Checks on Astrometry

The internal astrometric accuracy (between the stacked images themselves) is typically 0.04 arcseconds radially or 0.025 in any one direction. The external astrometric accuracy (between the stacks and the external reference frame) is typically 0.20 arcseconds radially or 0.14 in any one direction.

Internal Accuracy

The internal accuracy is checked by running SExtractor each stacked image in every band and obtaining catalogues of object positions. The positional catalogues for each band are matched to each other and common object identified. If the astrometry is perfect, then the position of an object in each band will be identical. In practice, there are astrometric residuals. Examining these residuals gives an idea of the astrometric uncertainties.

The figure at right shows checks on the internal astrometry between the D2.G and D2.R images.

The top left quarter shows the direction and size (greatly enlarged) of the astrometric residuals as line segments. This plot is an important diagnostic of astrometry because, while the residuals are typically quite small, there are outliers in any distribution. As long as these outliers are relatively isolated from each other and pointing in random directions all is well. Conversely, if there are number large residuals in close proximity to each other, all pointing the same direction, this indicates a systematic misalignment between the two images in question. The figure shows no such misalignments.

The bottom left quarter shows the astrometric residuals in RA and Dec. The red histograms show the relative distribution on the residuals in both directions. The 68%-tile of the residuals is 0.060 arcseconds radially.

The two right panels show the residuals in RA and Dec separately.

Over all, the astrometric residuals are about 0.04 arcseconds (68%-tile) in a radial sense or about 0.025 in any one direction. Note that there should be a factor of sqrt(2) between these two uncertainties.

External Accuracy

This is checked by matching the I-band catalogue for each field back to the astrometric reference catalogue. Again, the scatter in the astrometric residuals is a measure of the uncertainty and the presence or absence of any localized large residuals indicates a systematic shift.

The figure at right shows checks on the external astrometry. The panels have the same significance as above. The only difference is that the residuals are larger.

Back to Stephen Gwyn's CFHTLS pages
Send comments/suggestions/problems to:
Stephen.Gwyn@nrc.ca