GRIP (Gamma-Ray Imaging Payload)

Developed by California Institute of Technology (CalTech)

Balloon launched on 11/18/1987, from Australian Balloon Launching Station, Alice Springs, Australia

Purpose of the flight and payload description

GRIP was the acronym for Gamma-Ray Imaging Payload a balloon-borne gamma-ray telescope that operated in the energy range from 30 keV to 10 MeV, using the coded-aperture technique to produce sky maps at energies where traditional mirrors and lenses were no longer effective. The instrument, developed at the California Institute of Technology (CalTech) during mid 1980's decade, required two primary components: a mask consisting of opaque and transparent regions that spatially modulated sky flux, and a position-sensitive photon detector. Each source in the field of view cast a gamma-ray shadow of the mask pattern onto the detector, which could be deconvolved to form sky images using patterns built on skew Hadamard uniformly redundant arrays.

The detector system featured a 41 cm diameter, 5 cm thick NaI(Tl) disk as the primary detector, with scintillation photons detected by nineteen 3-inch photomultiplier tubes optically coupled to a glass window at the detector's rear. A maximum likelihood algorithm computed event locations in three dimensions from the detected light distribution, while deposited energies were calculated from the sum of all PMT signals. The position resolution was 1.2 cm FWHM at 122 keV and 1.0 cm at 662 keV, with energy resolution of 25% FWHM at 60 keV and 7% at 1 MeV.

Active anticoincidence detectors shielded the camera plate at the rear and sides, with a second NaI(Tl) camera plate forming the rear shield and thick plastic scintillator surrounding the detector periphery. This active shield system reduced background from atmospheric gamma-rays, celestial sources outside the coded field of view, Compton scattered photons, and iodine escape events. A passive lead collimator with 14° aperture further reduced diffuse background within the field of view.

The mask employed hexagonal uniformly redundant arrays built on a hexagonal lattice, possessing anti-symmetric properties upon 60-degree rotation with only the central cell unchanged. This antisymmetry enabled point-by-point background subtraction by comparing photon distributions from mask and antimask orientations. Continuous mask rotation during flight broke translational symmetry and restored unique sky mapping capabilities. Each opaque cell was a right hexagonal cylinder 4.8 cm flat-to-flat, made of 2.8 cm lead topped with 0.8 mm tin to absorb fluorescence photons. The mask was positioned 2.5 meters above the detector, providing 1°.1 angular resolution.

The instrumental effective area incorporated geometric detector area of 645 cm², instrument efficiency accounting for attenuation and detector response, and an imaging factor describing sensitivity loss from point spread function effects. A processing cut applied below 300 keV rejected preferential background events in the detector's rear half.

The telescope was mounted in an altitude-azimuth pointing platform with elevation maintained by stepper motor driven ballscrew assembly and azimuthal pointing controlled through magnetometer-based feedback in flux nulling mode. An on-board microprocessor calculated required parameters to track celestial coordinates. Orthogonal inclinometers mounted on the gondola frame provided tilt corrections applied during post-processing image reconstruction. Two CCD star cameras, one image-intensified, mounted on the telescope body provided optical aspect sensing during nighttime observations, with achieved aspect solutions typically accurate to better than 0°.5.

Details of the balloon flight

Balloon launched on: 11/18/1987 at 10:45 utc
Launch site: Australian Balloon Launching Station, Alice Springs, Australia  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon AF-392.73-080-NSCR
Balloon serial number: W23.50-3-03
Flight identification number: 236N
End of flight (L for landing time, W for last contact, otherwise termination time): 11/18/1987 at 21:24 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 11 h 30 m
Landing site: 400 miles W of Alice Springs, Australia

External references

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