(covers information from several alternate timelines)
The Argus Array (full name, Argus Subspace Radio Telescope Array) was a Federation subspace telescope in service during the mid-24th century. It was located at the edge of Federation space, three light years from Cardassian territory, in the Alpha Quadrant.
An unmanned structure, the Argus Array consisted of eighteen hexagonal subspace antennae clusters, each powered by an individual fusion reactor and all regulated from a central computer. It was capable of collecting detailed images of objects many light years away. (TNG: "The Nth Degree", "Parallels")
In different quantum realities, the Enterprise-D returned to visit the Argus Array again in 2370 to investigate why it was not transmitting data. In these parallel universes visited 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. In one universe, the Cardassians had destroyed the array; in another, the Bajorans did so. (TNG: "Parallels")