FAR-ULTRAVIOLET SPECTROGRAPHS - THE IMPACT OF HOLOGRAPHIC GRATING DESIGN

One of NASA's major scientific initiatives in astrophysics, the Far Ul traviolet Spectroscopic Explorer, is an orbiting observatory designed to perform high-resolution spectroscopy of stellar objects between 910 and 1150 Angstrom. To achieve the 100 cm(2) of effective area require d to observe faint quasars, the mission has adopted an unconventional design that couples grazing-incidence Welter optics and an aberration- corrected Rowland circle spectrograph. The projected cost of the satel lite has, however, become very high, driven in large part by the cost of the grazing optics and the side effects of compensating for their r elatively poor performance. The logic that leads to the current design is reanalyzed, and it is argued that because of the technical develop ments of the last 7 years, grazing incidence is no longer the most att ractive alternative. In particular, the application of aberration-corr ected, spherical substrate, holographic gratings now greatly simplifie s the design and thereby reduces the cost. A new design is presented t hat substantially outperforms the old and that is easier to fabricate.