Photopolarimeter-Radiometer (PPR)

Description

The Photopolarimeter-Radiometer (PPR) experiment was designed to measure the linear polarization and intensity of reflected sunlight and the intensity of thermally emitted infrared radiation from Jupiter and the Galilean satellites. The primary science objectives of the PPR were to: (1) determine the vertical and horizontal distribution of cloud and haze particles in the Jovian atmosphere, including their size, shape, and refractive index; (2) determine the energy budget of Jupiter and the variation in amount and spatial distribution of reflected solar radiation and emitted thermal radiation for Jupiter and its satellites, including the thermal structure of the Jovian atmosphere and the vertical distribution of absorbed solar radiation therein; and, (3) measure and map the photometric, polarimetric, and radiometric properties of the Galilean satellites and other targets of opportunity.

To achieve these goals, the PPR utilized a 10 cm aperture, 50 cm focal length Cassegrainian Dall-Kirkham ("scene-view") telescope with a 2.5 mrad (0.14 degree) instantaneous field of view. A view of space (used for radiometric measurements) was provided by reflection from a chopper mirror and a planar ("space-view") telescope mirror. Both instruments were mounted on the despun section of the spacecraft. For polarimetry and photometry measurements, the flux from the scene-view telescope was focused onto a circular field stop. Light passing the field stop was then modified by passage through optical elements located on a filter/retarder wheel.

For polarimetry measurements, the light passed through both a halfwave retarder and a spectral filter; for photometry, only the spectral filter. A relay lens then deflected the light through a Wollaston prism, serving as a polarizing beam-splitter, resulting in two spatially-separated and orthogonally-polarized beams which were then focused onto two silicon photodiodes. The chopper motor, operated at 30 Hz, was only used to radiometric measurements. This alternately directed light from the scene-view and space-view telescopes through a field stop before passing through a radiometry filter. After passing through the filter, the light would be reflected radially outward from the filter/retarder wheel by an ellipsoidal mirror, collected by a condenser system, and focused onto a lithium tantalate pyroelectric detector.

In photopolarimetric mode, measurements were made in three spectral bands centered at 410.0, 678.5, and 944.6 nm. In photometric mode, seven spectral bands centered at 618.7, 633.3, 648.0, 788.7, 829.3, 840.3, and 891.8 nm were used. In radiometric mode, five channels, four centered at 16.8, 21.0, 27.5, and 35.5 micrometers and one for wavelengths >45 micrometers, were used.

For typical satellites encounter altitudes (~1000 km), spatial resolution of PPR measurements was expected to be 2.5 km, comparable to the resolution of much of the better Voyager imaging. Calibrations of polarimetric and photometric measurements were done using both internal and external calibration lamps. Calibrations of radiometric measurements were made using an external calibration lamp as well as using the view of space as a reference.