One of the instruments on the Apollo Telescope Mount attached to Skylab was a telescope designed to photograph the solar disk in x-ray light (wavelength 5 to 60 Angstroms). This imaging x-ray mirror is an engineering prototype fabricated at the Marshall Space Flight Center in 1967. It was built to refine machining techniques for producing the required optical characteristics for the two-element, double reflecting grazing incidence x-ray telescope (SO56), which was built at the Goddard Space Flight Center (GSFC). The mirror was transferred to NASM in May 1971 and loaned to the Deutsches Roentgen Museum in Remscheid, Germany. The artifact was on display there until the end of 1981. It was returned to NASM in February 1982 and restored by the Optics Division at GSFC. Following that it was on display in the Stars Gallery until October 1997.

The mirror consists of an open cylinder fabricated from aluminum (6061T6), nickel plated to a depth of approximately 0.7 mm (0.03 inches). The interior of the tube, the optical surface itself, is machined to form paraboloid and hyperboloid surfaces designed to focus incoming x-rays onto the film plane. The reflecting surfaces are polished to 1/2 wavelength of visible light on the front paraboloid surface and to 1-1/2 wavelengths of visible light on the hyperboloid surface. The parabolic and hyperbolic co-axial surfaces are in tandem and consitute the two optical surfaces of a classical Cassegrain reflecting telescope; they are designed for reflection beyond the critical angle so that x-rays will be deflected and not absorbed.