To determine the responsivity of the instrument the standard stars have been observed in all LW and SW filters several times during the mission with different selections of the pixel field of view. An overview of the observations performed is given in Table 2.
In addition, a photometric monitoring of the so-called ISOCAM TFPR (Total Flux Photometric Reference) field was initiated in revolution #279. Observed regularly 8 times until revolution #770, this field was selected because it was also repeatedly observed by IRAS for photometric calibrations and so it was adopted for ISOCAM calibration given also its high visibility throughout the mission. The field contains a starburst galaxy and a QSO, both expected to be very stable on time scales of months or even years. So these measurements were taken as independent checks for the stability of the photometry found with the calibration stars and as a means of comparing the ISO absolute calibration with the calibration of IRAS.
The conversion of the measured signal in ADU/G/s to
monocromatic flux densities in Jy assume the spectral shape to be
lambda x F_lambda = constant,
or equivalently nu x F_nu = constant. For each CAM filter a reference wavelength
is selected to which these flux densities are referred.
Note that the result of this calculation stills need to be corrected
for the actual colour of the object, once this is known.
The spectral response function (SRF) of CAM-CVF was measured on a small subset of calibration standard stars. The photometry of the stars observed in CVF mode was done in the same way as for the filter photometry. As far as the SW detector is concerned, only HIC 96411 (= HR7469 = HD 185395) was used for computing the SRF. On this star four individual CVF scan observations are available. As far as the LW detector is concerned, several observations of four stars (HIC94890, HIC96441, HIC94376, and HIC87833) were used for computing the SRF. The star fluxes have been derived using aperture photometry (with apertures small enough to avoid the ghost reflection). They were observed more often with the 3 arcsec p.f.o.v., but also 1.5 arcsec and 6 arcsec p.f.o.v observations were done in order to check the photometry.
Model SEDs for all these stars are available from the ISO Ground Based Preparatory Program (GBPP) for the star HIC94890, and from the Cohen, Walker, Witteborn et al. (CWW) absolute calibration programme for the other three stars.