SUM

        SUM

    This  program is used to sum many images, handling a weight image in or-
    der to produce the average image later on.
              It computes     X(i,j) = X(i,j) + F * Z(i,j)
              and             Y(i,j) = Y(i,j) + F
    for  all  non  blanked  pixels  of  the  input  image  Z.   When  called
    repetitively  with  the same input/output images X and Y, X contains the
    weighted sum of all Z images, and  Y  the  weight  of  each  pixels.   A
    parameter is provided to initialise the process.

    The last step to obtain the average image is to divide X by Y using pro-
    gram COMBINE with option DIVIDE.  You can also use SIC command LET to do
    so, but SUM and COMBINE will handle properly blanking values.