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5.3 Point sources versus extended sources  

All targets used for flux calibration are point sources in the ISOPHOT wavelength range. For an extended source which is much larger than the aperture or pixel, the surface brightness is derived by involving the effective solid angle of the aperture or pixel.

For the P subsystems, the effective solid angle for each aperture corresponds to the area of the aperture in the focal plane projected on the sky. The areas were determined from the measured physical dimensions of the apertures.

The conversion form point source flux in Jy to a surface brightness can be written for PHT-P as follows:

 

(5.10)

where is the obscuration factor for the ISO secondary mirror, in sr is the effective solid angle of the aperture or pixel, for a given filter f. In OLP Version 7 the PHT-C surface brightness is derived from the in-band powers of each pixel. The computation for PHT-C is as follows (with the same notations):

 

(5.11)

A model of detector footprints (or beamprofile) is used to determine the effective solid angles of the C100 and C200 detector pixels for each filter band. The model takes into account the ISO primary mirror, secondary mirror, tripod, and the linear sizes of the detectors as measured pre-flight.

For PHT-S the values of for each pixel were determined from in-flight calibration observations of the detector footprints. These values are included in the conversion from signal to extended source flux density, see Eqn. 5.9.