Lyman/FUSE, the Far Ultraviolet Spectroscopic Explorer, is a proposed low earth orbit mission to explore the
1OO-16OGi spectra of diverse astronomical sources. The simultaneous design goals of high spectral resolution, high
sensitivity, high signal to noise ratio, limited slit imaging capability, wide spectral coverage, compactness, and high
stability were a formidable challenge. In the design, a Wolter type II glancing incidence telescope (70 cm aperture,
f/1O) feeds two spectroscopic channels. A boresighted fme error sensor is used to point the instrument. In the
4oo-16oc1i range, a 1.84m Rowland circle spectrograph uses five near normal incidence gratings with a common
MAMA detector. To achieve acceptable resolving power and limited imaging with a fast telescope, the grating
aberrations must be significantly reduced below those obtained with toroidal or ellipsoidal gratings. A modified
ellipsoidal grating is used to achieve resolving powers of 30000 in the three high resolution bands covering 91O-125Q.
A separate EUV channel will explore the 1OO-35O range with lower spectral and spatial resolution. The throughput
in this range is improved by more than an order of magnitude over the Extreme Ultraviolet Explorer (EUVE), when
EUVE is operating in its spectroscopic mode. The optical design and results of ray tracing studies are presented, as
well as the expected effective area in each channel.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.