Back-up film door assembly that was to have been used on planned second Apollo Telescope Mount (ATM). This door provided access for renewing instrument film supply by astronauts. The ATM was the major scientific instrument aboard Skylab, which was put into Earth orbit in May 1973 and operated for 8 months. ATM included eight major scientific instruments as well as a number of smaller experiments, and were designed to be operated and serviced by three sets of visiting astronauts. These instruments observed the sun in a broad range of the spectrum from the visual through the x-ray regions. ATM also included a white-light coronagraph, which examined the Sun's outermost atmosphere. It was transferred to NASM by NASA's Marshall Space Flight Center in 1986.