Short answers, pulled from the story.
The Apollo Telescope Mount flew aboard Skylab as a crewed solar observatory, observing the Sun in wavelengths from 2 to 7000 Angstroms. Its eight instruments covered soft X-rays, ultraviolet, and visible light, and it was operated by astronauts from 1973 to 1974.
The ATM originated as a project tied to the Apollo spacecraft before it became part of Skylab. It grew out of the Apollo Applications Program, a late 1960s NASA initiative that sought new uses for the infrastructure developed for the Moon landings.
Six of the ATM's instruments recorded data on film, and astronauts had to conduct spacewalks to manually retrieve the film canisters from the instruments. The heaviest canisters weighed 40 kilograms and could hold up to 16,000 frames of film.
The Apollo Telescope Mount's film instruments accumulated more than 150,000 successful exposures across the Skylab missions. Almost 30 film canisters were loaded, used, and returned to Earth inside the Apollo capsule at the end of each mission.
A backup Apollo Telescope Mount spare, with instruments still mounted to its frame, was restored in 2015 and placed on public display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The restoration required repairing Kapton layers that had degraded over four decades.
As of 2006, the original ATM film exposures were on file at the Naval Research Laboratory in Washington, D.C., accessible to interested researchers. The more than 150,000 frames were returned to Earth aboard Apollo capsules at the end of each Skylab mission.