The routine needs to know the total number of counts from the source, the counts per unit area from the background, the off-axis position of the source and the instrument used.
The signal to noise ratio is defined as:
Where, Ns is the total no. of source counts, Nb is the background flux per unit area, r is the box radius andS/N = Ns * FRAC(r) / SQRT( Ns * FRAC(r) + Nb * 2 * PI * r**2 )
is the fraction
of source counts which are contained within a circle of radius
r.
is a function of the detector involved e.g. WFC, XRT and
the distance of the source from the optical axis.
RADOPT finds the maximum of this function by trying a series of increasing radii until a maximum is found.