3.1 Chopping

There were three different chopper modes available for single pointing photometry. The decision to chop or stare and the selection of the chopper mode depended on the structure of the source and the nearby sky background. The different chopper modes are illustrated in Figure 3.1.

Figure 3.1: Chopper modes. The curves represent the chopping cycles. To the right the projection on the sky is illustrated. The chopping starts in the direction. For triangular and sawtooth chopper modes the peak-to-peak deflection is twice the chopper throw.

Rectangular - In this mode, the satellite pointed to the position between the source and an off-source position, and the chopper moved alternately between these two positions. The source was always in the positive beam in spacecraft Y-direction^3.1. Chopper throws (peak to peak) between 30 and 330 in steps of 30 were possible. There was the additional constraint that the chopper throw had to be greater than or equal to the selected aperture diameter^3.2 or the array side length (to avoid overlap of the beams) and less or equal than 360 minus the aperture diameter or array side length (to avoid beam vignetting). For instance, PHT-P observations with the 10 aperture allowed the use of all possible throws, namely 30, 60, 90, , 330; observation with the 79 aperture allowed the use of the following throws: 90, 120, 150, 180, 210, 240, 270; any observation with the 180 aperture allowed only the 180 throw. For C100 the throws of 150, 180 and 210 were allowed, and for C200 only a throw of 180 was possible.

Triangular - Chopping occured to two positions on either side of the source. The satellite pointed to the source position, and the chopper moved from one off-source position to the source and then to the other off-source position and finally back to the source, and so forth. In this way, the source was observed twice during each cycle and each of the off-source positions once. Chopper throws (distance between on-source and off-source positions) between 15 and 150 in steps of 15 were possible. There was the additional constraint that the chopper throw had to be greater or equal to the selected aperture diameter or the array side length (to avoid overlap of the beams) and less or equal than 180 minus half the aperture diameter or half of the array side length (to avoid beam vignetting). For instance, observing with the 10 aperture allowed the use of the full set of throws, namely 15, 30, , 150; observing with the 79 aperture allowed the use of the following throws: 90, 105, 120, 135 (to be compatible with the vignetting rule). The 120 aperture was the largest one to be used for the triangular chopping mode. It was not allowed to use C200 with this chopper mode, and for C100 the only throw was the 150 one.

Saw-Tooth - As in the triangular mode there were two off-source positions, and the satellite pointed to the source. However, after having moved from one off-source position to the source and then to the other off-source position, the chopper moved back to the first off-source position. During the transition between the two off-source positions the source was inside the beam for a very short time. Here the same chopper throw limitations applied as for triangular chopping.

In summary: for the C200 detector it was only possible to choose rectangular chopping mode with a single chop throw of 180. For the C100 detector the only chopper throws were 150, 180, and 210 for rectangular chopping. In case triangular and sawtooth chopping with C100 was selected, the only available throw was 150.

The different chopper modes determined the number and locations of reference positions observed with respect to the on-source position (see Figure 3.1). Chopping was recommended, if the sky background was expected to be comparable to the source flux, or stronger.

By chopping, source and background were measured with one telescope pointing. Otherwise, an additional pointing had to be made, which meant an additional observation using the same AOT on the background position. Depending on the structure of the background, e.g. flat background, infrared cirrus, or with strong gradients, one of the different chopper modes had to be selected.

The fastest chopper frequency was constrained to 0.5 Hz. This occurred for the brightest sources. For fainter sources the frequency was lower due to the requirements to fit at least 4 ramps into one plateau; therefore the selected reset intervals (4) determined the chopper frequency. In-flight it was found, that the fastest chopper frequencies meant significant signal losses of the differential signal with regard to the stabilised signal difference given by staring observations. Therefore, telescope nodding was recommended as a better alternative for slow signal modulation of very bright and intermediate sources against the sky background (see Section 3.3).