The COS optics correct for the spherical aberration of the HST primary mirror, but not for the mid-frequency wavefront errors (MFWFEs) due to zonal (polishing) irregularities in the
HST primary and secondary mirrors. As a result, the COS spectroscopic line-spread function (LSF) has extended wings and a core that is slightly broader and shallower than a Gaussian. The extended wings of its LSF limit the ability of COS to detect faint, narrow spectral features. The effect is greater at short wavelengths, and it may have consequences for some COS FUV science. The most severely impacted programs are likely to be those that
Initial results from an analysis of the on-orbit COS LSF at the original lifetime position are reported by Ghavamian et al. (2009) in COS ISR 2009-01. They find that model LSFs incorporating
HST MFWFEs are required to reproduce the absorption features observed in stellar spectra obtained with COS.
Figure 3.6 shows model LSFs computed for grating G130M at 1309 Å. The solid line represents a model incorporating the spherical aberration of the
HST OTA. It is well-fit by a Gaussian with FWHM = 6.5 pixels. The dot-dash line represents a model that includes the MFWFEs at the original lifetime position while the dashed line represents a model that also includes the MFWFEs at the second (new) lifetime position. The model at the original lifetime position has a FWHM of 7.9 pixels, slightly larger than that of the solid curve, and broad non-Gaussian wings. The model at the second lifetime position has a very similar FWHM. The non-Gaussian wings can hinder the detection of closely-spaced narrow spectral features. Model LSFs for all of the COS gratings at the original lifetime position are available on the
COS Web site. Model LSFs for the second lifetime position will be added in the near future.
When a substantial fraction of the power in an LSF is transferred to its extended wings traditional measures of resolution, such as the FWHM of the line core, can be misleading. For example, an observer assuming that the resolving power R = 16,000 at 1200 Å quoted for the G130M grating represents the FWHM of a Gaussian would mistakenly conclude that COS can resolve two closely-spaced narrow absorption features, when in fact it may not be able to. Nevertheless, the FWHM is a convenient tool, and we use it to describe the COS gratings in tables throughout this handbook. When using these tables keep in mind that the quoted resolving power
R is computed from the empirically-determined FWHM of the line core, and careful modeling may be needed to determine the feasibility of a particular observation or to analyze its result.
Figure 3.7 shows the resolving power of the FUV gratings for three cases: the first assumes a Gaussian LSF of FWHM = 6.5 pixels with no MFWFEs from the
HST OTA (solid lines), the second is an LSF model with the MFWFEs included for the original lifetime position (dot-dash lines), and the third is an LSF model with the MFWFEs included for the second lifetime position (dashed lines). In the second case the FWHM of the LSF is calculated directly from the line profile by taking the width at half the peak (from Table 1 of
COS ISR 2009-01). The MFWFEs reduce the resolving power of the G130M and G160M gratings by ~20%. The G140L profile is least affected by the MFWFEs, due to its lower dispersion. Preliminary measurements of the resolving power of the G130M grating at
λ < 1150 Å are presented in
Section 5.1.4.
The broad core and extended wings of the COS LSF increase the limiting equivalent width for absorption features in COS spectra. Figure 3.8 shows the limiting equivalent widths as a function of wavelength for a 3
σ FUV detection of absorption features at S/N = 10 per pixel. A series of Gaussian spectral features with nominal Doppler parameters of
b = 0, 10, 25, 50 and 100 km/s have been convolved with both a Gaussian instrumental LSF and the modeled on-orbit COS LSF for the G130M and G160M gratings. The results are similar for the NUV gratings, although the effect of the MFWFEs is more moderate for the long-wavelength G285M grating.
Figure 3.10 shows the fraction of enclosed energy within the LSF, measured from the center of the profile, for both the FUV and NUV channels. The differences between the modeled on-orbit LSFs (MFWFEs included) and the Gaussian LSFs without MFWFEs are apparent in both spectroscopic channels. Though inclusion of the MFWFEs at longer NUV wavelengths widens the FWHM of the on-orbit LSF models only slightly, the wider wings decrease noticeably the spectral purity and the contrast level of the observed spectra.
Table 3.2: Distance from Line Center (in Pixels) Versus Enclosed Energy Fraction and Wavelength for the G130M Grating.
1
Table 3.2,
Table 3.3, and
Table 3.4 present the enclosed-energy fractions for gratings G130M, G160M, and G140L respectively. The G130M and G160M data include the effects of scattering and are for both the original and second lifetime positions. The G140L data are taken from Ghavamian et al. (2009) and are for the original lifetime position. The G140L data do not include the effects micro-roughness.
