Instrument Science Reports (ISRs)

We describe some details related to changes in image scale induced by velocity aberration of starlight. We investigated whether an update was warranted to the HST keyword VAFACTOR, which quantifies the dimensionless factor by which the image scale is changed by velocity aberration at the mid-point in time of an exposure. Nominally, the factor differs from unity by ±1E-4 or less. We quantify each of the following effects on VAFACTOR: a) using the classical formula instead of the relativistic one, b) evaluating it at an inappropriate time, e.g. at the start of an exposure, and c) evaluating it at mid-exposure instead of averaging it over the entire exposure. Respectively, the three effects can produce errors in VAFACTOR as large as a) 1.0E-8, b) 0.8E-6 T, and c) 4.5E-9 T², with the exposure time T expressed in minutes. Whereas the first effect is negligible and the second effect is only hypothetical, the third effect has the potential to be marginally significant in rare circumstances, but we expect will never matter in practice. Based on this analysis, no updates are indicated to the formulas used for FITS keywords VAFACTOR (HST) or VA SCALE (JWST).

Most discussion of charge-transfer-efficiency (CTE) losses involves the parallel transfer of charge in the y-direction down the chip columns (y-CTE). Serial charge-transfer efficiency (x-CTE) refers to the horizontal transfer of charge. Serial CTE losses were first assessed in WFC3/UVIS in 2014 (WFC3/ISR 2014-02), where it was found that bright stars were shifted by ~0.0015 pixel away from the readout amplifier. Now that the WFC3/UVIS CCD has spent more than three times as long in space, imperfect serial CTE should have three times the impact on the images, so we revisit the effect, characterizing serial CTE in a similar manner to our models for parallel-CTE by using a combination of warm pixels, cosmic rays, saturated stars, and overscan pixels. Overall, serial CTE has a much smaller impact on images than does parallel CTE. As of 2023, parallel CTE can have a ~5% impact on bright sources and a >50% impact on faint sources, and serial CTE can have a ~1% impact on bright sources and a ~3% impact on fainter sources. The first few pixels in the serial CTE trails are much sharper than the parallel trails, but there is a very faint component to the serial trails that extends much farther than the parallel trails — even wrapping around to the next row. We develop a pixel-based model for the trapping and release of charge in the serial register and release a stand-alone beta-version of this pixel-based serial-CTE correction. Most images are not significantly impacted by the x-CTE effect, however HST users that require high-precision astrometry could benefit from this correction — at the very least, so that they can quantify its impact on their science.

The WFC3/IR detector has experienced a cumulative photometric sensitivity loss of ~1 - 2 % since installation, indicating that time-dependent corrections are necessary for highest accuracy photometry. We evaluated the changing photometric sensitivity using staring mode observations of globular clusters, scanning mode photometry of an open cluster, and grism observations of four CALSPEC standard white dwarf stars. Staring mode observations of CALSPEC standards were used for testing and validation. Sensitivity change appears to be wavelength-dependent, with the greatest losses in the bluest filters. A new IMPHTTAB reference file, with updated inverse sensitivities, will be delivered in 2024. In the interim, we provide factors users can apply to manually correct their photometry.

We present phase retrieval measurements of the focus of the Hubble Space Telescope (HST) found using the WFC3/UVIS and ACS/WFC detectors from February 2019 to December 2023. From our analysis we predict that by the end of 2024 WFC3/UVIS will be at -0.66 micron despace and ACS/WFC will be at -1.48 micron despace.

This report examines Charge Transfer Efficiency (CTE) flux losses in the Wide Field Camera 3 UVIS detector aboard the Hubble Space Telescope. Spanning approximately 14 years of observations from October 2009 to February 2024, the study analyzes CTE flux loss trends across various total background levels and source fluxes. In addition to analyzing the present state of CTE flux losses, we provide updated coefficients for the empirical model for point source photometry corrections in both non-CTE-corrected (FLT) and CTE-corrected (FLC) data. Between 2009 and 2023, the rate of CTE flux loss for a 500-2000 e− source, farthest from the readout, in FLT data with a 1-3 e−/pix background, is measured to be ∼0.05 Δmag/2051 pix/year. The recommended minimum total background level to mitigate CTE losses remains at 20-25 e−/pix. At that level, we find that 500-2000 e− sources, farthest from the readout, in 2024 FLT data can suffer ∼23% flux loss/2051 pix. The FLC data provide some relief, but we measure a ∼12% flux loss/2051 pix in 2024. There continues to be a slight over-correction in some FLC results that contain backgrounds above 40 e−/pix. In 2024, 8000-20000 e− sources farthest from the readout in a 40, 60, or 90 e−/pix background are over-corrected by ∼1, 2, and 3%, respectively.

The Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) has captured over 310,000 images in its near 15-year lifetime. Some of these images are affected by guide star failures, which can cause a smearing of the sources in the image. Although the images are manually flagged by WFC3 team members for such anomalies, machine learning is more practical for observatories that will be far more data rich, and where manual flagging will be inefficient or even impossible. In order to remedy this problem, we trained a convolutional neural network (CNN) to identify WFC3/UVIS images affected by guide star failures. The CNN's training and validation data were taken from May 2009 to May 2022. We developed a data processing pipeline to log-scale, down-sample, and normalize the images. Our best model achieved true negative and true positive rates of 90% and 91% on our validation data. We investigate the model's misclassifications, deployment tests, and rotational dependency. In addition, we present shortcomings from other trained models and ideas for future work. Our code and model parameters can be found on Deepwfc3's GitHub.

Our study focuses on the enhancement and validation of IR bad pixel tables (IR BPIXTAB) for Cycles 29 and 30, along with the revision and distribution of two existing IR BPIXTABs to rectify anomalies observed in 2021 and 2022. We address the issue of an unusual spike in cold and unstable pixels detected during Cycle 28, attributing it to algorithmic discrepancies within the pipeline utilized for BPIXTAB generation. Through meticulous analysis, we identify that the variance in input darks, sourced from three distinct BPIXTAB reference files, significantly impacts the prevalence of flagged cold and unstable pixels. By generating three separate IR BPIXTABs for Cycle 28 with consistent reference files, we observe a substantial reduction in the flagged pixel percentage (from 2.85% to less than 1%), affirming the presence of an algorithmic error rather than an intrinsic detector issue. Employing the most recent accurate IR BPIXTAB, labeled 53514239i_bpix, we successfully mitigate the abnormal pixel spikes observed in Cycle 28, ensuring consistency across subsequent cycles. Furthermore, we provide an updated Cycle 28 IR BPIXTAB, utilizing input darks referencing 53514239i_bpix, to reflect the corrected cold and unstable pixel counts.

Spatial scanning and slitless spectroscopic observations with the Wide Field Camera 3 (WFC3) IR channel are used to quantify its photometric stability and provide independent estimates of the rate of sensitivity evolution. Spatial scans of stars in the open cluster M35 observed with the F140W filter reveal a sensitivity loss at the rate of 0.065 +/- 0.006 % per year and also result in the first measurement of a sensitivity loss of 0.157 +/- 0.021 % per year in the F098M filter. Observations of spectrophotometric standard stars with the WFC IR grisms, G102 and G141, demonstrate photometric sensitivity losses at the annual rates of 0.115 +/- 0.008 %/yr and 0.061 +/- 0.007 %/yr respectively. The imaging and the grism modes of WFC3/IR show similar sensitivity evolution over comparable wavelength ranges.

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