APRIL 21, 2020
COS NEWSLETTERS

April 2020 STAN

In this newsletter we announce a new CalCOS release, provide updated dark rates, and announce the availability of line spread functions and cross-dispersion spread functions for cenwaves introduced in Cycle 26. 


New CalCOS Release

On April 8, 2020, an updated version of CalCOS was released as version 3.3.9. This CalCOS release includes updates in anticipation of an improved time-dependent sensitivity (TDS) correction which is cenwave dependent rather than just grating dependent. Two time-dependent sensitivity tables (TDSTABs) were delivered along with the release of this CalCOS version: 43n19046l_tds.fits for the FUV and 43n19048l_tds.fits for the NUV. At this time, no updates were made to the contents of the TDSTABs, so there are no changes to any data. However, the format of the reference file was expanded to include a "CENWAVE" column whose entries are identical to the appropriate grating in the TDSTAB.

In order to perform custom calibrations with data retrieved from the archive after April 8, 2020, CalCOS v3.3.9 will need to be used. This version can be installed by following these instructions to update your current AstroConda environment or by following the pipeline instructions here with more documentation about why to use the AstroConda pipeline releases here. Additionally, CalCOS v3.3.9 is not backwards compatible and will only work with the new TDSTAB format. Therefore, if using CalCOS v3.3.9, please re-retrieve data from the MAST archive.

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New Dark Rates Adopted for ETC v28.2

The dark rates for the COS FUV detectors (FUVA and FUVB) and the COS NUV detector are monitored regularly.

The dark rate of the FUV detector, primarily segment A, experiences occasional increases from its nominal baseline. See COS ISR 2019-11 for discussion of the variable, spatially structured component of the FUVA dark rate and recent efforts to more accurately account for it. In the past 12 months of monitoring, the FUV dark rate has decreased closer to baseline values, including the spatially structured FUVA component. As a result, the FUV dark rates adopted by the COS Spectroscopy ETC version 28.2 have been lowered by about 17% for FUVA and 4% for FUVB. The new dark rates are 2.01E-6 counts/sec/pixel and 1.85E-6 counts/sec/pixel for FUVA and FUVB, respectively. The dark rates for Spectroscopic Target Acquisition have also been lowered to 3.02E-6 counts/sec/pixels and 2.97E-6 counts/sec/pixel for FUVA and FUVB, respectively.

A minor error was corrected in the NUV dark rate calculations that had resulted in a slight underestimation of the NUV dark rate since Cycle 22. The error was small compared to routine variations in the dark rate, so there were no significant consequences. The NUV dark rate adopted in the ETC has been raised to 1.11E-3 counts/sec/pixel.

PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their Phase II proposals. Further details about the dark rate monitor, including links to the latest plots of the dark rates against time, may be found at the COS Monitoring page.

 

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Line Spread Functions and Cross-Dispersion Spread Functions for Cenwaves G140L/800 and G160M/1533

Cenwaves 800 and 1533 were introduced in Cycle 26 for COS FUV. Cenwave 800, observed with the G140L grating, optimizes low-resolution spectroscopy below 1150 Å while extending to 1950 Å. Cenwave 1533, observed with the G160M grating, extends coverage at the short-wavelength end of the grating to overlap with the longest wavelengths covered by G130M/1222. See the March 2019 STAN for additional details.

Line spread functions (LSFs) and cross-dispersion spread functions (CDSFs) have recently been tested and made available for these modes on the COS website. LSFs give the detailed distribution of light along the dispersion axis, while CDSFs describe the distribution of light along the cross-dispersion axis. Additionally, instructions for how to use the LSFs to fit lines have been placed on the website.

 

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Please Contact the HST Help Desk with any Questions

https://hsthelp.stsci.edu