COS has a simple optical design that minimizes the number of reflections required to disperse and detect ultraviolet light in its two optical channels. The instrument is optimized for high-throughput spectroscopy of point sources but may also be used to observe extended objects, albeit with limited spatial information and degraded spectral resolution.
Light enters COS through a 2.5 arcsec diameter circular aperture and encounters an optical element that enables far-ultraviolet (FUV; 1150 << 2050 Å) or near-ultraviolet (NUV; 1700 << 3200 Å) observations. In the FUV channel, the light illuminates a single optical element - a concave holographically-ruled diffraction grating. An optic selection mechanism configures either the low-dispersion grating or one of two medium-dispersion gratings for the observation. The grating disperses the light, corrects for the HST spherical aberration, and focuses the light onto a crossed delay-line microchannel plate (MCP) detector. The same selection mechanism may also be used to place a mirror in the light path in place of the grating for NUV observations. The COS FUV and NUV optical paths are illustrated schematically in Figures 1 and 2.
For further information, see Chapter 1 of the COS Instrument Handbook.