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Last modified: 23 August 2018

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 multi-variate optimal weighting formalism. In the Stacked Observation Detections Table, source positions and uncertainties are determined from the wavdetect algorithm and adjusted by the MLE fit.


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. The positions are derived from the resuts of the wavelet source detection algorithm (wavdetect) and adjusted by the maximum likelihood estimator fit. 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 statistically averaged source position uncertainties 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.

Stacked Observation Detections Table:
ra, dec, err_ellipse_r0, err_ellipse_r1, err_ellipse_ang

The position of each detection in an observation stack is defined by the ICRS right ascension and declination of the center of the source region in which it is located, determined by the MLE-adjusted wavdetect result, and the major and minor radii, and position angle of the error ellipse, parameterized by the MLE's Markov chain Monte Carlo draws, defining the uncertainty in the source position.

[NOTE]
Note

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.

[NOTE]
Note

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

Per-Observation Detections 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.


Stacked Observation Detections Table:
theta_mean

The mean source region aperture off-axis angle, θmean, from all observations in a stack.


Chip Coordinates

Per-Observation Detections Table:
chipx, chipy

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

The effect of the mean dy, dz, dtheta offsets from the aspect solution are included to determine the source's chip coordinates in CSC2. While this does not have a large impact on many observations in the archive, for more recent observations, 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, and need to be accounted for.


Last modified: 23 August 2018
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