Focas: faint object classification and analysis system.

The FOCAS (Faint Object Classification and Analysis System) apparatus for the detection and classification of images on astronomical plates which is intended to be used for the automatic assembly of catalogs of faint objects, is presented. The system has been used to process plates obtained by the KPNO 40-m telescope under good seeing conditions for 12 selected areas, which were digitized in 20-micron steps with a 20-sq micron pixel in a 6000 x 6000 raster corresponding to a field 37 arcmin on a side. The FOCAS acts to reduce the digitized image to a list including the magnitude, position and classification data for objects in the field as faint as the 24th magnitude, by a process including the segmentation of objects, object area assembly, and object assembly. Objects are then classified as noise, stars or galaxies on the basis of conventional nonparametric statistical pattern recognition techniques, and catalogs of objects meeting given criteria can be obtained by various programs. System tests have shown detection efficiencies of 97% and classification capabilities for 90 and 80% of the galaxies and stars, respectively, for apparent magnitudes down to 24, although the optimality and accuracy of the classifier have not been established. FOCAS has been used to assemble catalog of 86,000 objects which is 3 to 4 mag deeper than existing surveys.