This feature is included so that the graphics overlay will be more visible and under the user's control. Often a value greater than 3 is helpful. This ratio is the average exposure of the background frame, divided by the average exposure in the detect cell. This option was devised to minimize the problem of spurious detections along chip edges. If this parameter is non-zero, a user supplied exposure map must be specified via the parameter 'expstk'.
Units are pixels.
Allowed values are 1 or an integer divisible by 3. Value must be evenly divisible by three, or equal to one. Set to 0 for normal operation. A typical scenario in which the users may select to use offsets is when their data file is a sum of two or more observations, with different pointings. The nominal pointing in such data file is usually poorly defined and the off-axis angle could be calculated from an undesired origin, leading to suboptimal selection of the detect cell size.