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COS Data Handbook 2.00
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COS Data Handbook > Chapter 4: COS Error Sources > 4.3 Factors Limiting Flux and Wavelength Accuracy

4.3
4.3.1 Flux Accuracy
The accuracy of the absolute flux calibration of COS spectroscopic data is limited by several factors including:
calcos currently uses a “boxcar” extraction procedure. This ignores the instrumental profile perpendicular to the dispersion, and treats all elements within the extraction box equally. To ensure that all of the signal is included, the width of the extraction box is typically larger than the spectrum and, as a result, includes more background signal than necessary. In order to avoid introducing artifacts into the extracted spectrum, the algorithm rejects all data in a column that contains a single bad pixel, thereby throwing away some of the data. On-orbit data have been used to create profiles perpendicular to the dispersion, and the possibility of adopting an optimal extraction algorithm is being investigated.
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. Initial 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.

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