Excerpt:
The first usage of X-ray telescopes for astronomy was for observing the Sun, the only X-ray source in the sky producing an abundance of signal. Because the Sun is so bright in X-rays, the focusing element can be small, and photographic film can be used as a detection medium. The first X-ray picture of the Sun employing a rocket-borne telescope was taken by John V. Lindsay of the NASA Goddard Space Flight Center and collaborators in 1963. The first orbiting X-ray telescope flew on Skylab in the early 1970's and recorded over 35,000 full-disk images of the Sun over a 9-month period.