1. Characterization of the Post-Launch Line Spread Function of COS
Parviz Ghavamian
A. Aloisi, C. Proffitt, D. Lennon, G. Hartig, G. Kriss, C. Oliveira, D. Massa, T. Keyes
(STScI)
T. Delker (Ball Aerospace)
S. Osterman (Colorado)

2. The COS LSF: Ground Tests
During TV03/TV06, light from PtNe lamp was passed into COS after passing through RAS/Cal stimulus, which mimicked low- frequency errors from HST OTA (spherical aberration, astigmatism, coma...)
COS gratings correct for these LFEs, designed to produce R ~ 20,000 across 80% of bandpass in the resulting M-mode spectra (FUV and NUV)
The LSF profile in the FUV was well described by Gaussian shape during TV03/TV06
In NUV, LSF profile has non-Gaussian wings due to MAMA detector response; optical models give FWHM ~ 2.5 pixels
Gaussian
6.5 pixels FWHM

3. COS LSF: Effect of Mid-Frequency Errors
During SMOV, the shape of the on-orbit COS LSF was found to differ from the profile in ground testing, due to the presence of zonal (polishing) errors on primary and secondary HST mirrors
Zonal errors introduce mid-frequency errors (~18 nm) into beam entering COS; not included in RAS/Cal testing
Result is a lowered, broadened core and broad non-Gaussian wings on LSF
Mid-freq. WFEs are strongest at shortest wavelengths of COS (~1150 Å), dimin- ishing with increasing wavelength, and becoming negligible beyond ~ 2500 Å 3
COS/NUV Model LSF: Mid-Frequency WFEs included
Krist & Burrows (1995)
Fraction of flux falling outside of FWHM
LSF model
G130M
G160M
G185M
G225M
G285M
no WFEs
24%
36%
37%
39%
with WFEs (wav-avg)
41%
51%
49%
47%

4. Example: Impact of Mid-Frequency Errors on Spectra
FUV: Sk 155 in the SMC (O9 Ib, V=12.4), observed during SMOV with G130M, G160M gratings
E140H STIS echelle spectra exist (R ~ 114,000; E0.2x0.09) on archive and as part of Cycle 17 calibration data
Wings on LSF cause:
Significant filling-in of saturated absorption features
Merging of narrow absorption lines into wings of nearby saturated absorption features
Model the COS spectrum by convolving STIS E140H spectrum with model LSFs

5. Application of COS LSF models

6. Impact on COS Science
Programs using full resolution of FUV G130M, G160M and NUV G185M:
Close together lines harder to isolate
Weak, narrow absorption features (b < 35 km s-1) more difficult to detect at a given S/N; lower contrast between core and wings
Analysis of saturated absorption lines will require full consideration of LSF
Programs minimally affected are:
Those observing broad lines (b > 35 km s-1)
SED and continuum flux measurements
G140L observations, since they typically are done for SED measurements, or to observe sources with line widths larger than instrumental resolution
3σ Limiting Equivalent Widths
(assumes S/N=10 pixel-1 in continuum)
dotted: with WFEs
solid: no WFEs
Rough calculation; result may be improved with flux-weighted extraction, etc.