Grayscale Color Tables.The "-gray" directive described in the previous section defaults to a simple linear black-to-white color table (color table 0 below). More generally, there is a set of pseudocolor tables to choose from using the "-ct <number>" directive (e.g., "mViewer -ct 4 -gray SDSS_g.fits -0.5 max gaussian-log -out m51.png"):