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#### A.4.5.10 SkyCorr Form

The SkyCorr () form plots an error ellipse (or rectangle or other similar figure) on the sky defined by errors in the longitude and latitude directions, and a correlation between the two errors.

The error in longitude is considered to be premultiplied by the cosine of the latitude, i.e. both errors correspond to angular distances along a great circle.

The supplied correlation is a dimensionless value in the range -1..+1 and is equal to the covariance divided by the product of the lon and lat errors. The covariance matrix is thus:

       [  lonerr*lonerr       lonerr*laterr*corr  ]
       [  lonerr*laterr*corr  laterr*laterr       ]

This plot type is suitable for use with the `ra_error`, `dec_error` and `ra_dec_corr` columns in the Gaia source catalogue. Note however that Gaia positional errors are generally quoted in milli-arseconds (mas), while this plotter requires lon/lat errors in degrees, so to plot true error ellipses it is necessary to divide the Gaia error values by 3.6e6. In most plots, Gaia position errors are much too tiny to see!

Example SkyCorr plot

SkyCorr form configuration panel

The configuration options are:

Shading mode
  See Appendix A.4.6.
Longitude error
  Error in longitude in degrees, multiplied by the cosine of the latitude.
Latitude error
  Error in latitude in degrees.
Lon-Lat correlation
  Correlation between the errors in longitude and latitude. This is a dimensionless quantity in the range -1..+1, and is equivalent to the covariance divided by the product of the Longitude and Latitude error values themselves. It corresponds to the `ra_dec_corr` value supplied in the Gaia source catalogue.
Ellipse
  Ellipse graphical representation, selected from a range of options (includes also rectangles, crosses etc).

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk