COS Apertures
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.
Science Apertures
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.
Calibration Apertures
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.
Note: - 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 3.3.2
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