INFRARED TELESCOPE

The objective of the flight was to measure the absolute infrared radiance of Venus and Jupiter using a balloon-borne observational platform. The system was developed under project 7670 at the Optical Physics Laboratory of the Air Force Cambridge Research Laboratories in order to evaluate the suitability of using both planets as stable infrared calibration sources, especially in the atmospheric window between 8 and 14 micrometers.

The core of the observational setup was a 50-INCH CASSEGRAIN TELESCOPE coupled to a SPECTROMETER designed to minimize thermal radiation from the telescope's own mirrors. To achieve this, the system employed a double-beam chopping technique that alternated views between the target and the sky. The spectrometer included a circular variable filter cooled with liquid nitrogen and a copper-doped germanium detector cooled with liquid helium. The field of view was tightly defined by a 60 arcsecond circular aperture also maintained at cryogenic temperatures, and the system optics ensured a standardized low-background signal reaching the detector.

Data acquisition followed two methods. First, spectral scans were made over the wavelength range from 4.5 to 16 micrometers with a resolution of approximately 4 percent, using the entire planetary disk as the source. Second, raster scans were conducted at a fixed wavelength to measure the integrated radiance across the image of each planet. The raster scans covered a 150 by 150 arcsecond area in the image plane and consisted of 16 horizontal lines. The signal envelope from these scans was influenced by both the aperture size and a slight intentional defocusing of the telescope optics, which helped distribute the energy more uniformly without affecting the integrated signal.

The absolute radiance values obtained from raster scans were used to calibrate the spectral data. Radiometric corrections were applied for atmospheric attenuation, especially within the CO2 absorption band between 14 and 16 micrometers, where transmission at float altitude was approximately 55 to 60 percent. Signal quality varied across the spectrum, with a data gap between 8.5 and 11.5 micrometers due to filter leakage, and additional signal loss near 6 to 7.25 micrometers in Jupiter's spectrum. Overall, the experiment yielded radiance spectra for Venus and Jupiter that were in general agreement with previous ground-based observations, though with some discrepancies. These results support the idea that balloon-based measurements can avoid many of the limitations posed by atmospheric extinction in ground-based setups, providing a more stable basis for evaluating the use of planetary bodies as infrared calibration sources.

Balloon launched on: 7/1/1974 at  
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  
End of flight (L for landing time, W for last contact, otherwise termination time): 7/1/1974
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 8 h
Landing site: 19 miles SE of Casa Grande, Arizona, US

• Absolute Infrared Radiance Measurements of Venus and Jupiter Issue 494 of Environmental research papers, 1974
• Summary of Balloon Flights launched from H.A.F.B. NM (1962 thru 1987) U.S. Gov. National Archives