Chandra absolute astrometric accuracy
SummaryThis page gives the latest information the absolute positional accuracy of source coordinates in Chandra observations. This is based on measuring the distances between the Chandra X-ray source positions and corresponding optical/radio counterpart positions from two accurate catalogs ( Tycho2 or ICRS).
Distribution of aspect offsets for each detectorUsing data that have been processed (or reprocessed) with the alignment files released in 2007-May, we have determined aspect offsets for numerous observations with X-ray sources having well-determined radio or optical/IR counterparts.
The four plots below show the distribution of aspect offsets for observations from launch through 2003 to the present. The aspect offset is simply the distance from X-ray source centroid to the accurately known position of the optical or radio counterpart. This gives a good estimate of astrometric accuracy for observations without well-known counterpart positions.
Off-axis sources or observations with non-zero SIM offset
It is important to note that observations on ACIS or HRC-S at large off-nominal SIM-Z can suffer additional residual aspect offsets of up to 0.5 arcsec. This is due primarily to distortion of fid light images by the retroreflector collimator in the fiducial transfer system. HRC-I observations at large off-nominal SIM-Z can have residual aspect offsets of up to 3 arcsec due to the fid light distortion as well as a possible rotation of the detector relative to the geometry assumed in processing.
Sources which are at HRMA off-axis angles greater than 3 arcmin have additional offsets due to PSF blurring and asymmetry. A quick rule of thumb in this case is that the uncertainty in absolute astrometry is roughly 1/4 of the 50% encircled energy radius at the corresponding off-axis angle. This is based on the data tables available in the memo Plate Scale and Relative Chip Postions. A more accurate determination of uncertainties could be made using raytrace simulations with MARX or ChaRT
Time history of aspect offsets
The time-history of offsets for recent Chandra observations is shown below. There are no significant time-dependent residuals. For old archival data it should be noted that ACIS observations taken in 1999, and particularly before ~November 1999 could suffer larger offsets. Shortly after launch the alignment drifted rapidly due to outgassing of the ACIS mount tabs, and the calibration does not account for this effect.
Explanation of plotsThe above plots show the offsets between Chandra X-ray source coordinates and the corresponding optical or radio coordinates. The offsets are derived as follows:
For every "qualifying" observation, the positional accuracy monitor cross-correlates detected X-ray sources with several accurate optical/radio catalogs. For grating observations, the central 0.4 arcmin is used, while for imaging observations sources in the central 4 arcmin are considered. Currently a qualifying observation has:
How is the offset defined?The offsets (DY and DZ) 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. The exact equations are as follows:
# Define delta RA and dec in arcseconds delta_RA = (RA_catalog - RA_Xray) * cos(dec_catalog) * ARCSEC_PER_DEGREE delta_dec = (dec_catalog - dec_Xray) * ARCSEC_PER_DEGREE cr = cos(ROLL_NOM) # Nominal ROLL taken from observation header sr = sin(ROLL_NOM) DZ = cr * delta_dec - sr * delta_RA; DY = sr * delta_dec + cr * delta_RA;
What are the catalogs?
Archival notes and announcements
Note about aspect offset in early 2007: (May-2007)
Announcement of updated alignments (May-2002):
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