The tables below provide results from an optical model of the line spread functions (LSFs) for COS in the FUV and NUV. In the FUV, the LSFs and cross-dispersion spread functions (CDSFs) are available for different cenwaves and Lifetime Positions (LPs). LSFs give the detailed distribution of light along the dispersion axis, while CDSFs describe the distribution of light along the cross-dispersion axis. Both are expressed as functions of wavelength.
Understanding the Line Spread Function Files
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. The dominant effect in the observed spectrum is a broadening of the spectral features due to the finite resolution of the instrument. However, more subtle effects can add additional features and distort the profiles. 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.
Line Spread Function File Structure
The LSFs below are obtained with a Code V optical model and are validated using spectral data. For example, ISR 2018-07 discusses this process for select FUV cenwaves at LP4. The profiles are normalized to an integral of 1 and depend on wavelength. 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 files have the following format:
- NUV: LSF profiles are sampled every 100 Å, and the profile in each wavelength bin is sampled over 101 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
- FUV: LSF profiles are sampled every 5 Å, and the profile in each wavelength bin is sampled over 321 pixels
Line Spread Function File Usage
Suppose we are interested in fitting a feature at 1160.5 Å in a COS spectrum obtained with cenwave G130M/1291 at LP4. The input would be a spectrum of resolution significantly higher than that offered by G130M/1291 in this wavelength range that includes the feature of interest at 1160.5 Å and surrounding wavelengths. Spectra with a resolution about 3 times higher are a typical choice. We convolve the input spectrum with the appropriate LSF, which in this case is the 1159 Å column in the file. Note that in order to perform the convolution, the input spectrum must be broader than the LSF kernel (321 pixels) and must be sampled on the COS FUV pixels (i.e., the wavelength difference between subsequent pixels in the input spectrum must equal the first order dispersion coefficient for COS FUV). A walkthrough guide to working with the COS Line Spread Function (LSFs) is available on GitHub. Installation instructions and documentation can be found here.