The Neil Gehrels Swift Observatory

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Astrometry

The total astrometric error in the coordinates of a source in a UVOT SKY image can be obtained by taking the quadratic sum of the systematic error in the position and the statistical error in the position. The UVOT task uvotdetect returns statistical errors in the RA_ERR and DEC_ERR columns of its output file. These position errors are defined in Section 9.1.8 of the SExtractor User's Manual. Note that uvotdetect returns positional errors at the 1-sigma level. If we assume Gaussian errors then the values in the RA_ERR and DEC_ERR columns can be converted to 90% confidence intervals using

σ90 = 1.64485 × σ

where σ90 is the 90% confidence radius error and σ is the Gaussian 1-sigma error. Please note that σ is the quadratic sum of the 1-sigma errors in RA and Dec as given in the RA_ERR and DEC_ERR columns: σ = √( RA_ERR2 + DEC_ERR2 ).

The systematic error in astrometry performed on aspect-corrected UVOT SKY images is 0.42 arcsec (Breeveld et al. 2010, submitted). This is the 90% confidence radius. The Gaussian 1-sigma systematic error is 0.26 arcsec.

If the statistical error in the position of a source is not available (for example, when performing photometry using uvotsource) then the 90% confidence radius can be estimated using the raw count rate from the source using using

σ90 = √( 0.422 + (3 Cerr / C)2 )

where &sigma90 is the 90% confidence radius error in the astrometric position, Cerr is the 1-sigma error in the raw count rate from the source, and C is the raw count rate from the source. The raw count rate and raw count rate error values are output to the screen when uvotsource is run using the "output=ALL" or "output=RAW" command line options.

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