Appendix 1

The Imaging Photopolarimeter.



[187] THE ENTIRE SCIENTIFIC instrument complex of Pioneer was needed to provide the new look at Jupiter. However, it was the imaging mode of the imaging photopolarimeter (IPP) that returned colored images of Jupiter's cloud cover to reveal details never before seen. This appendix provides further technical details of this instrument.

The IPP consists basically of a positionable optics-detector assembly and an electronic equipment housing, supported on a central mounting frame. Special optical materials were selected to retain their transparency even though subjected to Jupiter's radiation belts of trapped energetic protons and electrons.

The optical system (Figure Ap-1) consists of a 2.54-cm ( 1-inch) diameter Maksutov type telescope, a calcite Wollaston prism polarization analyzer, multilayer filters to separate red and blue components of the reflected light from Jupiter, relay optics, and two dual, continuous channel-multiplier detectors each designed to sense two polarization components in one of two colors (a total of four channels). The field of view can be varied by use of three apertures on a carrier which also carries polarization processing elements (depolarizer and half-wave retardation plate) and an internal calibration lamp.

Analog signals from the detectors are digitized, buffered in the spacecraft's data storage unit, and transmitted together with instrument status information in either of two telemetry formats. After a command into the data taking mode has been received, the electronic logic processor automatically provides all internal commands required to sequence a complete measurement function with....


Figure Ap-1. The IPP optical system.

Figure Ap-1. The IPP optical system.


[188] .....the IPP and then return the instrument to standby. Additional commands are incorporated to adjust power supply voltages (gain), alter sampling rates, inhibit functions, change the direction in which the telescope steps, and so on.

A field of view of 0.028 degrees square is employed in the imaging mode of the IPP. The field of view is moved slightly (stepped) every roll of the spacecraft, unless inhibited from doing so by command. Step direction can be selected by command. A 6-bit telemetry format provides 64 shades of gray for imaging. In this mode only two of the four detector channels are used, and the light is depolarized prior to detection. Sampling takes place on the dark sky, and the resultant output is used to compensate for zero-level shifts and background caused by the radiation belt environment. Detector output is measured each 0.015 degrees (or 0.03 degrees if so commanded to the low sampling rate) of spacecraft roll, alternating colors. The spacecraft buffer stores imaging data collected over 14 degrees (or 28 degrees in the low sample rate) of each roll.


Figure Ap-2. The IPP imaging system.

Figure Ap-2. The IPP imaging system.


The manner in which the IPP scans to produce an image is shown in Figure Ap-2. Scan lines, analogous to the horizontal lines in a television system, are produced by the instrument looking in a fixed direction with respect to the spacecraft as the spacecraft spins. The start of each scan is controlled by a series of "spoke" commands which relate the start to the spin position, or, alternatively, the scan can be started automatically by the telescope receiving light from the limb of the planet using the "start data at threshold" mode. The equivalent of television vertical scanning is achieved by either stepping the instrument's telescope 0.5 milliradians with respect to the spin axis between each rotation of the spacecraft or, during the closest approach to Jupiter, holding the telescope fixed and letting the relative motion of spacecraft and Jupiter produce the scanning steps. This means that during the closest approach the scan lines can be overlapping, or have gaps between them, depending upon the relative motions of spacecraft and planet.

In the imaging mode, the data are converted to 64 levels of intensities (6 bits) and stored in a 61 44-bit buffer onboard the spacecraft. The instrument overwrites this buffer as it starts each "vertical" scan with each rotation of the spacecraft. The memory read-in time is approximately one-half second, and the spacecraft rotation period is approximately 12.5 seconds, which means that there are approximately 12 seconds available for reading out the data from the memory. To read out these 6144 bits in the 12 seconds that are available requires a data rate of 512 bits per second. The IPP instrument receives 50 percent service rate on the spacecraft's telemetry downlink. Thus a 1024 bits per second telemetry downlink to Earth is the minimum data rate at which all the IPP data taken can be returned to Earth.