Absolute astrometric accuracy through October 2000

We have done a new analysis of the absolute astrometric accuracy of X-ray sources in Chandra observations.  This includes sources for all four science instruments, processed using the current aspect pipeline and alignment calibration files. Observations for this analysis were required to have an X-ray source with off-axis angle less than 3.5 arcmin, detected at > 8 sigma, with an optical counterpart in the Hipparcos/Tycho , USNO, or ICRS catalogs. A specialized processing script was developed to automatically retrieve data from the Chandra archive, reprocess the aspect if necessary, and derive the X-ray source aspect offset.

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.


The "current" aspect offsets versus time for each science instrument are shown in the plots below. This applies to data processed with CXCDS software versions R4CU5UPD5.2 through R4CU5UPD11.1, inclusive. The FITS header keyword ASCDSVER in the event file specifies the processing version. In each plot, the red triangles show the Y offset and the blue squares show the Z offset, in arcseconds. These offsets are specified in the Chandra body coordinate system, since they reflect drifts in hardware alignments. For orientiation, at a spacecraft roll angle of 0.0 degrees, DY corresponds to Right Ascension, and DZ corresponds to Declination:

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.

Last modified: Wed Oct 25 15:25:39 EDT 2000

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