Instrument Science Reports (ISRs)

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.

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.

We present the STScI WFC3 project webpage, Transiting Exoplanets List of Space Telescope Spectroscopy, TrExoLiSTS. It tabulates existing observations of transiting exoplanet atmospheres, available in the MAST archive made with HST WFC3 using the stare or spatial scan mode. A parallel page is available for all instruments aboard JWST using the spectral Time Series Observation (TSO) mode. The webpages include observations obtained during primary transits, secondary eclipses and phase curves. TREXOLISTS facilitates proposal preparation for programs that are highly-complementary to existing programs in terms of targets, wavelength coverage, as well as reduces duplication and redundant effort. Reference for the quality of the HST WFC3 visits taken more than 1.5 years ago are made available via including diagrams of the direct image, white light curve and drift of the spectral time series across the detector. Future improvements to the webpage will include: Expanding program query to other HST instruments and reference for the quality of JWST visits.

We present new WFC3 UVIS CCD absolute gain measurements for December 2018 (Cycle 26) through June 2022 (Cycle 29). We employ the mean-variance technique using internal flat field image pairs taken in unbinned, 2×2, and 3×3 binned modes. As in previous programs, there are two epochs of data per Cycle, one in December and a second in June the following year. We find that the final gain values for the latest June 2022 unbinned data are 1.587 ± 0.007, 1.578 ± 0.004, 1.607 ± 0.005, 1.592 ± 0.006 e-/DN for amps A, B, C, and D respectively. In addition to analyzing the newest Cycles, we also looked at the full evolution of gain measurements over time. We find that while the gain measurement continues to increase Cycle to Cycle, it remains stable and consistent within 1-2% of past Cycles and the initial TV3 (ground) testing. We also investigate charge transfer efficiency (CTE) effects, and determine that increasing gain measurements is partially attributed to CTE degradation.

The infrared channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) is frequently used to obtain precision photometric measurements. We investigate the change in the sensitivity of the IR channel over time by comparing photometry of outer, less crowded regions of stellar cluster images at multiple epochs. The channel appears to be losing sensitivity at a rate of approximately 0.13±.02% per year with no apparent wavelength dependence, though the slope and observation dates vary from cluster to cluster. Staring mode observations of the standard stars appear to show more inconsistent results, even within the same filter, but this is likely due to the presence of systematics such as detector preconditioning from prior observations.

As of late-2019, MAST data products for ACS and WFC3 include improved absolute astrometry in the image header World Coordinate System (WCS). The updated WCS solutions are computed during pipeline processing by aligning sources in the HST images to a select set of reference catalogs (e.g. Gaia eDR3). We compute statistics on the alignment fraction for each detector and estimate the uncertainties in the WCS solutions when aligning to different reference catalogs. We describe two new types of Hubble Advanced Products (HAP), referred to as Single Visit Mosaics (SVMs) and Multi Visit Mosaics (MVM), which began production in MAST in late-2020 and mid-2022, respectively. The SVM products include an additional relative alignment across filters in a visit, and the drizzled images are used to generate point source and segment catalogs during pipeline processing. These catalogs supersede those produced by the Hubble Legacy Archive and will be the basis of the next version of the Hubble Source Catalog. The MVM data products combine all ACS/WFC, WFC3/UVIS, or WFC3/IR images falling within a pre-defined 0.2° x 0.2° 'sky cell' for each detector+filter, which are drizzled to a common all-sky pixel grid. When combining observations over a large date range, MVMs may have photometric errors of several percent or systematic alignment errors when combining visits with different catalog solutions. We therefore recommend these to be used as ‘discovery images’ for comparing observations in different detectors and passbands and not for precise photometry.

Using five years of observations from the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3), we assess the changing sensitivity rate of the two WFC3/UVIS charge-coupled devices (CCDs) and evaluate the photometric repeatability of the spatial scan observing mode in comparison to the standard staring mode. We perform aperture photometry on vertical, linear spatial scans of two white dwarf standard stars (GD153 and GRW+70D5824) taken on corner subarrays of the WFC3/UVIS detector, and compute the rate of photometric sensitivity decline from 2017 to 2021. To gauge the relative accuracy of scans, we compare sensitivity losses for staring mode observations over the same 5-year time scale and those acquired over longer time scales. After removing the time-dependence of the relative photometry, dispersion of the residuals is used as a proxy to measure repeatability of observing modes, and thus assess precision. We establish that spatial scans are more precise than staring mode observations. Scans with UVIS 1 show 2.4 times less residual noise than their staring mode counterparts; for UVIS 2, residual noise for scans is 2.5 times less than residual noise for staring mode. For scans, sensitivity losses are relatively flat independent of wavelength on both UVIS CCDs, with no evidence of contamination. UVIS 2 appears to have slightly higher losses (-0.17 +/- 0.01 %/yr) compared to UVIS 1 (-0.12 +/- 0.01 %/yr). When measured over the same time period, spatial scans and staring mode observations yield filter-dependent loss rates that agree well with each other in most filters within computed uncertainties.

This document describes and announces the public release of a software routine, hst1pass, that has been optimized for PSF-fitting photometry on undersampled HST images. Previous versions of the code have been written for individual HST instruments and released as a part of various instrument-specific ISRs. But over the last few years, the code has been generalized to work for all of HST’s main imagers (WFPC2, ACS/HRC, ACS/WFC, WFC3/UVIS and WFC3/IR). It also runs in aperture-photometry mode and, as such, can be run on _drz and _drc products or even non-HST images. The program itself is written in FORTRAN, but a simplified version in Python will be made available soon. In its typical usage, the user specifies some simple finding parameters, and the routine reads in (1) an HST image (_flt or _flc), (2) a PSF and, (3) a distortion solution. The routine then goes through the image pixel-by-pixel and returns a list of stars found and measured in the image. This star list can then be collated with similar lists, such as from a set of dithered exposures from the same program. In a future ISR the collation process will be described in more detail, but a simplified version of the collation software is provided here to facilitate preliminary analysis.

"Figure-8 ghosts" are a type of HST/WFC3 anomaly caused by light reflecting off a part of the WFC3/UVIS detector. Since anomalies affect both science and calibration data, it is vital that we flag them to properly support the WFC3 user community. Traditional methods of anomaly detection will become impractical as the volume of data sets increases in future missions such as the Roman Space Telescope. WFC3 data provides the opportunity to test anomaly detection with machine learning in current generation astronomical images. We trained five convolutional neural networks (CNNs) of varying architectures to classify figure-8 ghosts in WFC3/UVIS images. This report outlines the data selection, preparation, and augmentation to construct our training, validation, and test sets. Our models' accuracies range from 62% to 83%, showing that CNNs can be a powerful tool to detect other anomalies in WFC3 or other images. In addition to applying machine learning to a specific astronomical task and discussing future work, we consider astronomy's role in machine learning, i.e., innovating computer vision with large images, as a potential challenge for the future.

We compute new UVIS encircled energy (EE) curves from images of HST standard stars observed from 2009-2020, after correcting for temporal changes in the sensitivity of each CCD and filter. The latest UVIS photometric calibration (Calamida et al. 2021) makes use of the updated EE values presented in this report for two filters (F275W and F814W). We extend this analysis to five additional filters (F336W, F200LP, F350LP, F775W, and F850LP) to investigate differences between the 2009 EE, derived just after WFC3’s installation, and the 2016 chip-dependent EE, computed by averaging inflight observations over ~6 years. To recompute the EE, we rescale the calibrated FLC science array values using the new time-dependent inverse sensitivity (PHOTFLAM) keyword values, align the images in detector coordinates, and combine all images in a given CCD and filter. This process allows for a more accurate measure of the PSF at large radii out to 6 arcsec. We compare the EE values in a 10-pixel radius aperture, EE(10), used for the photometric calibration, and we find that the values for the two CCDs now agree to ~0.1% for each filter, compared to the 2016 solutions which differed by up to 0.5%. At UV wavelengths, the EE(10) is now lower by ~1% for both CCDs, in closer agreement with the 2009 solution. For two ‘red’ filters (F775W and F850LP), we compare the EE curves for a white dwarf and a G-type star but find no significant differences due to the color of the source. The new EE(10) values will be applied in a future update to the UVIS photometric calibration.