Interference Methods In The Testing And Fabrication Of New- Design Grazing Incidence Gratings

Recent work in the design of soft X -ray and extreme UV spectrographs has identified a new class of reflection gratings whose aberrations do not increase at grazing incidence. To achieve this characteristic, the groove spacings are varied in a continuous manner across a plano grating aperture. Ray traces of the concentric groove grating, the oriental fan grating and variations illustrate their potential for space astronomy and laboratory applications in the wavelength range of approximately 20,10 -1000 A. Both straight -groove and concentric -groove patterns have been successfully fabricated with varied line spacings through mechanical ruling techniques. A concave varied line -space grating has been measured to achieve 70% of its theoretical efficiency in the extreme UV. An interferometric method of fabrication is also discussed, in which the recording wavelength is scaled -up into the far UV or even visible region of the spectrum. This holographic grating is designated "Type V." Two interferometers capable of recording grating wavefront aberrations in the extreme UV and soft X -ray are proposed.