In the following, we call relative photometric error the sum of all the errors in the measurement of a source by comparison with a calibration source. Adding to the relative error the calibration error, or the error on the absolute flux of the calibration source, gives the absolute photometric error.
A set of standard sources has been established for the purpose of
the ISO mission. Their fluxes are known from ground based
measurement, either directly, or in combination with stellar
atmosphere models. The calibration errors for these sources are:
- 10% for the ``Royal" standards,
- 20% for the others.