PHOTOELECTRIC SKYLIGHT POLARIMETER

The payload was composed by a gain compensated photoelectric polarimeter employing a rotating calcite glan prism as polarization modulator, mounted on a stabilized balloon-borne gondola.

The polarimeter was developed to measure the degree of linear polarization and relative intensity variations in four narrow spectral intervals.

The instrument had a field of view of 3º. The principal design features are schematically shown at left. A collimated beam of polarized light passes through an optical train consisting of a rotating half-wave retardation plate, a stationary Glan prism, an interference filter with peak transmission in the neighborhood of the characteristic wavelength of the retardation place, and a quartz neutral density wedge. The emergent beam impinges upon the photocathode of an end-window type photomultiplier tube. The resulting sinusoidal photo signal passes through auxiliary logic and detection circuits and is recorded.

The system weighed about 10 kg and measures 25 cm x 25 cm x 25 cm was designed to operate on 28V DC to facilitate operation either from an aircraft or a balloon. For this test flight the polarimeter -encapsulated in 3.2-cm Styrofoam- was mounted onto a truncated, pyramidal gondola in such a fashion that it could scan through 270º symmetrically about the local nadir. A Wollensak time lapse camera was also onboard with its objective pointing toward the nadir, and the direction of the film transport parallel to the plane in which the polarimeter would scan. The color photographs, taken once every minute, have been utilized in two ways; namely, to determine the type and nature of the underlying terrain and surrounding sky and to fix the azimuthal orientation of the scan plane with the aid of the known trajectory of the balloon and prominent topographical features. This was later used to check the performance of the azimuth stabilization control.

Also was onboard a seven-channel oscillograph recorder as a backup system, a Barocoder, and telemetering equipment to receive and execute commands sent from the ground station and to transmit on the PCM telemetry system scientific and auxiliary data to the ground station. On-board banks of silver cadmium and solar cells furnished the required power. The gondola was fitted with an azimuth stabilization control driven by a torque motor, to allow that the polarimeter would always scan in the principal plane through the sun.

Balloon launched on: 5/28/1968 at 5:51 MDT
Launch site: Holloman Air Force Base, Alamogordo, New Mexico, US  
Balloon launched by: Air Force Cambridge Research Laboratories (AFCRL)
Balloon manufacturer/size/composition: Zero Pressure Balloon 600.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 5/28/1968 at 10:00 MDT
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 4 h 10 m
Payload weight: 290 kg

During the flight were performed multicolor measurements of the degree of linear polarization of light diffusely reflected by the Earth's atmosphere in the principal plane of the Sun.

This was the initial phase of a research program directed towards the evaluation of the feasibility of indirect optical experiments to study planetary atmospheres.

• Balloon measurements of the polarization of light diffusely reflected by the earth's atmosphere Planetary and Space Science Volume 17, Issue 6, June 1969, Pages 1307-1309
• Measurements and interpretation of the polarization of radiation emerging from the atmosphere at an altitude of 28 km over SW New Mexico (USA) Q.J.R. Meteorol. Soc., 99: 294-302
• Photoelectric Skylight Polarimeter Review of Scientific Instruments, Volume 34, Issue 7, p.764