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, which can be run optionally by the user (it is not used by default in omichain), omatt employs the USNO SA1 catalogue (it has to be provided by the user). The OM pointing vector and field of view are used as parameters to a search of the USNO, 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).