Focal Plane Chopper

The chopper consists of a small tilting mirror, driven by magnetic coils and positioned at the image of the telescope pupil. The chopping direction is the spacecraft Y-axis. For spacecraft coordinates the user is referred to the ISO Observer's Manual. For single-pointing photometry the chopper throw can be selected by the user. A detailed description of possible chopper throws is given in Sect. 3.1.

The chopper can be used for:

• differential measurements between two adjacent celestial positions, e.g. the source and the celestial background (see Section 3.1)
• absolute chopping between a celestial position and the internal calibration source (see Section 2.12)
• sampling in a scan or map at positions between spacecraft pointings.

For staring observations the chopper is set to its central position.