In the question At what time in the Apollo 14 mission did the maneuver shown in this image happen, and what exactly was happening? I've included the image and information below. The caption says that Apollo 14 CM & LEM and the Saturn IV B were "40,000 miles from Earth" at the time, or about 64,000 km. @X's answer estimates this to be at about
03:02:29 into the mission.
Question: How far away can spacecraft be seen with an optical telescope?
note: For the purposes of this question "optical" means near-UV through thermal IR, let's say 0.1 to 30 microns, and "seen" means it's existence and position determined in some way, either via recorded digital or film image, photomultiplier, or even reliable reporting by an experienced visual astronomer.
In a search for information for this answer I received this google books link of Comment: The Relevance of Space, by Arthur Kantrowitz, Bulletin of the Atomic Scientists 197, 4 April 1971, pp 32-33. It includes the image below, I've retyped the caption for easier reading.
Apollo 14 (at arrow tip) is nothing more than a speck in the sky at 40,000 miles from Earth. This picture of the spacecraft was taken by Justus Dunlap of Northwestern University’s Corralitos Observatory, and shows the capsule separating from the S4B rocket. The capsule itself is visible as a pinpoint at the apex of the fan and the fan itself is expanding rocket exhaust.