The PSF component contains information allowing the point spread function of an imaging instrument to be recovered by software. Due to the fact that the PSF may be best represented in quite different forms for different instruments and may be dependent on energy and epoch, the name and arguments of a subroutine to calculate values from a point spread function are stored, rather than storing the values themselves. The structure is:
PSF <EXTENSION>
ROUTINE_NAME <_CHAR*20>
LIBRARY_NAME <_CHAR*20>
Software then recovers a value from the PSF by calling the subroutine whose name is given in ROUTINE_NAME, which is located in the shareable library with logical name given in LIBRARY_NAME. A suite of PSF_* subroutines to perform these operations is described in document PROG_011.