X-ray all-sky monitors

Considerations for an all-sky monitor for the Ariel-5 mission are evaluated. The monitors with the best signal-to-noise utilize the pinhole-camera principle, and are also least expensive in hardware and data analysis costs. The sensitivity sufficient to sensibly monitor compact extragalactic sources requires a system with approximately 100 times the sensitivity of the Ariel-5 ASM. A system identical in size to the Ariel-5 ASM can be made with approximately 5 times better sensitivity, and can satisfy the minimum requirements for this mission if it spins. An all-sky monitor appears to be essential to assure that rare occurrences in compact objects can be observed with the pointed experiment, and to ascertain that large amounts of pointing time will not be wasted by observing at the wrong times.