January 2020 STAN
New COS/FUV Gain Sag Reference File Delivered
The COS FUV detector is susceptible to gain sag, a position-dependent reduction in its ability to convert incoming photons to electrons that becomes more severe with continued use. To counteract this effect, the location where FUV spectra fall on the detector (the lifetime position or LP) is periodically moved to an unused region. The move to LP4 took place on 2017 October 2 for all cenwaves except 1055 and 1096 of the G130M grating, which remained at LP2.
In the more than two years since then, gain sag holes (corresponding to a pulse height less than 3) have developed at LP4, particularly where Ly alpha airglow falls on segment B when cenwave 1291 is in use. Continued use of LP3 for some applications has also increased the effect of gain sag there. The sagged regions affect all cenwaves at the respective LPs.
The gain sag reference table (GSAGTAB) flags detector regions for which the pulse height of the gain has decreased below the threshold of 3; the boundaries of these areas depend on the detector voltage and date of observation. The CalCOS pipeline does not include them in the final calibrated x1dsum spectra. On 2020 January 16, we delivered a new GSAGTAB that accounts for new gain sag holes at LP3 and LP4. It affects LP3 data obtained since January 2018 and LP4 data obtained since July 2019. Users interested in the affected COS spectra should re-retrieve the data from the Mikulski Archive for Space Telescopes (MAST).
New COS/FUV Time Dependent Sensitivity Reference Delivered
The COS spectroscopic modes grow less sensitive with time, as shown on this monitoring webpage. The decline in sensitivity of each grating is modeled as a piece-wise linear function of time, with the slopes being smooth functions of wavelength. The parameters required to correct for the time dependent sensitivity (TDS) are incorporated in the TDSTAB reference file used by CalCOS. The TDS is monitored using observations of standard stars that are obtained as part of dedicated calibration programs.
Starting in Cycle 26 (Fall 2018), two new modes were introduced, G160M/1533 and G140L/800, which extended the coverage of these gratings to shorter wavelengths than existing modes. At that time, a separate TDSTAB reference file for these modes was created. It was populated with zero slopes as placeholders for the sensitivity decline. Based on the measured values for the nearby central wavelength settings, G160M/1577 and G140L/1150, the expected sensitivity decline for the new modes was small enough compared to the accuracy goal that zero was an acceptable approximation for the first 6 to 12 months of use.
The TDS monitoring observations for these modes were initiated in February 2019. The data from the monitoring program, along with the observations used to flux-calibrate the modes, obtained in mid-2018 were used to derive the TDS parameters for the new modes. The new modes show sensitivity declines of a up to about 4% per year, consistent with other modes using the same gratings. The TDSTAB reference file for the new modes was updated using the measured values for the declines in sensitivity and delivered on January 27, 2020. The recalibrated data available in the Mikulski Archive for Space Telescopes (MAST) will have the appropriate TDS correction applied to data obtained using the new modes. Users who are interested in G160M/1533 or G140L/800 spectra should re-retrieve the data from MAST.