The photometer optics delivers diffraction-limited image quality (Strehl ratio >95%). Therefore PACS preserves the image quality provided by the Herschel telescope and is diffraction-limited on it whole energy range. The photometer PSF (see Figure 3.1) is dominated by the telescope and characterised by:

For fast scans in normal and parallel mode, this PSF structure is smeared by detector time constants and data averaging. Quantitative information on the PSF is given in Table 1.

These PSFs and derived quantities reflect the intrinsic optical quality of Herschel+PACS. In a scanmap reduction they will be very slightly smeared, in particular at short wavelengths, because of the data averaging on-board (from 40Hz to 10Hz sampling), but also due to detector time constants telescope pointing jitter and drifts. For more detailed information on the PACS photometer PSF we refer to the PACS technical note PICC-ME-TN-033 (version 0.3, Nov. 10, 2009)

The photometer focal plane geometry, initially established on ground by scanning a back-illuminated hole mask across the bolometer arrays, has been adapted to the actual telescope by optical modeling. The in-flight verification required a small change in scale and a slight rotation to fit to the results of a 32x32 raster on the star αHer. In particular these measurements also show that the possibilities to further improve the calibration of detailed distortions are limited by short-term pointing drifts of the satellite. Residual measured inter-band offsets between blue/green (0.3 arcsec) and green/red (1.2 arcsec) have been characterised and are implemented in the astrometric processing chain. Current best point spread functions (PSF) have been determined on asteroid Vesta. The observed 3-lobe structure of the PSF (Figure 3.1) can be explained qualitatively by the secondary mirror support structure and has been verified in detail by ray tracing calculations taking the telescope design and known wave-front errors into account. The spatial resolution, expressed as encircled energy as a function of angular separation from PSF centre, is in reasonable agreement with expectations from telescope and instrument design.

Further details on the PACS PSF can be found in the technical note PICC-ME-TN-033 Version 0.3. The aperture correction factors for the are listed in the table in section Table 3.2.