NAME:
    jitter
 PURPOSE: (one line)
    Generate a jitter convolution kernel for one WFPC2 observation.
 DESCRIPTION:
   Given an observation, read its associated jitter file and create
     a convolution kernel that matches the tracking/jitter history
     of the object.  This is setup for either a sidereal object or
     planetary object.
 CATEGORY:
   CCD data processing
 CALLING SEQUENCE:
    jitter, visitid, imgid
 INPUTS:

 OPTIONAL INPUT PARAMETERS:
 visitid      : If specified, the images corresponding to the visitid
                will be refit. Otherwise, all images in kbobin.object
                will be refit based on the setting of the FORCE flag.
 imgid        : If specified, a single image, within the visit specified
                by visitd, will be refit. Otherwise all images are processed
                within the visit (or all images in kbobin.object). It is 
                an error to specify imgid without visitid.

 KEYWORD PARAMETERS:

 PATH  - Path to directory in which compressed fits files
         of the form rootname_c0f.fits.gz exist. 
         Default is /net/frakir/data1/grundy/hst_binaries/c0f.
 SAMPFACT- sub-sampling factor for kernel, default=3

 PROGRAM - String that identifies the program ID, default='u9zo' (Will's
             cycle 16 large program)

 OUTPUTS:
 kernel - Output convolution kernel

 COMMON BLOCKS:

 SIDE EFFECTS:
 Uses HST PSF images which may cause the memory and disk PSF caches to be
 populated. Modifies entries in KBOBIN data base table postable. These entries
 reflect the most recent values obtained from the fit,

 RESTRICTIONS:
   Ephemeris is geocentric, not HST-centric
   Assumes data is from WFPC2

 PROCEDURE:

 MODIFICATION HISTORY:
  2008/04/22, Written by Marc W. Buie, Lowell Observatory