COS Instrument Design

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.

Figure 1: The COS FUV optical path. Only one reflection is required to place the dispersed light onto the FUV microchannel plate detector. An optic selection mechanism configures one of the gratings for the observation. A mirror can also be inserted in place of the FUV grating to divert light into the COS NUV channel.

Figure 2: The COS NUV optical path. Four reflections are required to place the light onto the NUV MAMA detector. The NCM1 mirror is placed in the light path by the FUV optical element selection mechanism.

For further information, see Chapter 3 of the COS Instrument Handbook.