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Updated COS/FUV Wavelength Dispersion Solution Reference File for LP2 (DISPTAB) Released

The new COS FUV wavelength dispersion solution reference file (DISPTAB), 0bn1606sl_disp.fits, includes updated dispersion coefficients (zero points and slopes) for data taken at LP2 (i.e. between 23 July, 2012 and 09 Feb, 2015) with the G130M settings (1291, 1300, 1309, 1318, 1327) and G160M settings (1577, 1589, 1600, 1611, 1623). No updates have been made at this point to the G140L dispersion solutions. Updates to the LP3 dispersion solutions for the G130M and G160M settings will be released in the near future. Details specific to how the updated LP2 zero points and slopes were derived are given below. Further details about the techniques can be found in the May 2016 STAN

Users interested in COS FUV G130M or G160M data obtained between 23 July, 2012 and 09 Feb 2015 , are encouraged to re-retrieve their data from the Hubble Space Telescope Archive, hosted by MAST, at http://archive.stsci.edu/hst/ to ensure they have the highest quality data available.


While the techniques used for deriving the zero points and slopes for the LP2 dispersion relations were similar to the techniques used to derive the LP1 dispersion relations (see May 2016 STAN), there were a few major differences.


Figure 1.

As with LP1, diagnostic before and after plots for all of the G130M and G160M COS-to-COS cenwave comparisons are available here and COS-to-STIS comparisons are available here. Both show improvements in both the zero points and residual slopes for most cenwaves.

Figure 2 summarizes the mean shifts and standard deviations determined by cross-correlating COS exposures obtained in the same visit but with different cenwaves, with the old DISPTAB (green) and new DISPTAB (purple). For most of the G130M and G160M settings there is an improvement in both the mean and standard deviation, indicating that for most settings the overall uncertainties in the wavelength calibration are now within +/- 3 pixels (1 sigma). It should be noted that this does not include any target acquistion errors (+/- 3 pixels), which may introduce a constant wavelength offset between different COS spectra. The accuracy of the new dispersion solutions is also limited by detector walk and residual geometric distortion effects. These two effects are currently under study.



Figure 2: Mean shifts and standard deviations derived by cross-correlating COS exposures obtained in the same visit but with different cenwaves, with the old DISPTAB (green) and new DISPTAB (purple). The number of cross-correlation windows that were used for each reference cenwave is also given.