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Updated COS NUV Wavelength Dispersion

Solution Reference File Delivered


Due to lack of suitable calibration and/or science data, the zero-points of the dispersion solutions had not been updated with on-orbit values after the Science Mission Observatory Verification phase in 2009 for all G185M settings, and for select cenwave/stripe combinations of the G225M, G285M, and G230L gratings listed in the table below. In early 2016, a temporary wavelength dispersion reference file (DISPTAB) was delivered in which the on-orbit zero-point, or d values, from the closest stripe were copied into the appropriate columns (see the June 2016 STAN for more information). This preliminary update improved the zero-point accuracy from ~1 Å (100 km/s) to about 0.3 Å (30 km/s).

To improve the wavelength calibration further, we used newly acquired STIS data to update the G225M, G285M, and G230L settings listed in the table below. All the G185M cenwaves and stripes were also updated by using newly acquired COS data. We cross-correlated the COS data to the STIS data, which is used as a wavelength reference owing to its high wavelength accuracy (~10 km/s for medium-resolution gratings). We used the the resulting offset to update the a0 zero-point term in the dispersion solution, leaving the d zero-point term in the dispersion reference file as is.

This update now provides a wavelength zero-point accuracy of 15 km/s (0.17 Å) for the M-gratings, except G185M which has an improved accuracy of ~30 km/s (0.3 Å), and 175 km/s (1.8 Å) for G230L. Further results are discussed in COS ISR 2017-02, "Updates to the COS/NUV Dispersion Solution Zero-points", that will be released in the near future.


All cenwave and stripe combinations that were affected by this NUV Wavelength Dispersion Solution Reference File change are listed:

COS Grating Cenwave Stripes Updated
G185M 1786 NUVA, NUVB, NUVC
  1817 NUVA, NUVB, NUVC
  1835 NUVA, NUVB, NUVC
  1850 NUVA, NUVB, NUVC
  1864 NUVA, NUVB, NUVC
  1882 NUVA, NUVB, NUVC
  1890 NUVA, NUVB, NUVC
  1900 NUVA, NUVB, NUVC
  1913 NUVA, NUVB, NUVC
  1921 NUVA, NUVB, NUVC
  1941 NUVA, NUVB, NUVC
  1953 NUVA, NUVB, NUVC
  1971 NUVA, NUVB, NUVC
  1986 NUVA, NUVB, NUVC
  2010 NUVA, NUVB, NUVC
COS Grating Cenwave Stripes Updated
G225M 2186 NUVA, NUVB
  2217 NUVA, NUVB
  2233 NUVA, NUVB
  2250 NUVA, NUVB
  2268 NUVA
  2283 NUVA
  2306 NUVA
  2325 NUVA
  2339 NUVA
G285M 2996 NUVC
  3035 NUVC
  3057 NUVC
  3074 NUVC
  3094 NUVC
G230L 2950 NUVA
  3000 NUVA
  3360 NUVB

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