The Fly's Eye (1981-1993)

The Detector: a Multi-Faceted "Eye"

Between 1981-1986, the new Utah detector array at Dugway comprised of 67 modules constructed on the same design as the three prototypes used at Volcano Ranch in 1976. The new modules are housed on corrugated steel barrels on motor-driven rotary mounts. During down-time, the barrels are rotated such that the mirrors are pointed downward to protect the module from sunlight and the elements. During run-time on clear, moonless nights they are oriented as shown in the photograph of Little Granite Mountain.

As in the Volcano Ranch prototype, each detector module contains a spherical mirror of 1.6 m diameter, and 1.5 m.

During observation, the mirrors and the PMTs are oriented to divide the sky into 880 pixels. Each pixel covers 0.0066 steradians (~5° by 5°). The arrangement of the PMT-pixels is shown in the figure below. Because of the light cone, the actual shape of the field-of-view (FOV) for each pixel is hexagonal, leaving no gaps between PMTs. As an air shower crosses the sky, its trajectory is imaged onto the PMTs firing a succession of pixels, shown by the shaded hexagons in the same figure.

The coverage of the sky with hexagonal pixels is reminiscent of the compound eye of a fly. For this reason, the Utah experiment was named "The Fly's Eye".Extending the analogy, the read-out electronic (amplifier-discriminator) boards were referred to as "ommatidial boards" (OMB), as the neural tissues behind the retina are known as ommatidial nerves. The major parameters of the Fly's Eye Experiment are listed in the table below.

The Cornell detector is triggered by requiring a coincidence between any two adjoining pixels. The signals are piped to a bank of 3" cathode ray tube displays, and recorded on 70 mm film. This detector operated for several years but was not sensitive enough to detect UHE cosmic rays reliably. In particular, the 0.1 m2 lenses are too small to collect sufficient light, and the atmosphere in Up-state New York is too contaminated with water vapor and aerosols.

Fly's Eye Detector Parameters 
Number of mirrors 67
Diameter of mirrors 1.575 m
Focal length 1.500 m
Number of PMT (and Winston cones) 880
Mirror Obscuration by PMT cluster 13%
Mirror-cone efficiency product ~0.7
PMT type EMI 9861 B
Peak PMT quantum efficiency at 360 nm 0.21
Angular Aperture per PMT 91.5 mr
Solid Angle per PMT 6.57 msr
Number of electronic channels 2640
Charge dynamic range 105 linear
Time Resolution

25 ns