[Older version]

Payload Architecture

There are three basic payload components: (1) a telescope and baffling system, providing the angular resolution and rejection of straylight; (2) the Low Frequency Instrument (or LFI) - an array of tuned radio receivers, based on HEMT amplifier technology, and covering the frequency range $\sim$30 - 100 GHz; and (3) the High Frequency Instrument (or HFI) - an array of bolometers covering the frequency range $\sim$100 - 850 GHz. The LFI and HFI are both placed in the focal plane formed by the telescope, in such a way that the HFI detectors occupy the central area, while the LFI detectors are arranged in a ring around the HFI. This arrangement is a compromise which optimizes a number of thermal and mechanical constraints, while keeping off-axis aberrations to an acceptable level.

The temperature requirements of the two types of detector are very different, the HEMTs giving adequate performance at 20 K (achievable by a combination of passive and active cooling), while the bolometers must operate at temperatures between 0.1 and 0.15 K to achieve adequate performance. Thus, the LFI will consist of an array of corrugated horns feeding miniaturized receivers which are cooled by a H₂ sorption cooler. The HFI bolometers must be located inside a ``cold box" consisting of a series of nested radiation shields, and cooled by an open-cycle dilution refrigerator coupled to a 4 K Joule-Thomson cooler; the latter uses the main LFI 20 K cooler as a precooler. A schematic overview of the arrangement of the payload is shown in Figure 3.1. In the remaining sections of this Chapter we describe each of the three main elements (telescope, LFI, HFI) in turn, their integration into a working payload, and the constraints the payload sets on the technical design of the spacecraft.

 

Figure: A subsystem-level block diagram of the model payload, and a sketch of the geometric architecture of the payload.