As a result of this analysis, new alignment files have been prepared which apply to data taken after 01-Dec-1999. For HRC-S and HRC-I, the corrections are up to 2 arcsec and 1.5 arcsec, respectively. For ACIS, the corrections are less than 1 arcsec.
With these alignments in place, the RMS aspect offset (radial) is better than 0.6 arcsec, compared to the mission requirement of 1.0 arcsec.
DY = (RA_true - RA_Xray) * cos(Dec)
DZ = DEC_true - DEC_Xray
At a roll angle of 90.0 degrees, DY corresponds to Declination and -DZ corresponds to Right Ascension.
The predicted aspect offsets versus time for each science instrument are shown in the plots below. This corresponds to the expected offsets after reprocessing, using the latest available alignment files to correct for the observed long-term drift seen in the plots above.
The predicted radial offsets, DR = sqrt(DY^2+DZ^2), versus time are show below. The top plot shows all currently available offsets, which includes many sources with optical counterparts found in the USNO catalog. The coordinate accuracy is somewhat lower, and in some cases there is less confidence in the X-ray - optical identification. The RMS value of DR = 0.6 arcsec for this dataset. This is to be compared with the mission requirement of 1.0 arcsec. The lower plot shows only those sources with Hipparcos or ICRS coordinates. In this case the RMS value of DR is 0.4 arcsec, excluding the one obvious outlier. If this point is included, the RMS value of DR is 0.6 arcsec.
COMMENTS or QUESTIONS: Tom Aldcroft
Last modified: Wed Oct 25 15:25:39 EDT 2000
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