For each exposure the observer selects a detector (FUV or NUV), a grating, a central wavelength, an FP-POS setting, one of the two apertures (PSA or BOA), and a data-taking mode (
TIME-TAG or
ACCUM).
Chapter 13 provides detailed specifications for each grating and aperture. Note that the two channels cannot be used simultaneously as the NUV channel is fed by the NCM1 mirror on the FUV optics select mechanism (OSM1). As of this cycle, FUV science observations with the BOA aperture are considered “available-but-unsupported” (See
Section 2.4).
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COS is considerably more sensitive than STIS and earlier-generation HST instruments at comparable spectral resolutions, particularly in the far ultraviolet. Effective areas for targets observed through the PSA are shown in
Figure 5.1,
Figure 5.2, and
Figure 5.3. Note that the COS sensitivity changes with time (
Section 5.1.5), so the plots and tables in this handbook are based on predictions for the middle of Cycle 23 (April 2016). Please consult the
COS website for updated information.
Figure 5.3 compares the effective areas of the G130M and G140L gratings at the short-wavelength end of the COS bandpass. From this figure we can draw two fundamental conclusions. First, COS can obtain useful spectra at wavelengths between 900 and 1150 е. Second, the contrast between the throughputs at 1070 and 1150 е is roughly a factor of 100.
When G140L is used with CENWAVE=1280 wavelengths as long as 1200 е can fall on Segment B. For many targets the count rate at 1150 е will exceed the local bright limit, while the count rate at shorter wavelengths is perfectly safe. Consequently, turning Segment A off and using
FP-POS=4 is the recommended approach for observing bright objects below 1150 е with the G140L grating.
When grating G130M is used with CENWAVE=1055 Segment B records wavelengths shortward of ~1050 е, depending on the
FP-POS setting employed (
Section 13.3). Using this grating mode with Segment A turned off further reduces the danger of high count rates at longer wavelengths. Special configurations to allow proper wavelength calibration must be used (see
Section 5.7.4). Note that both the sensitivity and resolving power (
Section 5.1.4) of G130M are greater than those of G140L in this wavelength range.
For the G230L grating stripes B and C are still affected by second-order flux. When CENWAVE=3360 stripe B is contaminated by second-order light beyond 3200 е. In a spectrum of the planetary
nebula NGC 6833 obtained with
CENWAVE=3360 second-order light accounts for roughly 40% of the flux at 3320 е, and more that 50% at 3500 е (
Figure 5.4). Above 1700 е stripe C is more sensitive to second-order light than first-order by design (
Table 5.4), but on-orbit observations reveal that first-order light is detectable at a level of 5% from wavelengths greater than 3700 е at all central-wavelength settings (
COS ISR 2010-01).
In the FUV channel second-order light is present at long wavelengths (λ > 2150 е) in spectra taken with G140L
CENWAVE=1280 FUVA. These photons are rejected by the COS pipeline during processing, but available in the “net counts” column of the
*x1d*.fits files.
The spectroscopic resolving power (R = λ / FWHM) of each COS grating is listed in
Table 5.1, and plotted as a function of wavelength for the FUV gratings in
Figure 3.7 and
Figure 5.5. These values correspond to the FWHM of the model line-spread functions (LSFs) that are described in
Section 3.3, which have been validated with on-orbit measurements (
COS ISR 2013-07, Roman-Duval et al., 2013). Preliminary measurements and model predictions of the resolving power for the new 1055, 1096, and 1222 е central wavelength settings of the G130M grating are shown in
Figure 5.5. Recent adjustments made during Cycle 20 to the focus for the 1055 and 1096 central wavelength settings of the G130M grating have increased their short wavelength resolution by a factor of several over that available before Cycle 20. The quantitative values quoted here for this improved resolution are based on ray-trace models and actual resolution may be slightly different. However, preliminary comparison with on-orbit test data appears consistent with the predictions of these models. A comparison of data obtained with these revised 1055 and 1096 settings with archival
FUSE data for the same target is shown in
Figure 5.6. Users should note, however, that for each of these modes the focus values have been set to optimize the resolution over a limited part of their wavelength range, and it will be necessary to use multiple settings to get the maximum resolution over the full FUV wavelength range
(
Figure 5.5).
The COS LSF is not a Gaussian, so simple rules relating R to the observability of narrow spectral features may not apply. Careful modeling of the LSF may be required to determine the feasibility of an observation. See
Section 3.3,
COS ISR 2009-01, and
COS ISR 2013-07 for further information about the COS LSF.
Observations of HST primary spectrophotometric standard stars show that there is a significant time dependence of the COS sensitivity for some spectrographic modes. The reflectivity of the G225M and G285M NUV gratings, which are coated with bare aluminum (rather than MgF
2 over aluminum like the other gratings), showed a time-dependent degradation before launch that has continued on orbit, decreasing at a rate of ~3% and ~10% per year respectively, independent of wavelength.
The COS data-reduction pipeline calcos includes a time-dependent sensitivity calibration, and regular monitoring of standard stars is used to update the sensitivity reference files. The
ETC reflects the best estimate of the sensitivity for the middle of Cycle 24.
High S/N observations of standard stars have allowed us to uncover what is likely an optical effect that leads to a dip in the sensitivity at about 1180 е. Figure 5.8 shows the spectrum of WD0320-539 observed in Program 12086 with the G130M/1291 and G130M/1327 settings. The sensitivity dip, of ~10% of the continuum value, is seen near 1180 е in the G130M/1291 data and near 1190 е in the G130M/1327 data. This effect is not currently corrected by
calcos. The data shown in
Figure 5.8 were taken at the original lifetime position, but this feature is also present in data taken at the second lifetime position. Preliminary analysis indicates that this feature is not fixed in pixel or wavelength space, but that it moves to longer wavelengths with increasing
CENWAVE settings. More details about this feature will be posted on the
COS website as they become available.
The spatial resolution of COS is affected by the mid-frequency wavefront errors (MFWFEs) via the non-Gaussian wings they introduce (Section 3.3). The NUV channel corrects for the telescope’s spherical aberration, but not for the MFWFEs. For PSA observations the spatial resolution is ~ 0.07 arcsec for G185M and G230L, and 0.06 arcsec for G225M and G285M. For BOA observations, the spatial resolution is 0.29 arcsec for G185M, 0.22 arcsec for G225M, 0.24 arcsec for G285M, and 0.30 arcsec for G230L.
In the FUV the situation is more complex, because of the uncorrected astigmatism in the cross-dispersion direction. Figure 5.9 and
Figure 5.10 show the strong dependence on wavelength of the FWHM of the cross-dispersion profiles for each of the FUV gratings at the first and second lifetime positions, respectively. The spatial resolutions at LP1 and LP2 differ by not more than 3 pixels (0.3 arcsec). At wavelengths for which the profile width is large the spectra of two objects separated by less than the profile width in the cross-dispersion (XD) direction would be combined.
For additional information, please see “FUV Spectroscopic Performance” (COS ISR 2010-09, Ghavamian et al. 2010), “NUV Spectroscopic Performance” (
COS ISR 2010-08, BИland et al. 2010), and “COS/FUV Spatial and Spectral Resolution at the New Lifetime Position” (
COS ISR 2013-07, Roman-Duval et al. 2013).
After on-orbit alignment of COS in HST, fluxes of external targets in the PSA were found to be depressed at the short wavelength ends of the NUV stripes. For the medium-resolution gratings the reduction is about 20% at the first pixel and rises linearly to expected levels over approximately the next 160 pixels (
Figure 5.11). For G230L the reduction is about 15% at the first pixel and extends about 110 pixels. (The slope is the same as in
Figure 5.11; it is as though the ramp were shifted by 50 pixels.) The depression is thought to be due to vignetting of the beam at the NUV camera mirrors that image the spectrum on the detector. Corrections for this vignetting are not included in either the
ETC or the
calcos data-reduction pipeline. Users are advised to consult the
COS webpages, where additional information will be posted as it becomes available.
. 
