When evaluating the significance map on the first pass of SEARCH
mode its possible to miss the peak of a source because the optimum source
position lies between grid points. Although this is not a problem for any
image where the psf extends over more than a few pixels, there are
exceptions. For example, with a ROSAT PSPC image binned at one arcminute
resolution, an on-axis 
source centred on a corner of a pixel
may have its significance reduced by as much as 
if the grid
points are located on pixel centres.
One solution to this problem is to lower the the significance threshold on the first iteration. However, this obviously will result in more spurious peaks if the threshold is lowered too far.
A practical alternative for small image areas is to set the value of the SAMPLE parameter to greater than unity, which multiplies the density of grid points on the first iteration by that amount on both axes. While alleviating the problem above, it has the effect of prolonging the first iteration by the square of its value.