During the Thal rule of Skaro, deep-space telescopes were among the various technological advances made by the Thals as part of their interest in space travel. They existed in Skaro's orbit.
The telescopes were still in orbit at the end of the Dalek Prime's reclamation of Skaro, allowing the Daleks to use them to discover the planet Earth. (PROSE: The History of the Daleks [+]Loading...["The History of the Daleks (short story)"])