COS FUV Channel

The COS FUV channel covers the wavelength range 1150 - 1775 Å. The FUV channel employs concave diffraction gratings and a curved detector. It is fundamentally a Rowland spectrograph, modified to meet the specific needs of HST. There is one reflection between the aperture and the detector (see Fig. 1). The gratings have aspheric concave surface figures specified to compensate for spherical aberration. Holographically generated grooves provide dispersion and correct the astigmatism. Ion-etching creates a blaze that optimizes the grating efficiency over a narrow range of wavelengths. Two gratings, G130M and G160M, are used to cover the range 1150 - 1775 Å wavelength range at medium resolution (R = 20,000 - 24,000). Each medium-dispersion grating covers roughly 300 Å in one exposure. A third grating, G140L, covers the entire 1230 - 2050 Å region at lower resolution (R = 2500 - 3500). (The short wavelength cut-off of the low-dispersion grating is designed to avoid bright geocoronal Lyman $\alpha$ emission at 1216 Å.) The three gratings are mounted on a rotating mechanism similar in concept and function to the GHRS carrousel. The detector is a windowless microchannel-plate (MCP) array, with an opaque CsI photocathode, and a cross delay-line readout that has been adapted from the FUSE mission. A fourth optic mounted on the optics select mechanism 1 (OSM1) feeds light to the NUV channel. Table 1 summarizes the COS FUV spectroscopic modes.

 

 

Table 1: COS FUV spectroscopic modes.

Grating	Wavelength Range	Coverage per Exposure	Dispersion (Å / pixel)	Resolving Power ( $\lambda / \Delta\lambda$)
G130M	1150 - 1449 Å	300 Å	~0.0094	20,000 - 24,000
G160M	1406 - 1775 Å	375 Å	~0.0118	20,000 - 24,000
G140L	1230 - 2050 Å	820 Å	~0.0865	2500 - 3500

  

Figure 1: Schematic of the COS FUV channel. Light is received from the HST OTA through an aperture (shown at lower left), is dispersed by a concave diffraction grating (shown at lower right), and finally is recorded on a curved cross delay-line MCP detector (shown mounted within the optical bench at left) adapted from the FUSE mission. The NUV channel is fed by a fourth optic on the rotating optics select mechanism 1.