Instrument Science Reports (ISRs)

This report introduces the implementation of a new 2-dimensional saturation map for use in the calibration pipeline for the WFC3/UVIS detector. These changes were delivered in calwf3 v3.7.1 on December 7, 2023 for the reprocessing of all UVIS data in the Mikulski Archive for Space Telescopes (MAST). Similar to recent updates to the calacs pipeline, a new SATUFILE reference file will be used for flagging pixels in the data quality (DQ) array of calibrated images which exceed the full-well limit. The updated version of calwf3 with the new SATUFILE effectively flags the same saturated pixels as older versions of calwf3 using the previous method that applied a single saturation threshold from the CCDTAB reference file. Since the DQ flags do not change with this version of the SATUFILE, users will not need to retrieve the updated products from MAST. For products that are missing the SATUFILE keyword, the new calwf3 will revert to using the CCDTAB threshold value. In the future, improved pixel flagging will be possible by updating this 2D saturation map reference file.

This report describes a collection of six Jupyter Notebooks, released on the HSTaXe GitHub repository in Spring 2023, that demonstrate data reduction using STScI's official slitless spectroscopy software, HSTaXe. These 'cookbooks' present examples of how to preprocess data from ACS and WFC3 slitless-spectroscopic modes and use the core HSTaXe routines to extract 1D spectra. The specific preprocessing procedures explained here and in the cookbooks are meant to highlight three steps of the data analysis process users should consider to obtain optimal spectral extraction with HSTaXe. The three steps include a custom multi component background subtraction for WFC3/IR grism data, embedding subarray data into a full-chip image, and checking that the active World Coordinate System (WCS) of dispersed images matches the corresponding direct images. In addition to these preprocessing steps, we also address installation methods, the general cookbook workflow, advanced fluxcube extraction, and HSTaXe output files.

We have constructed the first sky images for the WFC3/UVIS G280 grism from both the calibrated, flat-fielded individual FLT exposures, as well as their corresponding CTE-corrected FLC frames using public on-orbit science exposures retrieved from the Mikulski Archive for Space Telescopes (MAST). We characterize the sources of stray light present in the G280 science frames, and provide guidance for minimizing stray light depending on the levels of precision needed for observers’ science cases. We search for the potential presence of multiple spectral components—however we determine that a single component should be sufficient to model the scattered light present in the G280 science exposures. We find the stray light scatters in an expected pattern for the WFC3/UVIS detector and we do not find additional spectral components from OII emission in Earth’s atmosphere. After processing data using our sky model with HSTaXe, we reduce the median of the background distribution for our test cases to one compatible with 0.0 electrons per second (e−/s), providing bounds for cases with both low- and high-levels of scattered light, i.e. from 0.0494 to 0.0025 e−/s and 0.3747 to 0.0002 e−/s, respectively. We also show that our sky model reduces the spatial variations across the two UVIS chips, where the change in background depends on the pixel’s chip position. We present the procedure used for our analysis and for the generation of the sky frames. We provide existing resources for WFC3 spectral analysis and applying these calibration frames with HSTaXe. In the future, we plan to examine changes in the spatial trends of the sky with time and if possible, provide a means to predict background levels given observing conditions necessary for specific science cases.

The dither patterns available in APT were designed with only one instrument in mind — the instrument that is “prime”. We explore here how effective the prime-instrument-based “box” patterns are for observations taken in parallel. To this end, we develop a metric to describe good and bad pixel-phase coverage. Not surprisingly, we find that a pattern that has been optimized for one detector observed in prime is often quite poor for another detector observed in parallel. We construct some additional patterns in the form of POS-TARGs that achieve a good sub-pixel dither for both prime and parallel observations for ACS/WFC and the two WFC3 cameras. It is worth noting that on account of distortion, there are sometimes tradeoffs between achieving good pixel-phase coverage and mitigating artifacts (bad pixels, blobs, persistence, bad columns, etc). In the process of this exploration, we discovered that the then-current ACS box dither likely got corrupted by post-SM4 changes in the SIAF files. We have since corrected those dither specifications to provide the intended sub-pixel phase sampling. The current document now provides 2-point and 3-point dithers in Appendix B that are good in prime/parallel instruments, in addition to the 4-point dithers. Users can group N dithers into sets of 2, 3 or 4 to achieve a good N-pt dither in both prime and parallel.

Timing Jitter in the WFC3/UVIS shutter is the non-repeatability in the exposure times from one exposure to another. We determine the timing jitter of the UVIS shutter from a series of G280 spectra taken in one HST orbit of calibration program 17265. The actual exposure time varies by 2.43 ± 0.32 ms, for exposures with nominal lengths of 1-s, 2-s, and 4-s. The shutter exceeds its specification for repeatability (10 ms) by a factor of four. The jitter is approximately ten times smaller than our reported value for 0.5-s exposures, in which the shutter sweeps without stopping during its open state. On the other hand, the jitter for 0.7-s exposures is about double the reported value. The 0.7-s exposure is the shortest one with a temporary stop in the open state, and as a result is most affected by the vibration of the shutter system. The shutter’s tendency to vibrate more for blade B than blade A does not affect the timing jitter noticeably. The spectra, obtained on the UVIS 2 CCD, exhibit a small loss of charge associated with saturation.

In this report we discuss the modified procedure for generating a superbias reference file for the Wide Field Camera 3 (WFC3) UVIS detector. Changes to the procedure include processing bias files individually, flagging cosmic rays, and replacing undefined (NaN) values. We compare the 2020 superbias currently in the Calibration Reference File System (CRDS) to a 2020 superbias created using the new procedure. We find a negligible increase of 0.02 ± 0.10 e- in the average 2020 superbias level compared to the original procedure and thus will not deliver a new version of the 2020 reference file to CRDS. The 2021 and 2022 superbiases were generated using the updated procedure and we compare the 2022 superbias to the 2020 CRDS superbias in this report. The 2022 average superbias value is 0.34 ± 0.07 𝑒! for UVIS 1 and 0.39 ± 0.07 e- for UVIS 2. We analyzed the superbias level from 2009 to 2022 per readout amplifier, finding a gradual increase due to dark current accumulated during readout and charge transfer efficiency losses in the detector. The current rate of increase per chip is 0.016 ± 0.001 e-/year and 0.033 ± 0.002 e-/year for UVIS 1 and 2 respectively. The 2021 and 2022 superbias reference files have been delivered to CRDS and are in use in the calibration pipeline. Observers can request updated products through the Mikulski Archive for Space Telescopes (MAST).

For the absolute flux calibration of the WFC3/UVIS detector, the aperture-corrected photometry of standard stars observed in staring mode is compared to the predicted photometry of simulated observations. Spatial scans offer greater precision than staring mode observations, but currently cannot be used directly for the absolute calibration of the instrument, as existing software used for generating synthetic observations lacks the capacity to model rectangular photometric apertures used for spatial scans. In this report, we introduce a novel method for calculating aperture corrections for spatial scans, and present the results of preliminary tests of this methodology. We find that ratios of observed-to-synthetic flux are constant over time, validating the implementation of current time-dependent zeropoints. However, the data exhibits a wavelength- and chip-dependent offset between observed and synthetic count rates. This offset may be due to underlying factors complicating the observed photometry, aperture corrections, or both. Until this discrepancy is resolved, spatial scans will not be directly used for the photometric calibration of the WFC3/UVIS instrument. Meanwhile, we provide calculated offset values for each chip and filter as evidence of our initial efforts. A future report will utilize deep exposures from an upcoming calibration program (Program 17271) to examine encircled energies at large radii in order to further refine the process of calculating aperture corrections for spatial scans

An LED provides WFC3/UVIS with the capability to increase background levels with user-directed flashes that can help to improve charge transfer efficiency (CTE) in images with low backgrounds. Analyzing data from 2012-2021, we completed several tests to check for changes in power and illumination pattern of the LED onto the UVIS detector over time. By studying the subarray and full-frame post-flashed dark images over time we find there is a decrease in the measured post-flash mean over time. This change is in line with previously observed changes in WFC3 sensitivity for filters with similar spectral properties as the LED, measured to be about 0.2% per year (Calamida et al. 2021, Marinelli et al. 2022). We interpret this to indicate that there is no systematic decay in the LED illumination that cannot be attributed to previously reported detector sensitivity changes. Following that result, we investigated whether there was any time dependence in the post-flash reference files if binned by yearly or biyearly cadences. We used these newly created time-dependent reference files to produce FLC images for various GO science data and determined that the time-dependent post-flash reference files do improve the quality of the science data without significantly affecting the measured noise. We report that the average mean measurement of our science images change up to 2.64% with the time-dependent reference files while the standard deviation change is measured to be less than 1/1000 of a percent. We determined that using yearly reference files was the most accurate calibration available as it balances the number of input files, which affects the signal-to-noise, and the time covered by the input data, which affects the change in measured post-flash. We therefore delivered new yearly post-flash reference files to the WFC3/UVIS data calibration pipeline calwf3. They have been available as of December 2022, and data requested from MAST since their delivery is post-flash-corrected with the new reference files.