The task omatt takes an OM OSW source list, with source positions in pixels, and converts them into sky coordinates. In addition, the whole image is corrected for geometric detector distortion and rotated so as to become north-aligned. The task requires the user to enter the name of an OSW source list and its corresponding OSW image. The positional accuracy obtained in this way is of the order of 2 arcsec, this error being due in part to the residuals of the geometric distortion correction.
In its second part, omatt can use the USNO SA1 catalogue to compute and apply an astrometric correction. An excerpt from the catalogue for the field of interest can be provided by the user. In SAS v13.0, omichain can produce this catalogue file if the proper configuration for remote access has been set (see SAS on-line documentation). The OM pointing vector and field of view are used as parameters to a search of the USNO file, which lists all catalogue stars in the field of view. Two synthetic images are constructed from the source list and the catalogue stars. A two-dimensional cross-correlation is performed on the images, and this 2D cross-correlation function is searched for its maximum, which gives x and y offsets in pixels between the OSW source list and the catalogue. This offset is used to correct the catalogue tangent plane coordinates. The nearest catalogue star to each source in the source list is then found. If this distance is smaller than a tolerance times the positional error, then it is assumed to be that catalogue star. If sufficient catalogue stars are identified, then a least-squares fit is performed to the catalogue positions, to yield more accurate astrometry. The source positions in RA/Dec are then written into the source list, along with the positional error (the pixel coordinates are retained in the source list). This correction is also applied to the sky north aligned images.
The task omsrclistcomb can also perform an astrometric correction similar to omatt. Now, the whole combined list is used. This allows us to obtain such a correction for sources detected in images with few sources (e.g. obtained with UV filters) where a solution cannot be found by omatt.