The Coronese Historical Telescope Array under construction in 5401AT (note that this image was taken in 5406 by a similar array in HIP 116317)
Cliotelescopy is the use of long-distance observation to study historical events. An telescope array in the Inner Sphere can observe events in the Periphery as they were six thousand years earlier (relative to the Inner Sphere's simultaneity plane). One on the far side of the Terragen Sphere can see even further into the past.
Cliotelescopy is rarely used to observe the Inner Sphere, both because light from the founding of these worlds has passed beyond the periphery and because they have kept detailed historical records for millennia. The Outer Volumes provide much better targets. Light from these regions is still passing through the Terragen Sphere, and many colonies may have poor record keeping or information loss due to blights or wars. The light from the first establishment of a colony on the Periphery will be received a few years or decades later, by telescopes in the nearest colonies. Using the Argus Array and telescopes based further out, the past history of most colonies established so far can be observed in significant detail. Only a few of the most distant colonies are too far away to be observed yet, but the light will reach a telescope array in due course.
All Archailects use cliotelescopy as a matter of course. It has been a crucial tool in studying the emergence of the Amalgamation, the Oracle Machines, and the Gehenna event. Cliotelescopy is also used to keep an eye on colonies that are members of the Sepirotic empires, but are not connected to the Wormhole Nexus. Almost certainly the Sephirotic Archailects use cliotelescopy to keep an eye on each other as well. Although the Sephirotic Archailects use the data collected by the Argus Array for this purpose (and many others) it has been established that they have constructed many independent godtech telescope arrays of their own; ironically this fact has been determined by modosophont-level cliotelescopy.
Hobbyist cliotelescopy is popular, and many modosophonts travel deeper into the Terragen Bubble to observe crucial events in the colony's history in 'relativistic realtime', as one brochure put it. The Coronese Historical Telescope Array is one of the most popular destinations for tourists.