imcombine: 

If you have more than 2 shifted images, then median. If 2, then average.
The names are just suggestions.   If you have more than 3 images, create
an "@" list or use your output list for the imalign task.

        input = "sht001.imh,sh002.imh,sh003.imh," List of images to combine
       output = "median_scale.imh" List of output images
      (plfile = "")             List of output pixel list files (optional)
       (sigma = "")             List of sigma images (optional)\n
     (ccdtype = "")             CCD image type to combine (optional)
     (subsets = no)             Combine images by subset parameter?
      (delete = no)             Delete input images after combining?
     (clobber = no)             Clobber existing output image?\n
     (combine = "median")      Type of combine operation
      (reject = "none")         Type of rejection
     (project = no)             Project highest dimension of input images?
     (outtype = "real")         Output image pixel datatype
     (offsets = "none")         Input image offsets
    (masktype = "none")         Mask type
   (maskvalue = 0.)             Mask value
       (blank = 0.)             Value if there are no pixels\n
       (scale = "mode")         Image scaling
        (zero = "none")         Image zero point offset
      (weight = "none")         Image weights
     (statsec = "[210:250,80:120]") Image section for computing statistics\n
  (lthreshold = INDEF)          Lower threshold
  (hthreshold = INDEF)          Upper threshold
        (nlow = 1)              minmax: Number of low pixels to reject
       (nhigh = 1)              minmax: Number of high pixels to reject
       (nkeep = 1)              Minimum to keep (pos) or maximum to reject (neg
       (mclip = yes)            Use median in sigma clipping algorithms?
      (lsigma = 3.)             Lower sigma clipping factor
      (hsigma = 3.)             Upper sigma clipping factor
     (rdnoise = "0.")           ccdclip: CCD readout noise (electrons)
        (gain = "1.")           ccdclip: CCD gain (electrons/DN)
      (snoise = "0.")           ccdclip: Sensitivity noise (fraction)
    (sigscale = 0.1)            Tolerance for sigma clipping scaling correction
       (pclip = -0.5)           pclip: Percentile clipping parameter
        (grow = 0)              Radius (pixels) for 1D neighbor rejection
        (mode = "ql")