The program wenssabp1 is the main part of the reduction batch. Each individual field in the mosaic (e.g. 80 in a 90 cm mosaic at declination 37 degrees) is reduced separately, just like a normal 12 hour synthesis observation. First, model lists are generated for each field, which will be used in the self-calibration process (the selfcal loop is the major cycle in the reduction procedure). The program will find peaks in the map down to a given search level, usually 6 W.U. (1 W.U. is 5 mJy) and gives values for peakflux and position. These models are subtracted from the map and a half resolution map is made to check if everything went satisfactory. This procedure can only be used for point sources and slightly resolved sources. Because selfcal is an iterative process several ``updates'' have to be made in order to get a stable modellist which is still in agreement with the data. If there are extended sources in the field they will have to be reduced with a CLEAN algorithm. The CLEAN process also generates a model list, which will be combined later with the model list obtained from the selfcal process.
The selfcal process introduces intrinsic position errors of all sources.
Because the algorithm is correcting for wavefront distortions due to the
irregular atmosphere (i.e. shifting the phase in the plane), the result
will always be a shift in the map (i.e. the Fourier Transform of the
plane). The shift is a general shift over the entire map, so relative positions
remain the same. In order to find the exact position shift needed a list with
known position calibrators is used (Patnaik et al., 1992).
The fields which contain these position calibrators can be restored directly. Other fields which do not contain these sources can still be restored, because the neighboring fields overlap strongly. Positions of point sources in these overlapping regions are compared and used in a bootstrap method which tie all the remaining fields together. The list of all position shifts is used in the last part of the reduction batch (wenssabp2). This program is used to make the last clean map of each individual field. The final step in the reduction is making one large mosaic map which is made out of all the individual maps. With calculating and proper weighting the primary beam attenuation of each map position relative to each pointing centre a map is made that is typically 10 by 14 degrees large. This map should have a uniform sensitivity over the entire field. Only at the edges they show effects of noise, because there is no data available of all neighboring fields. This data only becomes available when the mosaic next to the just reduced mosaic is finished. The mosaics are not the final products, but were only formed such that an efficient observation schedule could be made. Therefore the last part of the map making process consists of cutting out frames from these mosaic maps, that are 6 by 6 degrees large and lie on a 5 degree grid centred on the new Palomar Sky Survey field centres.
When the data reduction is completed, the final frames are converted to FITS format and stored on DAT tapes. These tapes are then transported to Leiden in order to build the catalogue.