Multi-aperture photometry uses the PHT-P sub-instrument either in staring or chopping mode. This observing mode is similar to PHT03 but here the user selects different apertures to be used in connection with only one filter. The minimum number of apertures is 2, and the smallest selectable aperture is the diffraction limited one at the wavelength of observation.
In this mode one can distinguish a circumstellar disc or a shell from the star itself and analyse the radial structure of these objects down to the diffraction limit of the ISO optics. A similar case is the separation of the disc component and the nucleus of galaxies. The contribution of an extended component around a point source can be derived by comparing the ``curve of growth'', which is the signal versus aperture, with that of a true point-source.