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  1. (ACS ISR 2023-06) The ACS/WFC Focus-Diverse ePSF Webtool

    November 10, 2023G. S. Anand et al.
    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.
  2. (ACS ISR 2023-05) HSTaXe - ACS & WFC3 Cookbook Tutorials

    September 19, 2023B. Kuhn et al.
    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.
  3. (ACS ISR 2023-04) Dithering for ACS and WFC3 Primes and Parallels

    October 18, 2023J. Anderson and N. A. Grogin
    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.
  4. (ACS ISR 2023-03) Measuring the Column Dependence of Read Noise in ACS/WFC Bias Frames

    June 22, 2023A. M. Guzman and M. C. McDonald
    The noise in bias frames for all four readout amplifiers in the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) is dependent on row number. This is because dark current accumulated during readout increases across the detector, influencing and increasing the read noise as a function of row number. In this report, we investigate bias frames taken with the ACS/WFC to explore the column dependence of read noise for each of the amplifiers for different anneal periods. Analyzing the data we find that there is no column dependence of read noise and that the read noise values for the physical pre-scans is approximately 0.5 e^- lower than in the science arrays because there is no readout dark accumulated in this area. We further investigate 1) the evolution of read noise over an anneal period, 2) a linear decrease in read noise within the initial columns per amplifier, and 3) pixels in elevated read noise columns. We conclude that 1) there is no visual trend of read noise over an anneal period, 2) amplifiers A and C have an initial linear decrease of read noise in the science arrays, and 3) masking unstable hot pixels in a column will decrease its read noise values.
  5. (ACS ISR 2023-02) Systematic Effects in ACS/WFC Absolute Gain Measurements

    March 27, 2023G. S. Anand et al.
    The ACS team takes calibration data yearly to measure the absolute gain for the four ACS/WFC amplifiers. Initiated in Cycle 25, this yearly monitoring of the gain allows us to monitor the health of the CCD. Here, we present a reanalysis of the gain with a revised photon transfer curve method, as well as with a new "full-frame" methodology that exploits the use of Poisson statistics on pairs of Tungsten-lamp exposures. Testing for the presence of systematic effects reveals: 1) an increase in the measured gain as a function of increasing row number away from the serial register, and 2) an increase in the measured gain as a function of a increasing flux level of the underlying data. We show evidence that the cause of the former systematic is likely CTE-related effects, and that the cause of the latter may be related to charge-diffusion processes. As the latter has also been tied to the brighter-fatter effect in the literature, we encourage future investigations along this matter for ACS/WFC images. After accounting for the two systematic effects, we find that our measured gains are ~5% lower than what is currently in the CCDTAB reference file. However, we see that the relative amp-to-amp gains are consistent with the values in CCDTAB to better than ~0.5%. Hence, while we do not recommend any changes to the relative amp-to-amp gains, the ACS team is considering future adjustments to the absolute gain levels (along with appropriate changes to the photometric zeropoints to compensate).
  6. (ACS ISR 2023-01) Corrections to Commanding Overheads for ACS/WFC Exposures

    February 17, 2023J. E. Ryon and N. A. Grogin
    In this report, we study corrections to the commanding overheads of ACS/WFC exposures. Commanding overheads are time delays associated with operating the instrument, which should be included in the DARKTIME keyword value. Previous work determined that corrections to the commanding overheads (as well as DARKTIME) on the order of a few seconds was necessary. We revisit these corrections in light of the fading trends of hot pixel dark rates discussed in Ryon et al. (2022), and utilize the fading trends themselves to determine that no commanding overhead corrections are required for the four ACS/WFC observing modes (full-frame/subarray and unflashed/post-flashed). We have updated the WFC CCDTABs to reflect this result.
  7. (ACS ISR 2022-08) Improved Identification of Satellite Trails in ACS/WFC Imaging Using a Modified Radon Transform

    December 20, 2022D. V. Stark et al.
    We present a new approach to identify satellite trails (or other linear artifacts) in ACS/WFC imaging data using a modified Radon Transform. We demonstrate that this 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, galaxies) in most cases. Comparing with a set of 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, we identify the Radon Transform parameter space and image properties where our algorithm is unreliable, and estimate a false detection rate of ∼10% elsewhere. We apply our method to ACS/WFC data taken between 2002 and 2022 to determine both the frequency of satellite trail contamination in science data and also the typical trail brightness as a function of time. We find the rate of satellite trail contamination has increased by approximately a factor of two in the last two decades, but there is no clear systematic evolution in the typical trail brightness. Our satellite trail identification program will be made available as part of the acstools package in the near future.
  8. (ACS ISR 2022-07) Fading Hot Pixels in ACS/WFC

    December 20, 2022J. E. Ryon, N. A. Grogin and M. C. McDonald
    In this report, we study hot pixels that do not accumulate dark current at a constant rate over the course of an exposure in the ACS/WFC detectors. The dark rate drops with time in many of these hot pixels, i.e., they fade during an exposure. We also find that the degree of fading is correlated with dark rate itself, and that there is no obvious evolution in the degree of fading over the lifetime of ACS. Very hot pixels are least likely to be properly dark-corrected with CALACS, so we now flag pixels >=20~e-/s as unusable for science purposes in the WFC dark reference files and data quality arrays of science products.
  9. (ACS ISR 2022-06) ACS/WFC CTE photometric correction: improved model for bright point sources

    December 19, 2022M. Chiaberge and J. E. Ryon
    We present an updated model to correct aperture photometry of point sources for CTE losses. The model is similar to the CTE photometric correction currently available but with a few major improvements that allow more accurate results for bright stars. The standard CTE photometric correction uses a linear dependence on source brightness, background level and time. Here, we present the improved model using a curvilinear dependence on point source brightness. The overall statistical accuracy of the new model is as good as, or better than the previous one. In this ISR we present the data analysis and describe the data science modeling.
  10. (ACS ISR 2022-05) Update of the STIS CTE Correction Formula for Spectra

    October 25, 2022R. C. Bohlin and S. Lockwood
    The correction formula for Charge Transfer Efficiency (CTE) that is used in the HSTCAL CALSTIS pipeline CCD data reduction has not been significantly updated since 2006. Correcting for CTE losses is crucial to the goal of 1% precision in the STIS spectrophotometric fluxes that are the basis for all HST and JWST flux calibrations. Precision in the CTE correction is especially relevant for the faintest flux standards, where the amount of correction can exceed 20%. The comparison of new datasets of STIS spectra and ACS photometry of faint stars reveals the required updates to the parameters of the STIS CTE correction formula. After replacing the original with the new parameters, the change in the spectral energy distribution (SED), i.e. the flux decrease in physical units, for a very faint star NGC2506-G31 ranges from 4 to 6% over most of the G430L and G750L spectral wavelength ranges.
  11. (ACS ISR 2022-04) The Impact of CTE on Faint Sources in ACS

    October 24, 2022J. Anderson
    ACS has now spent over 20 years in the harsh radiation environment of low-earth orbit, and the charge-transfer efficiency of the detector has suffered accordingly. Considerable attention has been devoted to developing pixel-based reconstructions and empirical corrections for traditional aperture photometry, which work well for moderate-to-high S/N sources, but thus far the impact of CTE on low S/N sources has not been explored. We examine the survival of faint point sources as a function of background in the ACS/WFC detector. Many HST programs involve stacking multiple exposures to find and measure sources that are too faint to be detected with significance in individual exposures. The trends explored here can help correct such observations and more effectively plan for future observations.
  12. (ACS ISR 2022-03) Improved Absolute Astrometry for ACS and WFC3 Data Products

    August 23, 2022J. Mack et al.
    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.
  13. (ACS ISR 2022-02) One-Pass HST Photometry with hst1pass

    July 05, 2022J. Anderson
    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.
  14. (ACS ISR 2022-01) Revisiting ACS/WFC Sky Backgrounds

    January 03, 2022G. Anand, N. Grogin and J. Anderson
    In this report, we analyze the sky backgrounds of all broadband ACS/WFC images with exposure times ≥200s and compare these observed values to those predicted by the online Exposure Time Calculator (ETC). Our goal is to provide the best available estimates for the sky background for purposes of ensuring reliable charge transfer efficiency (CTE) corrections. We find that the average sky-background rates (e-/ks) provided by the ETC are on average slightly overestimated and provide plots for each filter to help observers better anticipate their sky backgrounds. We include a summary of recommendations for observers to ensure that their images reach the minimum advised background levels for dependable CTE corrections.
  15. (ACS ISR 2021-02) Long-term Monitoring of the ACS Tungsten Lamp Brightness

    October 27, 2021Y. Cohen and N. A. Grogin
    In this report, we present a study of the history of the ACS tungsten lamp brightness in the time between the 2009 Servicing Mission 4 (SM4) and 2021. For our analysis, we leverage all available archival tungsten lamp exposure data from the internal flat field monitor calibration programs. We present the brightness history in twelve filters, using both zeropoint-adjusted and -unadjusted measurements. We find that, between SM4 and now, the brightness of the lamp has decreased by as much as ~3% in the reddest filters, and has increased by as much as ~2% in the bluest filters, with the exact amplitude of the change depending on whether or not the zeropoint adjustment has been applied. Relatedly, we also evaluate the absolute signal levels of recent tungsten exposures in order to assess whether any adjustments are necessary. We find that, in order to bring all the filters to a more uniform average signal level and desirable signal-to-noise ratio, the exposure times for several filters required slight adjustments. We implemented these changes to the calibration program starting in the middle of HST Cycle 28 (late 2020), and we find that the new exposures achieve the desired signal levels. The ACS Team plans to closely monitor the tungsten lamp brightness going forward.
Last Updated: 06/02/2023

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