The absolute flux calibration of the photometer is based on models of standard stars in the 0.6-15 Jy flux range in the three filters: α Boo (Arcturus), α Tau, β And, α Ceti & γ Dra (see Dehaes et al. 2010) and on thermophysical models for a set of more than 10 asteroids (Müller & Lagerros 1998, 2002), building up on similar approaches for ISOPHOT (Schulz et al. 2002) and Akari-FIS (Shirahata et al. 2009). Together they cover a flux range from below 100 mJy up to 300 Jy. Both types of sources agree very well in all 3 PACS bands, and the established absolute flux calibration is consistent within 5%. Neptune and Uranus with flux levels of several hundred Jansky are already close to the saturation limits, but have been used for flux validation purposes. At those flux levels, a reduction in response of up to 10% has been observed. For comparison, the latest FIR flux model of Neptune is considered to be accurate to better than 5% (Raphael Moreno, priv. comm. & Fletcher at al. 2010).
For the 5 primary PACS standard stars, the measured absolute flux accuracy is within 3% of predicted values in the blue and green filters, and within 5% of the predicted values in the red filter. See PACS Photometer - Point-Source Flux Calibration, Müller & al, 2011, for more information on the PACS photometer flux calibration accuracy.
Absolute flux calibration uncertainties should improve over the mission, with better statistics of available celestial calibration observations.
The calibration itself comprises: (i) flat-fielding, (ii) responsivity correction -- conversion of engineering units, volts, to Jy/pixel, and (iii) gain drift correction to account to small drifts in gain with time.
The flux calibration of the PACS photometer assumes: