3.5 Oversampled Linear Scans and Maps

These observing modes also performed observations on a regular one-dimensional grid, like rasters, but they were differential measurements. The chopper was used for oversampling between individual spacecraft positions, where chopping was performed along the scan line. This meant that all these observations were performed using the chopper and several spacecraft positions, which was in contrast to the staring raster mode. The same celestial position was observed during several raster pointings allowing for elimination of temporal changes in detector response (see Figure 3.6).

Figure 3.6: Illustration of the PHT32 sampling procedure for the C100 array and an oversampling factor in Z direction of 2/3, i.e. a step of 3/2 pixels or half the array size. In the illustration the mapping starts at the upper-left raster point, and the sampling steps performed with the chopper (numbered 1 to 13) are indicated at that position. Note that at each spacecraft raster point only the central C100 pixel is shown.

The scanning direction was parallel to the spacecraft y-axis. Its orientation with respect to equatorial coordinates depended on the date of observation and was not known before the observation was scheduled. As a fixed orientation of the scan line implied strong scheduling constraints due to a fixed observing time, a tolerance angle for the actual scan line as large as possible had to be provided.

Scans were used to obtain flux density profiles, or to observe bright sources with Nyquist sampling.

Maps were composed of a series of parallel scans which overlaped and were dedicated to observe extended sources or complex far infrared sources with the optimum spatial resolution. As in scans, chopping was only performed in scan direction, the sampling in the orthogonal direction was obtained by stepping the spacecraft in a fine mode so that the detector pixels were oversampled.