What photons are included in the 'good photon' file?
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 events data are screened against the aspect error in the attitude file. The aspect error has been lowered successively from 120 arcsec to 2 arcsec (SASS5_4_8, 20-DEC-1992) and 1 arcsec (SASS5_6, 17-MAR-1992) for the PSPC and to 1 arcsec (SASS5_5_2, 10-MAR-1992) for the HRI. As long as the formal aspect error is below 1 arcsec, the GSOC attitude solution is reliable within about 10 arcsec (see above). For aspect errors in the range 1-10 arcsec the situation is not unique because the star sensor usually is tracking only one star, which cannot be identified unambiguously. We urge the user to assure himself of the integrity of the data by inspecting the attitude error entries in the attitude file.
Users analyzing data summed up from observations taken several months apart should be aware that the boresighting and irreproducible errors (see the PSPC FAQs) may produce position differences of up to 20 arcsec between the observations. These differences can be corrected by first analyzing the datasets separately and then shifting them to a common reference frame using the detected X-ray sources.
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