February 2020 STAN
This STAN provides information regarding the updates made to the STIS Exposure Time Calculator (ETC) for Cycle 28 Phase I, news regarding the sensitivity recalibration of the E140M grating and the delivery of new PHOTTAB and RIPTAB files in support of this recalibration, updates to the NUV-MAMA dark rate, and the status of a new suite of Python STIS Defringing Tools. Additionally, STIS posters presented at the 235th Winter AAS in January 2020 can be found here.
STIS ETC Updates for Cycle 28 Phase I
ETC 28.1 was updated to use an evaluation date for calculations from mid-cycle 27 (MJD 58940; 2020-Apr-01) to mid-cycle 28 (MJD 59305; 2021-Apr-01). New underlying Time-Dependent Sensitivities (TDSs, discussed here) for the STIS MAMA modes have been delivered, featuring changes on the order of a few percent from previous TDSs at most. The CCD modes continue to use the same underlying TDSs from 27.2, though all TDSs have been re-evaluated at the new MJD.
The CCD Gain=4 Readnoise was updated from 8.4 e- to 8.7 e- and the STIS NUV MAMA dark rate was increased from 0.002 cts/s/pix to 0.0022 cts/s/pix. Likewise, we extrapolated the STIS/CCD dark rate from [0.021, 0.026, 0.031] to [0.022, 0.027, 0.032] counts/s/pixel at the [top, middle, bottom] of the detector to reflect the new evaluation date. These trends were derived from temperature-corrected darks, and the "low" value at the top of the detector was adjusted upward by 1σ of the temperature correction distribution to give a conservative value for the detector's lowest dark rate region (near the read-out amplifier and E1 position).
Improvements to the STIS Enclosed Energy (EE) tables were made in accordance with STIS ISR 2018-06, resulting in a more accurate estimate of the flux enclosed in the extraction region.
The default extraction height for 1st-order MAMA observations (G140L, G140M, G230L, G230M, PRISM) was changed from 7 to 11 pixels to reflect default extraction values in the pipeline. In addition, a selection of new Phoenix M-dwarf models have been made available to the ETC.
New Sensitivity Calibrations for E140M
In February 2018, a special calibration program (PID 15381) observed the HST standard star G191-B2B in order to recharacterize the blaze shape of E140M and obtain a snapshot of the current absolute sensitivity. The blaze function is the characteristic sensitivity with wavelength across a spectral order. While it is well known that all four echelle gratings aboard STIS require time and position dependent shifts to previously-measured blaze shapes in order to properly flux calibrate the spectra, E140M began showing evidence of an underlying shape change (STIS July 2018 STAN). With these new data, we have confirmed the shape change of the E140M blaze functions (Figure 1) and remeasured the grating's sensitivity curves. Our method makes use of a relatively new model atmosphere for G191-B2B to more robustly identify the stellar continuum; therefore, we have also rederived the sensitivity curves in the post-Servicing Mission 4 (post-SM4) calibration dataset taken in 2009 (PID 11866). New blaze shift coefficients have also been measured. Corrections in recent data, where shape changes are largest, are as much as 10% in the middle of echelle orders and 20% at order edges (Figure 2). As a consequence of this work, spectral order 86 (covering ~1711-1729 Å) will once again be part of the standard flux calibrated x1d files produced by the CalSTIS pipeline for all post-SM4 E140M datasets.
Delivery of new PHOTTAB and RIPTAB reference files with updated sensitivity shapes and blaze shift coefficients is imminent, and this document will be updated once the files have been delivered to CRDS. If the updated curves are needed more immediately, please contact us via the Help Desk at https://hsthelp.stsci.edu.
Figure 1: Comparison between sensitivity curves derived from data taken in 2009 from program 11866 (green dashed lines) and the data taken in 2018 for program 15381 (blue solid lines) for four representative E140M spectral orders. Both datasets have been adjusted to remove the effects of the time-dependent sensitivity (stored in TDSTAB) to allow direct comparison. Note that the longer wavelength sides of each order generally demonstrate the most significant sensitivity change.
Figure 2: Flux calibrated spectrum of standard star BD+28D4211 (thin lines) taken two months after the G191-B2B observations. The CALSPEC FOS spectrum (heavy, gray line) is shown for comparison. Left: The previous CALSTIS pipeline product showed flux mismatches in overlapping wavelength ranges of neighboring orders and, in this part of the spectrum, a systematic overestimate of the stellar flux. Right: The newly derived sensitivity file corrects both calibration errors.
Update to STIS/NUV-MAMA Dark Rates
For the NUV-MAMA dark subtraction, STIS calibration pipeline software, CalSTIS, scales the NUV dark reference image by a factor based on the NUV dark time correction table (TDCTAB) reference file. The factor in the TDCTAB is determined by models fit to observed dark rates over time, where the model parameters include time and detector temperature. CalSTIS has been using an older version of TDCTAB created on February 2011 for more than 8 years, but the STIS team found that the model dark rates have been somewhat inconsistent with the observed rates since 2014. We therefore delivered a new TDCTAB last year on June 20, 2019 to allow better fits to the past and future observed dark rates. Specifically, two additional "break points" were added to the existing 3 in the updated TDCTAB. The addition of these two break points were based on our findings that the observed NUV-MAMA dark rates depart from the predicted rate (shown as the black curves in the plots below) for two date ranges: (1) Since Jan 2014 (MJD = 56660), we found that the prediction generally overestimates the observed rate resulting in oversubtracting the dark; (2) On Oct 2018 (MJD ~ 58420), there was a gyro-related safing event and the NUV-MAMA detector had to power cycle. As of February 2020, the updated TDCTAB provides model dark rates that are in good agreements with the observed dark rates. We will continue to monitor the difference, and update the TDCTAB if necessary.
Porting the STIS/CCD Defringing Tools to Python
With the deprecation of STScI IRAF/PyRAF support, the STIS Team is in the process of translating the PyRAF stsdas.hst_calib.stis CCD defringing tools to Python and making them available in an upcoming release of the stistools package. These routines allow users to prepare and apply tungsten lamp flats to G750L and G750M science observations, mitigating fringes at wavelengths ≳7000 Å.
The processing steps, as outlined in the STIS Data Handbook, start with preparing the science observation through basic 2D reduction or obtaining the default cosmic-ray-rejected CRJ data (for G750L) or geometrically-rectified SX2 data (for G750M) from MAST. Then, the paired contemporaneous or library CCDFLAT observation is processed to remove its overall blackbody response via normspflat. This normalized flat is then shifted and scaled using mkfringeflat to match each science observation. Finally, defringe is used to divide the science data by the fitted fringe flat.
Work continues to bring the Python routines into specification and validate results. We expect these tools to be available to the community this Spring.