After passing the focal plane, the light from HST first encounters the COS entrance apertures, which are mounted on the Aperture Mechanism. There are four apertures on the 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. Because COS is a slitless spectrograph, the spectral resolution depends on the nature of the astronomical object being observed. Though not optimized for observations of extended objects, COS can be used to detect faint diffuse sources with lower spectral resolution than would be achieved for point (< 0.1 arcsec) sources.
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 PSA should be used for most normal science observations. The BOA is used for observations requiring flux attenuation. The BOA contains a neutral density (ND2) filter that attenuates the flux by about a factor of 200 at 2000 Å. The BOA-transmission is wavelength dependent. Furthermore, the bright object aperture degrades the spectral resolution by a factor of three or more from nominal design levels. Both science apertures will be fully calibrated and available for use in cycle 17.
The two calibration apertures are the wavelength calibration aperture (WCA) and the flat-field calibration aperture (FCA). The FCA is for calibration only and will not be available for observers. The WCA is offset from the PSA and in the cross dispersion direction and can 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. The wavelength calibration spectrum can be used to assign wavelengths to the locations of detected photons for science spectra obtained through either the PSA or BOA.
- Both the PSA and WCA are available for use at the same time for TIME-TAG mode observations with FLASH=YES ("TAGFLASH" mode).
- The WCA aperture is used only for user-specified target=WAVE wavelength calibration observations.
- The BOA cannot be used with the "TAGFLASH" mode because the WCA is blocked when the BOA is in position.
- At the same time, the BOA is open to light from the sky when the PSA is being used for science (and vice versa); therefore bright object screening for the field-of-view must include both apertures.
For further information, see sections 3.3.1