Characterization of the Post-Launch Line Spread Function of COS

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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
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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

LSF model G130M G160M G185M G225M G285M
no WFEs 24 24 36 37 39
with WFEs (wav-avg) 41 37 51 49 47
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Example Impact of Mid-Frequency Errors on Spectra

• FUV Sk 155 in the SMC (O9 Ib, V12.4), 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

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Application of COS LSF models
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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 lt 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 gt 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

3s Limiting Equivalent Widths (assumes S/N10
pixel-1 in continuum)
Rough calculation result may be improved with
flux-weighted extraction, etc.
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• http//www.stsci.edu/hst/cos/performanc
  e/spectral_resolution