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Introduction

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.

FUV Lifetime Position = 1

Tabulated Theoretical Line Spread Functions

G130M G160M G140L
1222 1577 1105
1291 1589 1230
1300 1600 1280
1309 1611  
1318 1623  
1327    

Tabulated Theoretical Cross-Dispersion Spread Functions

G130M G160M G140L
1222 1577 1105
1291 1589 1230
1300 1600 1280
1309 1611  
1318 1623  
1327    

FUV Lifetime Position = 2

Tabulated Theoretical Line Spread Functions

G130M G160M G140L
1055 1577 1105
1096 1589 1280
1222 1600  
1291 1611  
1300 1623  
1309    
1318    
1327    

 

Tabulated Theoretical Cross-Dispersion Spread Functions

G130M G160M G140L
1055 1577 1105
1096 1589 1280
1222 1600  
1291 1611  
1300 1623  
1309    
1318    
1327    

FUV Lifetime Position = 3

Tabulated Theoretical Line Spread Functions

G130M G160M G140L
1222 1577 1105
1291 1589 1280
1300 1600  
1309 1611  
1318 1623  
1327    

Tabulated Theoretical Cross-Dispersion Spread Functions

G130M G160M G140L
1222 1577 1105
1291 1589 1280
1300 1600  
1309 1611  
1318 1623  
1327    

FUV Lifetime Position = 4

 Tabulated Theoretical Line Spread Functions

G130M G160M G140L
1222 1533 800
1291 1577 1105
1300 1589 1280
1309 1600  
1318 1611  
1327 1623  

Tabulated Theoretical Cross-Dispersion Spread Functions

G130M G160M G140L
1222 1533 800
1291 1577 1105
1300 1589 1280
1309 1600  
1318 1611  
1327 1623  

FUV Lifetime Position = 5

 Tabulated Theoretical Line Spread Functions

G130M
1291
1300
1309
1318
1327

Tabulated Theoretical Cross-Dispersion Spread Functions

G130M
1291
1300
1309
1318
1327

FUV Lifetime Position = 6

 Tabulated Theoretical Line Spread Functions

G160M
1533
1577
1589
1600
1611
1623

Tabulated Theoretical Cross-Dispersion Spread Functions

G160M
1533
1577
1589
1600
1611
1623

NUV

NUV

All Gratings
(Recommended)

All Gratings
(Oversampled)

 

Last Updated: 06/02/2023

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