# 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.