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COS Data Handbook v.3
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COS Data Handbook > Chapter 4: COS Error Sources > 4.3 Factors Limiting Flux and Wavelength Accuracy

4.3
The COS calibration accuracies are given in Table 1.3 of the COS Instrument Handbook. In this section, several factors limiting those accuracies are discussed.
4.3.1 Flux Accuracy
The accuracy of the absolute flux calibration of COS spectroscopic data is limited by several factors including:
Due to on-board Doppler corrections, a given pixel in ACCUM data will contain data from nearby pixels, which will cause a slight smearing of the fixed pattern noise.
4.3.2 Wavelength and Spectral Resolution Accuracies
There are several issues that may affect the COS wavelength calibration and spectral resolution, and these are explained in detail in COS ISRs 2010-05, 2010-06, 2009-01, and 2010-09. Some of these issues are outlined here.
OSM motions, or drifts, can cause the spectrum to shift in the dispersion direction by as much as 2-3 pixels (~1 resolution element for NUV, approximately one-half resolution element for FUV) in the first 20 minutes after an OSM is moved. tagflash wavecals correct for these motions to accuracies ≤ 0.5 pixel. However, it is only possible to correct ACCUM data for the mean OSM motion that occurred during the exposure and, in rare circumstances, this may result in a slight degradation in the spectral resolution of ACCUM data.
The accuracy to which the source is centered in the science aperture along the dispersion direction can result in small displacements in the absolute wavelength scale corresponding to the plate-scales of 0.22 arcsec per FUV pixel and 0.25 arcsec per NUV pixel. Measurements for ACQ/IMAGE centering accuracies are of the order of 0.05 arcsec, and accuracies of other types of COS acquisition can be of the order of 0.1 arcsec or more. One can calculate the resulting wavelength accuracy using the plate-scale and dispersion given in Table 1.4 and Table 1.1 respectively.
As discussed in the COS Instrument Handbook, the BOA degrades the target image, resulting in a reduction of the spectral resolution by a factor of three or more.

COS Data Handbook > Chapter 4: COS Error Sources > 4.3 Factors Limiting Flux and Wavelength Accuracy

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