The entrance optics fulfill the following tasks: they create an image of the telescope secondary mirror (the entrance pupil of the telescope) on the focal plane chopper; this allows spatial chopping with as little as possible modulation in the background received by the instrument.

It also provides for an intermediate pupil position where the Lyot stop and the first blocking filter, common to all instrument channels, can be positioned. It allows the chopper, through two field mirrors adjacent to the used field of view in the telescope focal surface, to switch between a (chopped) field-of-view on the sky and two calibration sources (see also Figure 2.3).

The chopped image is then re-imaged onto an intermediate focus where a fixed field mirror splits off the light into the spectroscopy channel. The remaining part of the field of view passes into the photometry channels. A "footprint" of the focal-plane splitter is shown in Figure 2.3.

The calibration sources are placed at the entrance of the instrument to have the same light path for the sky observation and internal calibration. This is essential for removing detector baseline drifts as well as possible, a serious task with a warm telescope and the associated high thermal background. To eliminate non-linearity or memory problems with the detector/readout system, the calibration sources are low emissivity gray-body sources providing FIR radiation loads at two slightly different signals around the level of the telescope background. This is achieved by diluting the radiation from a (small) black source with a temperature near the telescope temperature inside a cold diffusor sphere with a (larger) exit aperture. The temperature of the radiator (~80K) is stable to within a few mK.

Differential measurements are required to extract faint signals from celestial sources from the dominant thermal background radiation of the warm (~80K) Herschel mirror. For this purpose a small tilting mirror, the chopper, flips alternately on the astronomical source and on a nearby sky position, with a variable throw up to 6 arcmin on the sky for the spectrometer and 3.5 arcmin for the photometer. This allows full separation of an object field and a reference field.

The chopper is also used to alternatively look at the two internal calibration sources (ICS) which are located at the left and right side of the instrument FOV (see Figure 2.3) for frequent calibration measurements.

The chopper is capable of following staircase waveforms with a resolution of 1", and delivers a duty-cycle of ~90% at chop frequency of 5 Hz. The chopper axis is stabilized in its central position by flexular pivots and rotated by a linear motor. The chopper design allows a low heat load in the PACS FPU.