acf(1) --- Imcat Users Guide --- section: imtools --- last changed: Thu Jul 10 HST
acf - calculates 2-D autocorrelation function of a fits imageSYNOPSIS
acf [options...]DESCRIPTION
'acf' reads a fits file from stdin and writes the autocorrelation function to stdout. If the input image is fin(r) then we compute fout(r) = sum_r' fin(r') fin(r' + r) / (N1 * N2) and resulting image is wrapped so that zero lag is at pixel (N1/2, N2/2).
Options are:"-p " calculate power spectrum instead
"-c f1 f2 " cross correlate named fits files
"-P pixtype " output pixtype (FLOAT_PIXTYPE)
"-n " no MAGIC substitution
"-m magicval " replacement for MAGIC pixels (0)Power is defined so that white noise with variance sigma2 will produce P = sigma2. Power is translated so that zero frequency lies at N1/2, N2/2. With '-c fits0 fits1' we compute c(r) = sum_r' f0(r') f1(r' + r) / (N1 * N2) to compute instead c(r) = sum_r' f0(r') f1(r - r') / (N1 * N2) you should first rotate the first image by 180 degrees. Supply file name '-' to read from stdin. Default output image format is BITPIX = -32 (i.e. 4-byte float). AUTHOR
Nick Kaiser: kaiser@hawaii.edu