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Simple Design Optimized for Spectroscopy

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; 815 << 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 places in the optical path 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. For NUV obsercations, the same selection mechanism may also be used to place a mirror in the light path in place of the grating, reflecting light onto a second optic selection mechanism. The COS FUV and NUV optical paths are illustrated schematically below.

LAST UPDATED: 08/27/2021

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