Following a successful program to investigate the physics of ultra-hig
h-pressure proportional counters, a counter array has been developed f
or hard X-ray astronomy. A parallel investigation has evaluated the pe
rformance of a large-area phoswich scintillator detector for the same
purpose. The two detectors have been integrated in a balloon-borne pay
load, the Astrophysical X-ray Experimental Laboratory (AXEL). This pap
er describes the instrumentation aboard the payload.