The Argus Array
The Argus Array is a Federation subspace telescope operating in the 24th century. The Array was located three lightyears from Cardassian space.
The Argus Array malfunctioned in 2368, following contact with a Cytherian probe, which disrupted the Array's computer systems. The problem was fixed by the USS Enterprise-D (TNG: "The Nth Degree")
The Enterprise-D visited the Argus Array again in 2370 after it stopped sending data for the third time in a year. In a series of parallel universes encountered by Lieutenant Worf, the cause of the Array's shutdown was varied. In one, the Cardassians had reprogrammed the Array to spy on Federation outposts; in another, it had suffered a simple mechanical failure. The outcome of the malfunction also varied - in one universe, the Cardassians destroyed the Array; in another, the Bajorans did so. (TNG: "Parallels")