NIXT

The NIXT, or Normal Incidence X-ray Telescope, was a sounding rocket payload flown in the 1990s by Professor Leon Golub of the Smithsonian Astrophysical Observatory, to prototype normal-incidence (conventional) optical designs in extreme ultraviolet (EUV) solar imaging. In the EUV, the surface of the Sun appears dark, and hot structures in the solar corona appear bright; this allows study of the structure and dynamics of the solar corona near the surface of the Sun, which is not possible using visible light.

NIXT and its sister rocket, the MSSTA, were the prototypes for all normal-incidence EUV imaging instruments in use today, including SOHO/EIT, TRACE, and STEREO/SECCHI.

...
Wikipedia