The Primary Science Aperture (PSA) is a circular field stop 2.5 arcsec (700 μ
m) in diameter. It is located, not at the HST
focal surface, but near the point of the circle of least confusion. The aperture transmits ≥
95% of the light from a well-centered, aberrated point-source image delivered by the HST
optics. The PSA is used for almost all COS science observations. It is in place, ready to use, at the start of a new visit. Note that, when the PSA is in place, the Wavelength Calibration Aperture (WCA; see below) is also in place and available to acquire simultaneous wavelength-calibration spectra. External light entering the PSA and internal light entering the WCA are dispersed by the same grating. Thus, for a given grating and central-wavelength setting, no additional motion of the Aperture Mechanism is required to obtain a wavelength-calibration exposure.
Like the PSA, the Bright Object Aperture (BOA) is 2.5 arcsec (700 μ
m) in diameter, but it incorporates a neutral-density (ND2) filter. The transmission of the ND2 filter varies with wavelength (Figure 3.2
), but is roughly 0.6%. The BOA is offset from the PSA by 3.7 mm (about 13 arcsec) in the cross-dispersion direction opposite the WCA. For scientific observations the aperture block is shifted, via movement of the Aperture Mechanism, to place the BOA in the position normally occupied by the PSA. Thus, spectra obtained through either the PSA or BOA use the same optical path and detector region (for a given channel), and so may employ the same fixed pattern calibrations. Moving the BOA into place for scientific use shifts the WCA as well, precluding simultaneous use of the WCA for a wavecal exposure
. Before or after an observation through the BOA the Aperture Mechanism must be moved to properly position the WCA, so that a wavecal exposure may be obtained.
The COS FUV optical path is illustrated schematically in Figure 3.3
. To maximize throughput a single FUV grating is used to disperse the light, remove the spherical aberration introduced by the HST
primary mirror, and focus the beam onto the detector. Because the FUV gratings introduce astigmatism in the direction perpendicular to dispersion, the height of the spectrum varies with wavelength (Section 5.1.9
). Given the location of OSM1 in the HST
optical path it is possible for the FUV gratings to disperse, focus, and correct the beam optimally only for a point source that is centered in the aperture. Performance is degraded when the source is moved away from the aperture center. Fortunately, this degradation is low for displacements up to about 0.4 arcsec (Section 8.8
The COS FUV channel provides spectra from 900 to 2150 Å at low and moderate spectral resolution (Section 5.1
). The FUV detector is described fully in Chapter 4
, but it is important to note that it consists of two independent detector segments with a small physical gap between them. Light falling into the gap is not recorded. Though the gap prevents a continuous spectrum from being obtained at a single central-wavelength setting, the missing wavelengths can be recovered by obtaining additional exposures at other central-wavelength settings (corresponding to small rotations of the OSM1 mechanism; see Section 5.5
The COS NUV channel, illustrated schematically in Figure 3.4
, provides coverage from about 1650 to 3200 Å at low and moderate spectral resolution. If the NUV channel is to be used OSM1 is turned to place mirror NCM1 in the beam. NCM1 corrects the beam for the spherical aberration of HST
, magnifies it by a factor of four, and directs it to the NUV Collimating Mirror 2 (NCM2). The NCM2 collimates the light and directs it to Optics Select Mechanism 2 (OSM2). OSM2 holds five optical elements: the four NUV diffraction gratings (G185M, G225M, G285M, and G230L), and a mirror for target acquisitions or imaging.
To accommodate the NUV detector format, dispersed light from the NUV gratings is imaged onto the detector by three parallel mirrors (NCM3a, 3b, 3c). For the medium-dispersion gratings the spectra appear as three non-contiguous 35−
40 Å stripes on the MAMA detector, providing 105−
120 Å wavelength coverage per exposure. The low-dispersion grating provides ~400 Å per stripe. The layout of the stripes is shown in Figure 4.8
. The gratings can be shifted via slight rotations of OSM2 to cover the entire NUV wavelength band. The NCM3 mirrors are spaced such that several correctly-chosen exposures will produce a complete spectrum, from the low end of the short-wavelength stripe to the high end of the long-wavelength stripe.
For imaging observations OSM2 is turned to place a mirror in the light path instead of a grating (Figure 3.5
). When used in direct specular reflection this mirror is designated as MIRRORA.
For bright targets, the flux can be attenuated by adjusting OSM1 so that the order-sorting filter in front of the mirror reflects the light onto the detector. This configuration is referred to as MIRRORB
. COS imaging is described in Chapter 6