Updates to the COS Wavelength Calibration

Updates to the FUV Wavelength Calibration

The FUV wavelength dispersion reference file as well as the FUV lamp template reference file have been updated based on data obtained during SMOV. The updates are described in more detail in the upcoming COS ISR 2010-06.

Analysis of a limited number of datasets, using the updated reference files, seems to indicate that the zero point of the wavelength scales are improved by 1 resel (6 pix) on average for the G130M and G160M gratings and a little less for the G140L grating (4 pix). There are residual stretches of the wavelength scales that are not corrected by the current dispersion solutions. We are currently working on refining the wavelength scales so that these effects can be removed. Users who reprocess their data with the updated wavelength dispersion reference file are reminded to also use the updated FUV lamp template reference file.

Note that the COS FUV spectra are not currently flat fielded. This implies that the shadows of the grid wires on the detector are not properly removed by the COS pipeline. While the pipeline actually flags the grid wires in the single exposures, this information is not used when combining different FP-POS exposures into an x1dsum file. In addition the grid wire map currently used by the pipeline is based on pre-launch data and needs to be updated for on-orbit observations. In some instances, the shadows of the grid wires produce an artificial broadening of the absorption lines in the spectra. This effect might be confused with uncertainties in the wavelength scale and will be apparent if one compares datasets obtained with the same setting (grating/cenwave/fppos), but taken at different times. For this reason, users should be careful when combining the types of datasets described above.

Updates to the NUV Wavelength Calibration

The NUV wavelength dispersion reference file as well as the NUV lamp template reference file have been updated based on data obtained during SMOV. The updates are described in more detail in the upcoming COS ISR 2010-05.

Analysis of a limited number of datasets, using the updated reference files, seems to indicate that the zero point of the wavelength scales improves from up to 1 resel (3 pix) for the G225M grating to up to 3 resels for the G230L grating. For the G285M/2695 cenwave the zero point improves by more than 80 pix, since the OSM position for this setting was changed by 3 OSM steps during SMOV. Users with G285M/2695 data might want to reprocess their data with the updated reference files. In any case, users that reprocess their data with the updated wavelength dispersion reference file are reminded to also use the updated NUV lamp template reference file.

Note that for the G185M grating, the update to the wavelength dispersion reference file only takes into account the relative positions between the pre-launch and on-orbit wavecal lamp spectra. We are currently in the process of studying how to best complete the wavelength calibration update for this grating. In addition, we are also working on refining the wavelength dispersion solutions for the other NUV gratings.