Improved COS Medium Resolution FUV Line Spread Functions

TIPSJuly 21, 2011

Jerry Kriss

The COS LSF has Broad Wings

The broad wings on the COS Line Spread Function (LSF) were discovered during SMOV (Ghavamian+2009).

The wings are caused by mid-frequency wave-front errors (MFWFEs) produced by polishing errors on the HST primary and secondary mirrors.

Ghavamian+2009 used WFPC2 measurements of the OTA MFWFEs (Krist & Burrows 1995) and modeling of the COS optics to derive model LSFs that appeared to be a good match.

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The Modeled COS LSF

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Model LSF is a Good Match

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But, there are shortcomings …

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Si II Interstellar Line isn’t Black …

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Since the deconvolved absorption line isn’t black,

==> The wings on the LSF must be even broader …

Optimizing the LSF---How broad?

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Geocoronal Lyα in the Mrk 509 observation

Light filling the aperture (as scattered light does) extends to ±100 pixels

Optimizing the LSF---What Shape?

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In addition to the MFWFEs on the mirrors, there should be scattering from microroughness.

Hasan & Burroughs (1993), using WFPC observations, predict scattering due to microroughness as a power law for the PSF with index −2.5. (In the far field, diffraction will have a limiting power-law index of −3.) Integrating the PSF over 1 dimension to obtain an LSF for COS

will reduce this index by 1: a prediction of −1.5 for scattering due to microroughness, and a limit of −2.0 for simple diffraction.

Empirically test a grid of models with two free parameters: Core of the new empirical LSF is the same as the current model. The relative normalization of the power law. Power-law indices ranging from −1 to −2.5.

Optimizing the LSF---What Shape?

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Best fit index is −2.25

Normalization is 1.4% to 2.5% of the peak of the core LSF.

Transition radius is 19−23 pixels.

Additional power in the extended wings is ~3%.

Improvement is slight, but significant

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Sk 155/STIS E140H convolved with old and new COS LSFs

The New LSF Improves Deconvolutions

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Sk 155/STIS E140H compared to deconvolved COS spectrum

Now the ISM lines in Mrk 509 are black

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Conclusions

Adding extended power-law wings to the model COS LSFs with a power-law index of −2.25 improves the characterization of the LSF.

LSF wings this steep are even steeper than diffraction alone. This implies that scattering due to microroughness on the HST mirrors is negligible, even in the far UV, and that scattering within COS itself is also negligible.

The additional power in the wings is only ~3%, but this significantly improves modeling of saturated absorption features.

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References

The ISR is online at: http://www.stsci.edu/hst/cos/documents/isrs/ISR2011_0

1(v1).pdf

The improved LSFs are online at: G130M:

http://www.stsci.edu/hst/cos/performance/spectral_resolution/fuv_130M_lsf_empir.html

G160M: http://www.stsci.edu/hst/cos/performance/spectral_resolution/fuv_160M_lsf_empi.html

Preprint on Mrk 509 results appeared on astro-ph on July 5: http://arxiv.org/abs/1107.06616/16/2011 TIPS 14/14