taby_093_b.fits or taby_093_c.fits, use of the tabx files will result in a reproduction of the error currently occurring in SASS.
Currently only the following OGIP standards are supported by this task
For further details see OGIP calibration memo CAL/ROS/95-010
None known
Rehana Yusaf
HEASARC
NASA/GFSC
http://heasarc.gsfc.nasa.gov/cgi-bin/ftoolshelp
(301) 286-6115