November 2016 STAN
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.
The first of these differences is that there are a few cenwaves for which we had no data that would allow us to derive FUVA dispersion solutions by cross-correlating COS and STIS data. These cenwaves are G130M/1327, G160M/1589, G160M/1600, and G160M/1623.
While trying to find a way to handle the missing cenwaves, we evaluated the dependence of the COS/FUV dispersion values on the Optics Select Mechanism 1 (OSM1) focus position. The OSM1 is moved to a unique focus for each cenwave to maintain spectral resolution. Figure 1 shows that if we plot the dispersion values by focus position rather than by cenwave, then both LP1 and LP2 fall on the same ray-trace model line. We then used the zero points derived from the COS-to-STIS correlations as an initial estimate (LP1 zero points for missing cenwaves) and fine tuned them using COS-to-COS correlations. Additional adjustments were done to all zero points for G160M/FUVA to better align them. The improvement over the old dispersion solutions grows with increasing cenwave.
An additional caveat with these dispersion solutions is that we saw a hump in the residuals of the COS-STIS cross-correlation; this hump appears in detector coordinates at approximately pixel 3000 on the FUVA detector. This hump is visible in the COS-to-STIS comparison figures, and is most easily visible in the G160M/1611/FUVA figure. When we calculated dispersion values excluding windows below this pixel value, the resulting residuals were significantly improved compared to not excluding this region. The source of this hump is being investigated, and it is thought to be related to walk and or geometric distortion effects.
As with LP1, diagnostic before and after plots for all of the G130M and G160M COS-to-COS cenwave comparisons are available in COS ISR 2018-23, Appendix B and COS-to-STIS comparisons are available in COS ISR 2018-23, Section 4.1. 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.