Next: 3.7 ISOPHOT Modes of Up: 3 Instrument and AOT Previous: 3.5 Fine Calibration Sources

3.6 Focal Plane Chopper (FPC)

The Focal Plane Chopper (FPC) deflects the beam onto adjacent fields on the sky, or, in case of internal reference measurements, to the FCSs. The chopper can move the beam only along the spacecraft Y direction, positive chopper position commands yield throws in +Y direction. FCS1 is seen by the detectors when the chopper moves the beam beyond the (+) field at +460'', FCS2 is seen at -460''. The maximum scanning amplitude for the sky is $\pm\,180''$ . The chopper transition times are given in Table 3.4. 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 3.4:** Chopper transition times
Mode	transition time	transition time
	in ITK units	(s)
FCS	832	0.051
FOV	224	0.014

note: 1 ITK (instrumental time key) unit = 2^-14 sec.

The chopper can 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 may be chosen during a measurement. In the AOTs three distinct chopper modes can be selected:

RECTANGULAR mode implies chopping between 1 reference position and the source position with an equal time spent on the reference and source positions. The reference offset is in -Y direction. The telescope pointing is not on target but located in the middle between the source and the reference position.
$\rhd~$ A chopper cycle consists of 2 consecutive chopper plateaus:
ref1 $\Rightarrow$ on-source.
SAWTOOTH mode implies chopping between 2 reference positions and the source position with equal time spent on all 3 positions. The source position is coincident with the telescope pointing direction, and between the 2 reference positions.
$\rhd~$ A chopper cycle consists of 3 consecutive chopper plateaus:
ref1 $\Rightarrow$ on-source $\Rightarrow$ ref2.
TRIANGULAR mode implies chopping between 2 reference positions and the source position with a time equivalent to that spent on the source position split equally between the 2 reference positions. The source position is coincident with the telescope pointing direction, and between the 2 reference positions.
$\rhd~$ A chopper cycle consists of 4 consecutive chopper plateaus:
ref1 $\Rightarrow$ on-source $\Rightarrow$ ref2 $\Rightarrow$ on-source.

The first chopper position in the above cases is an off-position in -Y direction. There are strong constraints for the chopper throw which are listed in [10] pages 23-25.

The chopper mode and amplitude is fixed per AOT and not per measurement. For AOT PHT32 the chopper is indirectly commanded depending on the detector (C100 or C200) one chooses. In the case of C100 the chopper is commanded to perform in sawtooth mode 13 chopper steps with 15'' separation symmetrically with respect to the centre field if view. When C200 has been chosen, the chopper is commanded to perform in sawtooth mode with 7 chopper steps with 30'' separation.

Next: 3.7 ISOPHOT Modes of Up: 3 Instrument and AOT Previous: 3.5 Fine Calibration Sources

ISOPHOT Data Users Manual, Version 4.1, SAI/95-220/Dc