... The first, which is already in use for restoring HST images, relies on an approximation to obtained by substituting a Poisson variate for the read-out noise in Eq. ... Useful starting values for the Newton iteration ( 16 ) can be obtained by making approximations to the exponential in Eq. ... A C subroutine to compute using the saddlepoint approximation with a Newton iteration to determine the saddlepoint is given in the Appendix for starting values obtained with either the Pad? or Picard methods. ...