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Method

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:

  S/N = Ns * FRAC(r) / SQRT( Ns * FRAC(r) + Nb * 2 * PI * r**2 )

Where, Ns is the total no. of source counts, Nb is the background flux per unit area, r is the box radius and FRAC(r) is the fraction of source counts which are contained within a circle of radius r. FRAC(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.

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