A new type of Fresnel zone plate has been constructed which can focus ultraviolet radiation of any wavelength down to the soft x-ray region. It consists of a set of thin circular gold bands made self supporting by radial struts, leaving the transparent zones empty. Experimental tests at 6700, 4358, and 2537 A showed that the theoretical minimum angular resolution obeys the Rayleigh criterion, sinθmin = 1.22λ/D. The diameter of the zone plate is D=0.26 cm and contains 19 opaque zones, the narrowest of which measured about 20 µ across. The zone plate was better than the optimum pinhole in resolution by a factor of about 6 and in speed by a factor of 40. The zone plate produced pictures that compared favorably with those made with a lens of similar focal length and aperture. The lens was about 20 times faster than the zone plate at 4358 A, but at 1000 A the zone plate would have been far faster than the lens. Focusing tests are contemplated at 1000 A and at 100 A where lenses and mirrors, the conventional image-forming devices, may fail. The angular resolution at 2537 A was close to the theoretical value of 1.2×10−4 rad and held over a field of at least 1.75×10−2 rad, which is 2.0 times the angle subtended by the sun’s disk at the earth. A zone plate telescope, operating in the soft x-ray or extreme ultraviolet region, far above the earth’s atmosphere in an orbiting satellite, now seems possible.
ALBERT V. BAEZ, "Fresnel Zone Plate for Optical Image Formation Using Extreme Ultraviolet and Soft X Radiation," J. Opt. Soc. Am. 51, 405-412 (1961)