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.
There are four apertures on the COS Aperture Mechanism: two look at the sky for science exposures, and two are for calibration. The COS science apertures are field stops and are not traditional entrance slits like those used on STIS and earlier HST spectrographs. Thus, they do not project sharp edges on the detectors.
The two science apertures are the primary science aperture (PSA) and the bright object aperture (BOA). The PSA is a 2.5 arcsec diameter field stop that transmits 95% of the light from a well-centered, aberrated point-source image delivered by the HST optics. The BOA is a 2.5 arcsec diameter field stop used for observations requiring flux attenuation. The PSA should be used for most normal science observations.
The two calibration apertures are the wavelength calibration aperture (WCA) and the flat-field calibration aperture (FCA). The FCA is for calibration only and is not available for observers. The WCA is offset from the PSA in the cross dispersion direction and is used to obtain wavelength calibration spectra. Light from external sources cannot illuminate the detector through the WCA; instead the WCA is illuminated by one of two Pt-Ne wavelength calibration lamps.
For further information about the COS apertures, see Section 3.1.2 of the COS Instrument Handbook.
The COS FUV gratings are concave and have holographically-generated grooves to provide dispersion and correct for astigmatism. The gratings have aspherical surfaces to correct for HST’s spherical aberration. The FUV “M” gratings have been ion etched to produce triangular groove profiles for better efficiency. The G140L grating has grooves with a laminar profile. All FUV gratings are coated with MgF2over aluminum.
The NUV gratings are flat and were not constructed holographically. The NUV MAMA has low but measurable sensitivity at FUV wavelengths, and with some gratings second-order light could contaminate the spectrum. To minimize this effect, the optics are optimized for wavelengths above 1600 Å. Even so, it is possible for second-order light to appear in G230L spectra, especially in the long-wavelength stripe.
Full wavelength ranges for each central wavelength are listed in the COS Instrument Handbook in Table 5.3 for the FUV and Table 5.4 for the NUV. The resolving power, dispersion, plate scale, and wavelength range for each central wavelength are summarized in Section 13.3 of the COS Instrument Handbook.
The two COS detectors are photon-counting devices that convert light focused on their photosensitive front surfaces into streams of digitized photon coordinates. A discussion of the detector characteristics and performance of each detector is provided in Chapter 4 of the COS Instrument Handbook.
The FUV detector is a windowless, cross delay line micro-channel plate (MCP) stack optimized for the 1150 to 1775 Å bandpass. The active front surface of the detector is curved to match the focal surface radius of curvature of 826 mm. To achieve the length required to capture the entire projected COS spectrum, two detector segments are placed end to end with a small gap between them. The two detector segments are independently operable; loss of one segment does not compromise the independent operation of the other. Each detector segment has an active area of 85 x 10 mm digitized to 16384 x 1024 pixels.
The NUV detector is a multi-anode microchannel array (MAMA) optimized for spectroscopic observations in the 1700-3200 Å bandpass. Target acquisitions that are not performed with dispersed light in the FUV channel are performed in the NUV channel with this detector. The COS MAMA is the backup for the STIS NUV MAMA flight unit. The MAMA high-resolution (pixel sub-sampling) mode available with STIS is not supported with COS.