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The wavelength calibration of the FUV detector changes with the central wavelength setting and lifetime position at which spectra are obtained mainly due to the repositioning of the grating focus mechanism to maintain spectral resolution. For each lifetime position, a new DISPTAB file is required by CalCOS with updated dispersion solutions at each cenwave due to the new focus offset. Beginning with LP3, a new LAMPTAB file has also been created with the wavelength calibration lamp spectra taken at the new focus positions.

On 09 Apr, 2018, DISPTAB 24914346l_disp.fits and LAMPTAB 24914347l_lamp.fits were delivered for use at LP3.  The DISPTAB includes updated linear coefficients for 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. Users interested in COS FUV G130M or G160M data obtained between 09 Feb, 2015 and 02 Oct, 2017 are encouraged to re-retrieve their data from the Hubble Space Telescope Archive, hosted by MAST, at to ensure they have the highest quality data available.

The new lifetime-dependent LAMPTAB was created in an effort to further improve the wavelength solution when used in the COS calibration pipeline, CalCOS, resulting in a more accurate spectrum shift. The LP3 LAMPTAB was created using data from PID 14856. Since the lamp templates define the reference frames for the dispersion solutions, the new DISPTAB and LAMPTAB must be used together.

Details about the dispersion solution techniques can be found in the May 2016 STAN for LP1 and the November 2016 STAN for LP2.  Unlike for LP1 and LP2, there were not enough archival data to derive solutions at LP3, so we needed to craft a special calibration program. In the LP2 analysis, we found the K2Ve star ε Eri yielded excellent results and it was chosen to derive the calibrations for G130M/FUVA and G160M modes. However, because its Lyman Alpha emission line is too bright for the detector limits, we chose a different target for G130M/FUVB, AzV75, which is also used in the yearly wavelength monitoring program.

For the LP3 DISPTAB, the methodology for deriving the solutions was the same as we used at LP2. The linear dispersion coefficients are based on the COS ray-trace models, as well as the COS absolute focus values, scaled to match dispersion coefficients by cross-correlating COS and STIS. Figure 1 shows the dispersion values by focus position for all lifetime positions 1 through 3.  As with LP1 and LP2, diagnostic before and after plots for all of the G130M and G160M COS-to-STIS comparisons are available here, showing the overall uncertainties now fall between +/- 3 pixels (1 sigma). All cenwaves show improvements in both the zero points and residual slopes. A change in the zero point due to the creation of a new LAMPTAB at LP3 also required updating the "D" column for the G140L/1105/1280 and G130M/1222 PSA and BOA modes, based on the difference in the LAMPTABs from LP1 to LP3 zero points. The D column and D_TV03 columns for the new G130M and G160M solutions were absorbed into the new zero points. These column values are set to 0 where the linear dispersions were updated. For all updated modes, the BOA values were copied from the PSA. G130M/1055/1096, G140L/1230 values were copied from xaa18189l_disp.fits because no new LAMPTAB or DISPTAB data was observed for these modes.

Figure 1: Dispersion coefficients for G130M (top) and G160M (bottom) standard cenwaves as a function of grating focus position for LP1, LP2, and LP3. Filled circles are values from the COS/STIS dispersion solutions for each cenwave, dashed lines are the aligned ray-trace models used in creating the DISPTABs.

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