The aspect solution roll accuracy for Chandra data is approximately 25 arcsec. At this level the impact for X-ray observations is essentially negligible since it corresponds to a spatial offset of about 0.02 arcsec for a source at 3 arcmin off-axis angle. Given the combination of increasing PSF size at large off-axis angles and centroiding limitations due to photon statistics, aspect roll uncertainty is not a significant factor in Chandra data analysis. When merging observations it is not typically necessary to fit for roll offsets in the different datasets.
The roll uncertainty value of 25 arcsec is simply estimated as follows.
The positional offset for a guide star is typically less than
about 0.5" and the typical radial distance is around 30 arcmin.
Translating to a roll error (and including a factor of 1/sqrt(5) for
the 5 guide stars) gives sigma_roll ~ 25 arcsec. The factor of 10 increase
in the ACA field of view relative to HRC or ACIS is primarly what makes
the roll uncertainty be an unimportant error term.
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Last modified: 12/20/13
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