This is the sunward canister from a backup unit of the Apollo Telescope Mount. It includes the 8-foot cap, or "sun shield" that houses the light entry ports for the ATM instruments, the handholds, and the locking devices. Built by NASA's Marshall Space Flight Center, the telescope mount was attached to Skylab's multiple docking adapter. It held eight major scientific instruments and several smaller experiments that were operated from within Skylab. Astronauts used the instruments to study the Sun's surface and atmosphere in visible and ultraviolet light, as well as by x-ray emission. Most of the instruments recorded data on photographic film, which the astronauts replacedas needed.
NASA transferred the telescope mount to NASM in 1982. It was displayed in the Stars gallery from then until the exhibit closed in 1997.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.