![]() ![]() FUV: LSF profiles are sampled every 5 Å, and the profile in each wavelength bin is sampled over 321 pixels.In addition to the recommended file, which is sampled at each NUV pixel, the original NUV LSF file, which oversamples the profile with multiple data points per NUV pixel, is still available to ensure backwards compatibility NUV: LSF profiles are sampled every 100 Å, and the profile in each wavelength bin is sampled over 101 pixels.In the LSF files, each column refers to the center of a wavelength bin, and each row represents one pixel in the dispersion direction. The profiles are normalized to an integral of 1 and depend on wavelength. For example, ISR 2018-07 discusses this process for select FUV cenwaves at LP4. The LSFs below are obtained with a Code V optical model and are validated using spectral data. For example, COS LSFs are known to have non-Gaussian wings that are a consequence of mid-frequency wavefront errors (MFWFEs) produced by zonal polishing errors on the primary and secondary mirrors of HST. Since the non-Gaussian wings of the LSF contain a significant fraction of the total power, a model of the LSF is needed to perform accurate line profile fitting. By convolving a model spectrum with infinite resolution with the COS LSFs, it is possible to reproduce the spectral line profiles observed in COS spectra. In practice, any spectrum with resolution significantly higher than COS (e.g., STIS echelle spectra) can be convolved with the LSFs to reproduce COS line profiles. However, more subtle effects can add additional features and distort the profiles. The dominant effect in the observed spectrum is a broadening of the spectral features due to the finite resolution of the instrument. ![]() The LSF of a spectrograph describes the light distribution at the focal plane as a function of wavelength in response to a monochromatic light source. Understanding the Line Spread Function Files Both are expressed as functions of wavelength. LSFs give the detailed distribution of light along the dispersion axis, while CDSFs describe the distribution of light along the cross-dispersion axis. In the FUV, the LSFs and cross-dispersion spread functions (CDSFs) are available for different cenwaves and Lifetime Positions (LPs). ![]() The tables below provide results from an optical model of the line spread functions (LSFs) for COS in the FUV and NUV. ![]()
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