2.8 Focal Plane Chopper (FPC)

The Focal Plane Chopper (FPC) deflected the beam onto adjacent fields on the sky, or, in case of internal reference measurements, onto the two FCSs (see Figure 2.2). The chopper could move the beam only along the spacecraft Y direction, positive chopper position commands yielded throws in +Y direction. FCS1 was seen by the detectors when the chopper moved the beam beyond the (+) field of view of the field mirror at , FCS2 was seen at . The maximum amplitude for the sky was . The chopper transition times are given in Table 2.9. The first column indicates the mode: either from the centre field of view (CFOV) to one of the FCSs (FCS mode) or to a sky offset (FOV mode):

Table 2.9: Chopper transition times.

Mode	Transition time	Transition time
	in ITK units	[]
FCS	832	0.051
FOV	224	0.014

Note:   1 ITK (instrumental time key) unit = 2.

The chopper could be commanded to operate in staring mode (i.e. one position for the whole measurement) or in chopped mode whereby two or more viewing directions could be chosen during a measurement. In the AOTs three distinct chopper modulation functions could be selected (see also Figure 3.1):

• RECTANGULAR:
  chopping between two end positions, with an equal time spent on both positions.
• SAWTOOTH:
  chopper scan with the number of steps ranging from 3 to 301; minimum step size , maximum step size , and .
• TRIANGULAR:
  chopper scan with the number of steps ranging from 3 to 301; step sizes and limitations as for sawtooth chopper scan.

The first chopper position of a chopper cycle was always the largest deflection in -Y direction. The chopper mode and amplitude is fixed per AOT and not per measurement. The parameters of the modulation function per AOT are described in detail in Section 3.1.