Imaging Photopolarimeter (IPP)

Description

The Imaging Photopolarimeter (IPP) experiment (also on Pioneer 11) used during Jovian encounter made simultaneous, two-color (blue - 3900 to 4900 A, red - 5800 to 7000 A) polarimetric and radiometric measurements, and moderate-resolution (about 200 km at best) spin-scan images of Jupiter and the Jovian satellites. The polarimetric and radiometric work was performed using an 8- by 8-mrad field-stop aperture, while the spin-scan imaging used a 0.5- by 0.5-mrad aperture stop. Relative radiometric calibration was derived using an internal tungsten lamp. Long-term absolute calibration of the instrument was accomplished by means of a sunlight diffuser/attenuator element located in the spacecraft antenna structure. Primary radiometric calibration was obtained throughout the mission by periodically commanding the telescope to view this diffuse backlighted (sunlight) source. The experimental train for the IPP package consisted of the following elements: (1) a near-diffraction-limited 2.54-cm Maksutov catadioptric telescope of focal ratio f/3.4, (2) a focal-plane wheel containing field-of-view (FOV) apertures, depolarizers, calibration source, etc., (3) a Wollaston prism to split light into two orthogonally polarized beams, (4) a 45-deg dichromatic mirror that reflected wavelengths shorter than 5500 A (blue beam) and transmitted all light of greater wavelength (red beam), (5) for each spectral beam (two polarizations) a filtering-coated relay lens and folding mirrors, and (6) for each spectral beam, two Bendix Channeltron detectors (blue bialkili S-11 photocathodes and red S-20 photocathodes) to register the intensity in each polarization component. Polarization data also include the interplanetary region. The instrument experienced erroneous aperture cycling in October 1991 and was used occasionally for spacecraft roll-reference measurements until October 1993, when it was turned off to conserve spacecraft power for other experiments and subsystems.