Instrument Science Reports (ISRs)

Recently, the ACS team applied an Ubercal framework to assess the photometric repeatability of stars observed across the WFC detector using 15 years of post-SM4 calibration data in the globular cluster 47 Tuc (Ryan et al., 2024). A surprising finding was an apparent 0.05 mag global difference in sensitivity between the WFC1 and WFC2 chips, which had not been seen in prior tests of sensitivity variations around the field-of-view. Given the many degenerate variables within the Ubercal framework such as CTE losses, time-dependent sensitivity, and flat-field corrections, we obtained new calibration data to perform a straightforward test of the reported 5% flux offset between detectors. We observed three white dwarf standards with three filters at four positions on the detector (each on a different amplifier), but with the same number of x and y pixel transfers to mitigate differential CTE-related effects. For the F606W and F814W filters, the agreements are good to 0.4% on average, and always 1% or better in individual cases. The consistency of these two filters over all three stars and the four dither positions provides very strong evidence against the large global sensitivity offset between WFC1 and WFC2 as seen in the Ubercal work. Larger variations seen in the bluer F435W filter are likely a result of a sensitivity of the flat field in that filter to underlying spectral type, warranting a future solution.

We analyze the archive of ACS/SBC images taken of the calibration field at the center of M70 to develop empirical “effective” PSFs for the SBC's five long-pass filters. The F115LP PSF is the sharpest, and the redder PSFs show elongation roughly along the y axis. The F140LP and F165LP PSFs exhibit a two-peaked structure. About 80% of the images analyzed have nominal PSFs, while 20% exhibit some degradation, likely due to focus variations. We provide these PSFs in the STDPSF format and the distortion solution in STDGDC format so that the hst1pass routine can use them to perform PSF-based photometry and absolute astrometry for SBC images.

Recent analysis of 23 years of Hubble Space Telescope ACS/SBC data has shown that background levels can vary considerably between observations, with most filters showing over an order of magnitude variation. For the shorter-wavelength filters, the background is understood to be dominated by airglow; however, what precisely drives background variations is not well constrained for any filter. Here, we explore the causes of the background variation. Using over 8,000 archival SBC observations, we developed a machine learning model that can accurately predict the background for an observation, based upon a set of 23 observational parameters. This model indicates that, depending on filter, the SBC background is generally dominated by Solar elevation, Solar separation angle, Earth limb angle of observation, SBC temperature, and target Galactic latitude.

We have used 23 years of ACS/SBC data to study what background levels are encountered in practice, and how much they vary. The backgrounds vary considerably, with F115LP, F122M, F125LP, PR110L, and PR130L all showing over an order of magnitude of variation in background between observations, apparently due to changes in airglow. The F150LP and F165LP filters, which are dominated by dark rate, not airglow, exhibit a far smaller variation in backgrounds. For the filters where the background is generally dominated by airglow, the backgrounds measured from the data are significantly lower than what the ETC predicts (as of ETC v33.2). The ETC predictions for `average' airglow are greater than the median of our measured background values by factors of 2.51, 2.64, 105, and 3.64, for F115LP, F122M, F125LP, and F140LP, respectively. A preliminary analysis suggests this could be due to certain OI airglow lines usually being fainter than expected by the ETC. With reduced reduced background levels, the shorter-wavelength SBC filters can conduct background-limited observations much more rapidly than had previously been expected. As of ETC v34.1, a new option will be included for SBC calculations, allowing users to employ empirical background percentiles to estimate required exposure times.

We examined the long-term behavior of the superbias calibration frames for the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) aboard the Hubble Space Telescope (HST). Superbias frames are used to remove detector-level bias structure from science images and are currently generated after an anneal and delivered monthly. The primary goal of this study was to determine whether the frequency of superbias generation could be reduced without compromising calibration quality, potentially aligning with the Wide Field Camera 3 UVIS (WFC3/UVIS) approach of generating only one superbias per year. We analyzed superbias frames produced from 2007 through 2024 to investigate whether these calibration products have changed significantly over time, and whether the frequency of superbias generation and delivery could be safely reduced without loss of calibration accuracy. In addition to visual inspections and pixel-level comparisons, we employed Principal Component Analysis (PCA) to evaluate whether any long-term, global structure exists beneath the apparent noise in these frames. Our findings show that the superbias structure has remained fairly stable post-Servicing Mission 4 (SM4), a 15-year period, and no significant or unexpected global trends or systematic shifts were detected. However, due to unstable hot columns and increasing readout dark observed in ACS/WFC data, it is likely that these calibrations still benefit from more frequent superbias updates than the annual cadence adopted for WFC3/UVIS.

Using repeat imaging of a galaxy cluster taken over a seventeen-year baseline, we examine the impact that degraded Charge Transfer Efficiency (CTE) has on photometric measurements of extended sources using the ACS/WFC on HST. We examine how measured brightnesses depend on time since ACS installation, source location on the WFC detectors, source brightness, and local background level in individual exposures. We find that global brightness measurements using large apertures are generally reliable within ∼0.05 magnitudes across the WFC detectors if exposure backgrounds are above 20e−/pixel and sources are brighter than ∼ 300e− in a single exposure. However, brightness measurements on smaller scales can suffer deficiencies in excess of 0.1 mags (sometimes significantly more) in recent data unless sources are very close to the CCD serial registers (≲ 512 pixels), or brighter than ∼3000 e− in a single exposure. We also show how degraded CTE can result in artificial asymmetries in galaxy light distributions, which are largely mitigated if backgrounds are > 20e−/pixel and targets are not far (> 1536 pixels) from the serial registers. As expected, brightness measurements in later epoch data are best when using CTE-corrected images (FLC/DRC), but our results imply that the pixel-based CTE correction algorithm employed by the ACS reduction pipeline does not necessarily place charge back into its original location within extended sources. Based on this study, users are advised to keep backgrounds above the already recommended 30e−/pixel, ensure targets will have at least ∼ 300e− in a single exposure, and place targets close to the serial registers if analysis of their spatially resolved properties is needed.

In 2024, due to some operational changes, the Hubble Space Telescope began exhibiting undeclared loss of lock events. This loss of lock can result in the smearing out of light from the target field during an exposure, which leads to data degradation, which in turn may require data to be retaken. In this work, we investigate this 'roll-drift' effect in ACS/WFC images. We quantify the impact of roll-drift on measurable parameters in data by using simulations and existing data reduction techniques. We identify a threshold of one such measurable parameter beyond which data may likely be affected by roll-drift, so that users can quickly and easily assess whether their data needs further attention.

We present a dedicated study of CCD serial (x-direction) charge transfer efficiency (CTE) in ACS/WFC. Following past studies of parallel (y-direction) CTE, we use the serial CTE trails behind hot pixels in calibration dark frames to characterize charge trapping and release in the serial registers of the WFC detectors. Serial CTE trails are sharper and longer than parallel CTE trails. Many fewer charge traps come into play during serial pixel transfers than parallel transfers, which explains why parallel CTE is much worse than serial CTE. We find that serial CTE can cause losses of ~0.005-0.02 mag in stellar photometry and shift stellar centroids by ~0.01-0.035 pixels. The pixel-based algorithm in CALACS that corrects for parallel CTE losses in WFC data has been modified to include a correction for serial CTE losses. The PCTETAB reference file has also been updated to include serial CTE parameters. The pixel-based correction for serial CTE currently runs only on full-frame WFC images obtained after SM4 (May 2009). Shortly following the publication of this report, science data corrected for both parallel and serial CTE will be available in the MAST archive.

We implement an Ubercal model for the photometric repeatability for the Advanced Camera for Surveys, Wide-Field Channel for the F606W filter based on observations of 47 Tuc for post SM4. We have identified and measured 31,523 unique stars from 422 independent exposures using the hst1pass photometric toolkit, so each star has on average 59.3 measurements. Since these stars fall on distinct pixels (often in different chips), this dataset represents a unique opportunity to improve the relative photometric calibration. Specifically, we implement a sixth-order polynomial form for the low-frequency flat for each detector separately, time-dependent sensitivity losses for each amplifier, and a systematic uncertainty floor. Additionally, we find evidence that very short exposures (<~200 s) have a small bias in their photometry of 0.08 mmag/s, although the physical explanation of this is unclear. Further, we find that the sensitivity of the four amplifiers is degrading with time at a rate of ~1.5-2.0 mmag/yr depending on the amplifier. Similarly, we find that WFC2 sensitivity is depressed by ~0.05 mag with respect to WFC1. Finally, we see that the photometric scatter is still larger than expected, implying there are systematic effects still unaddressed, and we find strong evidence for serial CTE losses. Future work may consider other filters (such as F814W), linking to more fields (such as the deep fields), or including other systematic terms (such as characterizing the serial CTE).

Glint is a thin band of light that extends across the field of view (FOV) of images from the Advances Camera for Surveys Wide Field Channel (ACS/WFC) on board the Hubble Space Telescope. Glint has been thought to arise from internal reflections when a star is in or very near to WFC chip gap, but this idea has never been thoroughly tested. This project seeks to robustly determine the underlying cause of glint. To achieve this goal, the positions of bright stars relative to the WFC FOV in images with and without glint were examined using data from the Gaia DR2 catalog. We find that unlike other forms of scattered light, glint does not emanate radially from bright stars causing it. We identify regions on the WFC FOV where bright stars tend to fall when glint is present. These regions partially overlap the chip gap, but extend towards the middle of the WFC1 detector. Armed with the knowledge of these at-risk regions, observers can adequately plan observations to avoid glint compromising their data quality.

The Solar Blind Channel (SBC) typically exhibits elevated dark rate levels at temperatures exceeding 25.5°C. However, instances of rapid dark rate increases have been observed before the detector reaches this threshold. To more closely monitor these anomalies, the existing calibration program was expanded, scheduling 24 total orbits per year distributed across eight visits for Cycle 31 and beyond. With enhanced data availability, we provide further updates and analysis of the dark rate in this report. We investigated the influence of the South Atlantic Anomaly (SAA) passage on dark rates. Using orbital parameters from SPT products, we plotted the Hubble Space Telescope's (HST) locations during dark exposures relative to the SAA. Comparison of HST paths during elevated and stable dark rate visits revealed no significant correlation between proximity to the SAA boundary and dark rate increases. Furthermore, we examined the stability of dark rates in the vicinity of the SBC-LODARK aperture within the detector. Our findings confirm that this region maintains consistently low dark rates across visits, unaffected by the elevated dark current observed elsewhere in the detector. The SBC-LODARK aperture therefore continues to be recommended for small sources.

After the Hubble Space Telescope's Servicing Mission 4 (SM4) replaced electronics in the Advanced Camera for Surveys (ACS), images taken with the Wide Field Channel (WFC) detector presented a new feature: low-level, horizontal striping caused by 1/f noise from the replacement electronics. This "bias striping" effect is uniform across CCD rows and can be effectively removed using the acs_destripe tool in calacs. In this work, we use raw bias frames from 2009 through 2023 to measure the row-dependent striping noise intensity in an effort to investigate how bias striping noise has evolved over time. We utilize summary statistics such as variance, skewness, and kurtosis, as well as outlier measurements, to better understand the characteristics of the striping noise distributions, ultimately confirming that bias striping noise has remained relatively stable since its initial detection post-SM4.

Using mock observations, we investigate how imperfect charge transfer efficiency (CTE) affects the detection of point sources in imaging data from the Advanced Camera for Surveys Wide Field Channel (ACS/WFC). Using two public source detection algorithms, hst1pass and DAOStarFinder, we analyze the recovery of sources in both FLT and (CTE-corrected) FLC images, as well as an identical set of idealized images with perfect CTE. The simulations incorporate the expected worsening of CTE losses as a function of ACS lifetime and exposure background level. Optimal signal-to-noise ratio (SNR) for sources with brightnesses less than a few hundred electrons is obtained at backgrounds of ∼30e-/pixel, where there is a balance between decreasing CTE losses and increasing background Poisson noise. Consistently, the sensitivity of FLC/FLT images is maximized at these sky levels when using hst1pass for detection, although this trend is less apparent when using DAOStarFinder. Individual FLT exposures are slightly more sensitive than FLCs at low backgrounds, but this behavior generally reverses when multiple exposures are combined. Importantly, FLC exposures do not match the depth and SNR of idealized exposures with perfect CTE until backgrounds reach > 100e-/pixel. False detections become an issue when relaxing the allowed point-spread function goodness-of-fit in hst1pass although the false sources do not appear to be linked to CTE or its correction. False detections are very common in DAOStarFinder output and do appear to be associated with either the detection of CTE trails or amplified noise from the CTE-correction. However, simple cuts on source properties (SNR, sharpness) can reject most of these false detections.

In our study spanning 2015-2021, we examined sink pixels (SPs) in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) using various datasets. SPs, defined here as pixels with values ≤ -10 electrons, collect and trap significant charge during readout. Analyzing seven years of short dark data, we assessed SP creation and persistence. Approximately 5,430 created SPs were identified in WFC1 and 5,649 in WFC2, with creation rates of about 2.15 pix/day and 2.23 pix/day, respectively. These calculations allowed us to estimate around 44,068 SPs in the detector by the end of 2021, constituting approximately 0.25% of the science frame. We found it is rare for SPs to return to a typical, non-negative pixel value. We observed more flagged SPs near the serial register than the chip gap. Skewed y histograms, exhibiting a ''bounce-back" effect, were evident for both WFC1 and WFC2, while x distributions remained uniform. Examining CTE-corrected images from 2021, 2018, and 2015 revealed consistent trends, with the gradient getting steeper over time due to CTE losses, which is also worse for pixels further from the serial register. We simulated a short dark to assess whether CTE influences SP readout. While the gradient effect persisted, the "bounce-back" effect was not replicated. Accumulated CTE losses during readout seemed to contribute to the gradient in the y-position of SPs.

The effects of telescope breathing cause the focus level of HST to change as a function of time during each orbit. This results in modest, but non-negligible, changes to the point-spread functions (PSFs) of stars. For optimal PSF fitting photometry, the library PSFs must be adjusted to account for the changing focus levels. Anderson and Bedin (2017) used a phylogram-based technique to show that it is possible to accurately model focus-related variations in the PSF and thus determine the focus level for a given exposure with measurements of just a handful of bright, isolated stars in the individual WFC3/UVIS images. Bellini et. al (2018) extended this analysis to ACS/WFC, with similar conclusions. Using the results of the Bellini et al. (2018) analysis and subsequent extension to the 11 most popular ACS/WFC filters, we have developed a webtool and Python API to deliver focus-diverse effective PSFs (ePSFs) to the community. The webtool is available for public access at acspsf.stsci.edu.