This is a toggle switch which alters the way in which the background level is measured. There are two methods available. The first is interactive, and uses an aperture identical in size and shape to the object aperture. The aperture is positioned manually to select the region of sky to measure. The message `Interactive aperture in use' will signify that this has been chosen.
When using the interactive aperture, several sky areas can be sampled to improve the estimate of the sky. The sky estimates from each aperture are simply summed, and the mean of these is used when the object is measured.
The alternative is to use an aperture which is a concentric annulus around the object aperture. In this mode the sky is measured automatically every time a measurement is made. The message `Concentric aperture in use' signifies this choice. The size of the sky aperture is specified by the INNER and OUTER parameters. These are defined as multiplying factors of the object aperture size, or in optimal extraction mode of the clipping radius, e.g. if INNER were 2 and OUTER 3 then the sky aperture would start at two radii from the centre and end at three radii.
When starting PHOTOM one of these modes will be chosen as the default. This choice is controlled by the CONCEN parameter. If the positions of the objects is entered by a file of positions (command F), then the background is automatically taken with the concentric annulus, whatever the default value of CONCEN.
PHOTOM --- A Photometry Package