Selecting the CASH option at the SOPT prompt selects the
Cash maximum likelihood statistic (Ref 1) as the method of comparing
data and model.
where 
is the image position, 
are the regularly spaced
Poisson distributed data and 
the model.
Using the normal model (1), and ignoring
the 
term in (3), which is independent
of the parameters (and hence constant for a given dataset), we minimise,
the summation being performed over all pixels for which p is
significantly greater than zero (see discussion below). At each 
the statistic is evaluated repeatedly to find the optimum flux 
,
when,
