SQRT replaces each pixel in 'source' by the square root of the pixel value. If there are negative pixels, the square root of the absolute value is taken, unless the NOABS keyword is given, in which case negative pixels are set to zero, or unless the words SGN or SIGN are included. If either if these words are included on the command line, the square root of the absolute value is computed and given a negative sign (example: -4 turns into -2).