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September 25, 2020

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In this STAN we announce the deliveries of new reference files for the calibration of COS spectra obtained with the FUV blue modes G130M/1055 and G130M/1096 and with the NUV gratings G185M, G225M, and G230L.

New COS/FUV Blue Modes Reference Files Delivered

Cenwaves 1055 and 1096, observed with the FUV G130M grating, are known as the blue modes. Extending to shorter wavelengths than the original G130M modes, they provide coverage from 899 Å to 1196 Å and from 940 Å to 1236 Å, respectively, with a 15 Å gap in the middle due to the physical gap between detector segments. The resolving power offered by these modes peaks at 11,000-12,000 near the low ends of their wavelength ranges and falls to 2,000-4,000 at the longest wavelengths covered. This is smaller than the resolving power offered by G130M at longer wavelengths but significantly larger than that offered by G140L over these wavelengths.

The blue modes were introduced after the installation of COS, and their calibrations have not been as accurate as those of the other G130M modes. With recent increased usage, we completed improvements to the flux calibration, time-dependent sensitivity (TDS) calibration, and wavelength calibration of these cenwaves. The flux residuals improved from greater than 20% in some wavelength regions to within the 5% requirement. With the new TDS correction, residuals are within the 5% absolute and 2% relative requirements for almost all bins as a function of wavelength and time, with residuals approaching 5% for a small number of wavelength bins. The wavelength calibration uncertainties improved from ±6 pixels to better than ±3 pixels.

A new FLUXTAB, TDSTAB, and FLATFILE were delivered on 2020 July 21. (As part of the improvements discussed here, the blue modes now have their own FLATFILE instead of using the general one for the G130M grating.) A new DISPTAB and SPWCSTAB were delivered on 2020 September 17. Users who have obtained blue modes spectra are encouraged to re-retrieve their newly recalibrated data from the HST archive.


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New COS/NUV Time-Dependent Sensitivity Reference File Delivered

COS gratings lose sensitivity over time, as shown at this COS webpage. This time-dependent sensitivity (TDS) is characterized in two TDSTAB reference files read by CalCOS, one for the FUV detector and one for the NUV detector. After the initial conversion of counts to fluxes, the fluxes are further modified by a factor that changes with time. In the NUV, this factor depends on the grating and stripe.

The NUV gratings have lost sensitivity at slower rates than estimated in the previous NUV TDSTAB, delivered in Cycle 17. As a result, NUV fluxes had been slightly overcorrected by an amount that grew with time. While the discrepancies were minor for most of the period since the Cycle 17 delivery, they eventually moved outside of the 5% specification for many combinations of grating, cenwave, and stripe, exceeding 10% in some cases. All of the three currently monitored gratings were recalibrated: G185M, G225M, and G230L. (Monitoring of G285M ceased after Cycle 25 due to its high TDS and lack of use.) Based on a comparison with model white dwarf spectra, fluxes are now within the 5% specification.

A new TDSTAB was delivered on 2020 July 30. Users who have obtained NUV spectra with the affected gratings are encouraged to re-retrieve their newly recalibrated data from the HST archive


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