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ROSAT Guest Observer Facility
NAMEInstrument AffectedDescriptionSASS version
Aspect Reconstruction Error PSPC and HRI It has been found that in the Standard Processing, some of the data delivered to the observers may still contain time intervals during which the applied attitude solution shows unacceptably large errors. The aspect error in the attitude file, which is screened against was lowered to 1'' (SASS5_6, 1992 March 17) for the PSPC and to 1'' (SASS5_5_2, 1992 March 10) for the HRI.
Exposure map Error PSPC There are a few pointed observations with aspect errors less than 1'' but where the aspect solution drifts away from the nominal position by an appreciable amount. This has the consequence that an unfavourable mean aspect is calculated and not all aspect entries are projected into the aspect histogram and thus these times will not be counted in the exposure map. The situation has been remedied by implementing a new screening technique, accepting only aspects that are within limits from the nominal position (SASS5_9, 1992 August 20)
Roll angle error PSPC and HRI There is a systematic error in the roll angle with respect to the astronomical coordinate frame of the PSPC and HRI. The error is small, being for the PSPC. This corresponds to a position error of 6'' at the radius of the support ring. SASS 6
HV problem PSPC and HRI High Voltage (HV) checks are done by checking the HV ON/OFF flag, and checking whether the HV value itself is nominal. This information is stored in housekeeping frames in a rather obscure way. SASS was checking the wrong HV field. Consequently the HV value indeed remained unchecked and data intervals appeared in the accepted time intervals where the HV was turned on, but the HV had not yet reached its nominal value. As the gain of the PSPC is a strong function of HV, this leads to wrong pulse height information for the affected times. Typically the time between HV getting turned ON and the HV reaching its nominal value is about 18s. This effect thus will be of no great impact for most observations, but it will affect observations with HV ON and OFF occurrences and short observation intervals. This effect may be seen in the quality file and in the housekeeping plots. Every time when HV is switched ON a time span of 20s should be cut out of the accepted times. This bug was fixed in SASS release SASS5_7_7 (1992 June 15).
Temporal Gain Variation PSPC There is some temporal variation of the PSPC gain which is not included in the gain correction process currently applied to data in SASS. This effect showed up primarily as inconsistencies between calibration spectra of N132D taken at different epochs across the ROSAT mission. Another artifact of the problem is highly significant residuals in number of spectra compared to the expected model, these appear as large negative residuals below about 0.20 keV and large positive residuals between about 0.20 to 0.40 keV. See "Stability of the PSPC Spectral Calibration" by J. Turner None implemented in SASS; off-line corrections can be made using EXSAS or FTOOLS
PSPC TIMING PSPC A bug was found in the timing of individual events and was due to the preprocessing software. The origin of the problem was the spacecraft clock reset which followed the spacecraft tumbling incident of 1991 January 25. All PSPC data after that time will be affected (this problem occurs after each clock reset, which is done annually to accommodate roll-over of the counter). The problem leads to relative shifts of 1 s between adjacent PSPC events. Although the problem is not cumulative, throughout the data there may be relative time shifts of the order of 1 second. The effect of the problem is to invalidate some timing analyses which are done on seconds or subsecond time scales. The bug has negligible impact on spectral and spatial analyses. The bug was corrected in SASS5_7 (1992 April 22)
HRI TIMING (1) HRI The space-craft clock onboard the ROSAT satellite has undergone several resets during the first two years of the mission. There has been two reasons for a space-clock reset:
  1. The satellite is lost or enters safe mode.
  2. Annual reset due to an overflow in the on-board counter.
Until recently the processing of these data has introduced an fractional second offset in times assigned to the photons.
All HRI data processed at GSFC after 1992 October 22 (when HRI_4_5_2 was installed in the on-line processing) are free of timing problems.
The following are not documented in the ROSAT Users Handbook:
Spatial Gain Mis-correction PSPC The spatial gain correction is applied to PSPC photons by the SASS subroutine DCORG. Its purpose is to correct the detected photon energy bins for variations caused by differences in the separation of the PSPC anode wires. This correction depends on the electronically-corrected Y coordinate of the detected photons. In early versions SASS had been applying the spatial-gain correction based upon the electronically-corrected X coordinate, which introduced spurious variations in the PI channel assigned to each photon. See the discussion of the spatial gain correction in cal_ros_95_010 for more information. The subroutine DCORG was corrected to use the electronically-corrected Y coordinate in PSPC7_4, which was implemented in SASS7_8
HRI FBKMAP error HRI A bug was found in the code which produced the background map FBKMAP. The FBKMAP is used to determine source positions, intensities and variabilities. Fixed as of SASS 7_9_4 (RDF 3_9) (ALPHA VMS only)
Boresight problem PSPC and HRI Incorrect boresights were used for all ROSAT observations processed at GSFC after 4 October, 1997 and before May 8, 1998. Data processed with the incorrect boresight values were identified and reprocessed; see ROSAT Status Report #172
HRI ASPECT TIME HRI A bug occurs because the fractional time (an integer) assigned to the aspect time was divided by 8192 to get decimal seconds to add to whole integer seconds, whereas it should have been divided by 64. This bug was discovered in the fall of 1998. It results in some photons being assigned to an incorrect aspect interval which causes a slight smearing of the resulting point spread function. More information can be found at http://hea-www.harvard.edu/rosat/aspfix.html SASS 7_B


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Page author: Michael F. Corcoran
Last updated: Monday, 25-Jun-2012 15:16:20 EDT