Conference Posters

We are announcing the release of slitlessutils v2.0, a Python package designed for simulation and extraction of wide-field slitless spectra in Hubble Space Telescope imaging cameras. This release includes new capabilities and improved performance. The forward modeling is now performed using Numba, which reduces runtime by ~3-10x. A grouping algorithm has been added to organize sources found in all the input images. Support has been added for subarray modes and ACS/SBC prism observations. Reference files have been updated, collated, and standardized. Documentation has been revamped for clarity. In addition to the software release, we are making available a set of Jupyter notebook tutorials that guide users through using slitlessutils on all available modes. These are complete examples that can be customized for the most common grism/prism observations. The notebooks can be found in the STScI Notebooks GitHub repository. slitlessutils is now the recommended package for HST grism/prism extraction, with HSTaXe being retired and no longer supported.

The Solar Blind Channel (SBC) far-ultraviolet detector of the Advanced Camera for Surveys on the Hubble Space Telescope (HST) typically exhibits elevated dark rate levels at temperatures exceeding 25.5°C. However, rapid dark rate surges have been observed in SBC dark data before the detector reaches this temperature threshold. We present a new principal component analysis of the dark rate in science exposures which until now, has not been investigated. We aim to create a model for the dark current that can be used to remove dark signal from photometric measurements. This will provide users with clearer insight into the dark rate behavior in these images. We also present results from an expanded dark rate monitor program. With eight eppochs of dark data per Cycle, we provide a detailed analysis of the SBC dark rate. We investigated the influence of South Atlantic Anomaly (SAA) passage on dark rates. Using orbital parameters from SPT engineering products, we plotted the HST's 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. 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. Finally, we announce the creation and release of SBC empirical PSF models for all five long-pass filters. The PSF-fitting photometry software hst1pass has been updated to support these new PSFs with SBC imaging modes.

As the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope is damaged over time in the hostile environment of space, its CCDs suffer from an increasing number of charge traps that lower their charge transfer efficiency (CTE). Constraining how these degradations impact the measured properties of astronomical sources is crucial in order to carry out reliable scientific studies. Taking advantage of existing software to simulate the time-dependent impact of imperfect CTE, we use a large suite of simulations to analyze the impact of CTE on signal-to-noise ratio and blind detectability of point sources. Consistent with analyses using real observations, we show that optimal SNR and detectability for weak point sources occurs when backgrounds are ~30 e-/pix, where the sky level limits flux losses from imperfect CTE without adding significant additional Poisson noise. We also present situations where the depth of "CTE-corrected" FLC exposures does not match that of idealized mock exposures with perfect CTE, as well as how CTE can introduce false detections using some source-finding algorithms. While the impact of CTE on point sources in ACS data has been heavily studied over the years, its influence on measurements of extended sources is less constrained. We present the current results from a new analysis of CTE losses on extended extragalactic sources using calibration data of a galaxy cluster taken across multiple epochs spanning 17 years.

In this work, we focus on matching point source catalogs from Gaia Data Release 3 (DR3) and the Hubble Space Telescope's Advanced Camera for Surveys (ACS), aiming to enhance the scientific and calibration utility of both datasets. We develop a concise yet robust method for matching the sources across two catalogs by leveraging positional separation and magnitude information. Running this pipeline results in a high quality GAIA matched source catalog for nearly every HST/ACS image, which we will deliver to the community. We find that the overlap in magnitude range of well-measured stars in both catalogs ranges from about -11.5 to -16 in HST magnitudes, and that approximately 30% of the stars in ACS stellar fields have a matching GAIA source. Leveraging the huge volume of ACS data, this overlap fraction is sufficient for us to utilize our matched source catalogs to make improvements to ACS geometric distortion calibrations, as well as to perform interesting science using the interplay between GAIA proper motions and ACS data quality.

Glint is a thin band of light that extends across the field of view 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 ACS/WFC chip gap, but this idea has never been thoroughly tested. This project seeks to robustly determine the underlying cause of glint to ensure observers can properly plan observations to avoid it in the future. To achieve this goal, positions and brightness of stars in the fields of view of a large sample of ACS/WFC images were examined using data from the Gaia DR2 catalog. Images with and without glint were compared and no evidence was found to suggest that glint is correlated with the presence of stars at or near the ACS/WFC chip gap. The possibility that glint is caused by stars outside the field of view is being explored using similar methodology. Without a method to avoid the presence of glint in images, scientists are at risk of obtaining sub-optimal data, impacting their ability to achieve their scientific goals.

The Advanced Camera for Surveys (ACS) has been a workhorse Hubble Space Telescope (HST) imager for over twenty-two years, subsequent to its Servicing Mission 3B installation
in 2002. The ACS Solar Blind Channel (SBC) continues to offer unique access to high-resolution, high-sensitivity imaging in the far-UV with its Multi-Anode Microchannel Array (MAMA) detector. The once defunct ACS Wide Field Channel (WFC) has now been operating thrice as long (>15yrs) since its Servicing Mission 4 (SM4) repair than it had originally operated prior to its 2007 electronics failure. Despite the accumulating radiation damage to the WFC CCDs during their long stay in low Earth orbit, ACS remains heavily exploited by the HST community as both a prime and a parallel detector, and now by the JWST community for its matched spatial resolution at optical wavelengths. Ongoing detector-calibration efforts by the STScI ACS Team consistently deliver the highest level of performance to the ACS users.

We present results from the latest studies of detector performance for both WFC and SBC, including long-term monitoring of WFC and SBC dark current, WFC readout noise, and WFC CCD pixel stability. We also advertise updated ACS documentation and software tools for the user community.
Highlights presented here include:
1) adding a CALACS pipeline pixel-based correction for WFC serial ('x-direction') charge-transfer efficiency (CTE) trailing;
2) simulations of faint-source S/N to optimize background-level recommendations for WFC CTE mitigation;
3) a new initiative to measure SBC time-dependent geometric distortion via long-term monitoring of the NGC 6681 field; and
4) the recently-released AWS webtool for SBC and WFC focus-specific, spatially-dependent point-spread functions (PSFs).

Sink pixels (SPs) in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) on the Hubble Space Telescope (HST) are pixels containing a large number of charge traps. These pixels are spread randomly across the detector and exhibit consistent lower values compared to the background level of an image. Analyzing seven years of dark data, we assessed SP creation and persistence. About 5500 SPs were created in each of the two CCD detectors in ACS/WFC, which implies an SP creation rate of approximately 2.2 pixels/day in each. We also found it is rare for pixels to return to their previous average values over each annealing process. A higher fraction of SPs were detected near the serial register, and to better understand this skewed distribution, we simulated a short dark image. We found that CTE losses accumulated during the readout process do appear to create the gradient in the y-position of SPs, since the data was initialized with a uniform distribution and the result had a gradient in the direction we expected.

Imaging Spectropolarimetry is a new observing mode on the HST/ACS instrument, which has been offered to HST observers since Cycle 31. The polarization state of light observed from astrophysical sources can provide vital information that is otherwise unobtainable from intensity alone (e.g., the origin of emission; nature of emitting or scattering particles), and measurement of the polarization state of the light as a function of wavelength, known as Spectropolarimetry, can provide new and valuable constraints on the nature of the light source. While Spectropolarimetry is a technique that is accessible from ground-based observatories, the superb spatial resolution afforded by the HST/ACS combined with the slitless nature of HST/ACS grism spectroscopy opens up the possibility of studying extended polarized emission in a way that is not possible even with AO facilities on the ground. This mode could help to study interesting targets including (but not limited to) QSOs, AGN and Radio Galaxies, ISM Dust Properties, Pre-Planetary Nebulae, Proto-Planetary and Debris Disks, Supernovae and Supernova Remnants, and Solar System objects. In this presentation, we will discuss the results and ongoing work from the calibration programs used to commission and calibrate Imaging Spectropolarimetry on HST/ACS.

Additionally, we present updates on the software tools necessary to reduce/analyze any HST slitless spectroscopy data including that from the above-mentioned new mode. (a) The HSTaXe software has been updated recently to make it compatible with all the slitless spectroscopy modes of HST/ACS and HST/WFC3. We have also recently released a collection of six Jupyter Notebook tutorials/cookbooks (as discussed in ACS ISR 2023-05) that present examples of how to preprocess data from HST/ACS and HST/WFC3 slitless spectroscopic modes and use the core HSTaXe routines to extract 1D spectra. (b) We are introducing a new package -- Slitlessutils -- which is a set of tools for simulating and extracting wide-field slitless spectroscopic data, currently focused on data from HST/ACS and HST/WFC3 only. This package will improve upon some of the current limitations of HSTaXe (e.g., data simulations, and analysis of multi-orient data).

The Advanced Camera for Surveys (ACS) has been a workhorse Hubble Space Telescope (HST) imager for over twenty-one years, subsequent to its Servicing Mission 3B installation
in 2002. The ACS Solar Blind Channel (SBC) continues to offer unique access to high-resolution, high-sensitivity imaging in the far-UV. The once defunct ACS Wide Field Channel (WFC) has now been operating far longer (>14yrs) since its Servicing Mission 4 (SM4) repair than it had originally operated prior to its 2007 electronics failure. Despite the accumulating radiation damage to the WFC CCDs during their long stay in low Earth orbit, ACS remains heavily exploited by the HST community as both a prime and a parallel detector. Ongoing detector-calibration efforts by the STScI ACS Team consistently deliver the highest level of performance to the ACS users.

We present results from the latest studies of detector performance for both WFC and SBC, including long-term monitoring of WFC and SBC dark current, WFC readout noise, and WFC CCD pixel stability. We also advertise updated ACS documentation and software tools for the user community. Highlights presented here include:
1) a re-analysis of the absolute gain values among the four WFC CCD readout amplifiers at the default (GAIN=2) setting, using a new algorithm that has revealed systematic gain-estimation biases due to degraded CCD charge-transfer efficiency (CTE) and due to increased CCD pixel charge-diffusion at high intensity levels;
2) a revised modeling of the empirical WFC aperture-photometric losses due to CTE, incorporating nonlinearity in flux-dependence that has been observed for bright point-sources;
3) a recent bug-fix to the sub-pixel phase sampling of the oft-used WFC four-point compact dither pattern, as well as a recently designed suite of four-point compact dither patterns optimized for joint sub-pixel phase sampling of both the WFC detector and the WFC3 detectors (both for WFC3/UVIS and for WFC3/IR) when observing in parallel; and
4) an advertisement of the ACS operating mode newly offered for HST Cycle 31: high-precision optical/near-IR imaging spectropolarimetry at high spatial resolution (~0.1 arcsec) by

We present a new approach to identify satellite trails (and other linear artifacts) in ACS/WFC imaging data using a modified Radon Transform. Our approach is sensitive to features with mean brightness significantly below the background noise level, and it is resistant to the influence of bright astronomical sources (e.g., stars and galaxies) in most cases. Comparing with a set of 358 images with satellite trails identified by eye, we find a trail recovery rate of 85% and a false detection rate (after removing diffraction spikes that are easily filtered) of 2.5%. By performing an analysis using a much larger ACS/WFC data set where false trails are identified by their persistence across multiple images of the same field (e.g., cases of scattered light artifacts or chance persistent alignments of stars/galaxies) we identify the modified Radon Transform parameter space and image properties where our algorithm is unreliable (mostly image corners and extremely dense fields), and estimate a false detection rate of ~10% elsewhere. We apply our method to ~30,000 ACS/WFC images taken between 2002 and 2022 and find the rate of satellite trail contamination has increased by approximately a factor of two in the last two decades, with ~9% of images currently affected, but there is no clear systematic evolution in the typical trail brightness. Our new satellite trail identification program is available for public use as part of the acstools package.

The polarization state of light observed from astrophysical sources can provide vital information that is otherwise unobtainable from intensity alone. Examples of such information include (but are not limited to): The origin of emission (intrinsically luminous vs. scattered); Nature of emitting or scattering particles (e.g., relativistic electrons, small dust particles); Geometry, orientation, and detailed structure of an object; Magnetic field strengths and directions; and Scattering media properties such as sizes, morphologies, and complex refractive indices. Measurement of the polarization state of the light as a function of wavelength, known as Spectropolarimetry, can provide new and valuable constraints on the nature of the light source. While Spectropolarimetry is a technique that is accessible from ground-based observatories, the superb spatial resolution afforded by the HST/ACS combined with the slitless nature of HST/ACS grism spectroscopy opens up the possibility to study polarized extended emission in a way that is not possible even with AO facilities on the ground. This new observing mode, which has been commissioned in HST Cycle 30 and is being offered to HST observers in Cycles 31+, could help to study QSOs, AGN & Radio Galaxies (central illuminating sources & synchrotron emission), pre-planetary Nebulae, proto-planetary & Debris Disks, Supernovae & Supernova Remnants, Solar System Targets (e.g., comets, asteroids, moons, minor planets), ISM Dust Properties, Alignment, Photo-Dissociation Regions, Young O&B Stars & Associations, AGB Stars, Cataclysmic Variables, and Galactic Magnetic Fields. In this presentation, we will discuss the results from the calibration programs used to commission and calibrate a specific mode of Imaging Spectropolarimetry on ACS, which will provide a useful capability for the science community.

The Wide Field Channel (WFC) on Hubble’s Advanced Camera for Surveys (ACS), is composed of two charge-coupled devices (CCDs). While CCDs are popular and ubiquitous in astronomical imaging due to their high sensitivities and remain a valuable detector architecture, exposure to radiation in space can damage them over time. For WFC, data is read out by moving the electrons in each pixel to an amplifier at the corners of each chip, row by row, and pixel by pixel. When this occurs, electrons can get stuck in charge traps and improperly counted as belonging to a different pixel, causing inaccuracies in imaging, due to imperfect CCD charge transfer efficiency (CTE). While this has been characterized in the parallel or Y-direction (YCTE), and is now accounted for in Hubble’s image processing, this has not been characterized in the serial or X-direction (XCTE). By analyzing superdarks and other data used for calibration, we both prove the existence of XCTE in WFC/ACS, and aim to characterize its effects. The results from this analysis can then be used to help guide development of XCTE correction algorithms, and help users understand the impact XCTE has on their data.

We present updated calibrations for the prism modes of the Hubble Space Telescope's Advanced Camera for SurveysSolar Blind Channel (ACS/SBC). The prism calibration has been updated to correct for a ~30% error in the absolute flux calibration, similar to the correction made to the imaging modes in 2019. The flux calibration also incorporates newly derived time-dependent sensitivity corrections. We also present a set of Jupyter notebooks describing how to properly extract spectra using the hstaxe software, a full Python based translation of the now defunct IRAF aXe package.

The Hubble Space Telescope’s (HST) Advanced Camera for Surveys (ACS) has now been on-orbit and serving as the Hubble’s widest field optical and ultraviolet imagers for 20 years. It also offers other capabilities such as both wide-field slitless grism spectroscopy and polarimetry. As well as for ACS primary observations, it is also frequently used in parallel with other instruments including the spectrographs COS and STIS, and especially with the Wide-Field and Infrared Camera 3 (WFC3), which has both an infrared channel and an optical/UV channel (UVIS) that has a somewhat smaller field of view but somewhat higher resolution than ACS/WFC, and higher UV sensitivity as opposed to the sensitivity of ACS/WFC which is most sensitive in the redder optical wavelengths. Optimizing dither patterns for parts of both detectors when used in parallel are but one of the subjects we address. Over the course of these 20 years of operation, we have accumulated much experience in “best practices” for proposing and observing with ACS, distilling the much larger volume of information from Instrument Science Reports and other publications into a more condensed document in which we give advice on proposing for ACS observations in HST Phase I and on more detailed program design in HST Phase II. We include examples of requirements needing to be specified in Phase I if needed in Phase II. We present this document as a companion to the ACS Instrument Handbook and ACS Data Handbook and include some representative details. It may be accessed at https://hst-docs.stsci.edu/acsoam and will continue to be updated for HST Cycle 30 and future cycles as needed.

Radiation damage to the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) detectors results in individual pixels with anomalously high dark currents, or hot pixels. These hot pixels have been growing in number since the installation of ACS on Hubble. Recent work has revealed that the rate of accumulation of dark current in many hot pixels is not constant over the course of an exposure, but decreases systematically as a function of exposure time. This behavior is consistent from exposure to exposure. This presents a problem for accurate removal of hot pixels from science images during dark correction. We present further analysis of these fading hot pixels over the lifetime of ACS as well as approaches to mitigating their effects.