FAR-ULTRAVIOLET SPECTROGRAPHS - THE IMPACT OF HOLOGRAPHIC GRATING DESIGN

One of NASA's major scientific initiatives in astrophysics, the Far Ultraviolet 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 required 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 satellite has, however, become very high, driven in large part by the cost of the grazing optics and the side effects of compensating for their relatively poor performance. The logic that leads to the current design is reanalyzed, and it is argued that because of the technical developments of the last 7 years, grazing incidence is no longer the most attractive alternative. In particular, the application of aberration-corrected, spherical substrate, holographic gratings now greatly simplifies the design and thereby reduces the cost. A new design is presented that substantially outperforms the old and that is easier to fabricate.