Methodology
Air-fluorescence (calorimetric) technique: the detector images the nitrogen-fluorescence light of the extensive air shower and integrates the longitudinal development profile (Gaisser-Hillas fit) to get the primary energy nearly independently of hadronic-interaction models, unlike ground arrays. Fly’s Eye I: 67 mirrors, 880 PMTs (full operation 1981); Fly’s Eye II (1986): 36 mirrors 3.4 km away, enabling stereo reconstruction. Corrections: Cherenkov-light subtraction, Rayleigh/aerosol atmospheric attenuation (cross-checked against MODTRAN VII, <10% aperture effect), ~10% missing-energy correction. Systematics: fluorescence-yield uncertainty 20%; monocular geometry adds ~20% energy shift (corrected); total systematic uncertainty on energy ⇐ 40%; energy resolution 20-24% (stereo), 27-36% (monocular). Stereo-vs-mono cross-calibration agrees to ~2.5%. Duty cycle ~10% (clear moonless nights).