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Last modified: 13 December 2012

URL: http://cxc.harvard.edu/csc/columns/positions.html

Source Position and Position Errors


Source positions and positional uncertainties in the Master Sources Table are computed by statistically averaging observations of the same source using a multivariate optimal weighting formalism. In the Source Observations Table, source positions and uncertainties are determined from the wavdetect algorithm.


Equatorial Coordinates

Master Sources Table
ra, dec, err_ellipse_r0, err_ellipse_r1, err_ellipse_ang

Source positions in the Master Sources Table are the best estimate of the ICRS celestial position of the source at the epoch of observation. For release 1, the positions are derived directly from the wavelet source detection algorithm (wavdetect) with no further modelling. The ICRS equatorial coordinates of each source (ra, dec) are determined with an absolute uncertainty that is not to exceed 1.0 arcsecond (1σ) for an isolated point source with at least 10 counts located within 3.0 arcminutes of the optical axis; 2.0 arcseconds (1σ) for an isolated point source with at least 30 counts located within 10.0 arcminutes of the optical axis; and 5.0 arcseconds (1σ) for an isolated point source with at least 50 counts located within 15.0 arcminutes of the optical axis.

The uncertainties of the statistically averaged source positions are expressed in the form of error ellipses centered upon the source positions, projected from the celestial sphere onto a common tangent plane; calculation of the position errors are described in the How and Why topic 'Position Errors in the Master Chandra Source Table'. Three of the parameters specifying the geometry of each error ellipse are the semi-major and semi-minor radii (err_ellipse_r0, err_ellipse_r1), and the position angle θ that the major axis of the ellipse makes with respect to the tangent plane y axis (err_ellipse_ang). The semi-major and semi-minor radii correspond to the 95% confidence intervals along these axes.

Source Observations Table:
ra, dec, err_ellipse_r0, err_ellipse_r1, err_ellipse_ang

The position of each source in an individual observation is defined by the ICRS right ascension and declination of the center of the source region in which it is located, determined by wavdetect, and the major and minor radii, and position angle of the error ellipse defining the uncertainty in the source position.

Notes: The 95% confidence level error ellipse of the source position will be approximated by a 95% confidence level error circle in the first catalog release. This limitation will be lifted in a future release.

In the first catalog release, the 95% confidence level error ellipse is defined on a tangent plane projection. The 0 deg position angle reference is defined on that tangent plane to be parallel to the true North direction at the location of the tangent plane reference (refer to the tangent plane reference right ascension (ra_nom), declination (dec_nom), and roll angle (roll_nom)).

Off-Axis Angles

Source Observations Table:
theta, phi

The angular location of the source region (that includes a source) relative to the optical axis is defined by the off-axis angle θ and azimuthal angle φ.

Mean Chip Coordinates

Source Observations Table:
chipx, chipy

The location of the source region (that includes a source) in chip coordinates is defined by the effective CHIPX and CHIPY pixel positions corresponding to the off-axis angles (θ, φ).

Note that the chipx and chipy values are computed without including the effect of the mean dy, dz, dtheta offsets from the aspect solution. For many observations this won't have much impact, but as the mission has progressed the offsets have become significant and now the offsets are of order 15 arcsec range (~30 ACIS pixels) in both the dy and dz directions. The mean chip coordinates for some observations will be uncertain by this amount.


Last modified: 13 December 2012
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