  We present a theoretical analysis of the measuring algorithm we use when
applying the Diffracto-Astrometry technique to Hubble Space Telescope Wide
Field Planetary Camera 2 (WFPC2) saturated stellar images. Theoretical Point
Spread Functions (PSFs) were generated using the Tiny Tim software for the four
CCDs in the WFPC2 and for some of the available filters.
  These images were then measured with our Diffracto-Astrometry measuring
algorithm using only their diffraction pattern, and positions for the simulated
PSFs on each generated CCD-frame were obtained. The measuring algorithm
recovers the original positions reasonably well ($\pm 0.1\ \rm{to} \ \pm 0.4$
pixels). However, slight deviations from the original values are observed.
These also vary with position over the entire surface of the CCD. We adjust the
difference between the real and the measured position with a quadratic function
of the coordinates. The transformation coefficients also present a slight
correlation with the filter effective wavelength. Application of these
transformation coefficients allows us to determine the position of a stellar
image with a precision of a few hundredths of a pixel.
