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Results: 38

The Effectiveness of Post-flashing ACS/WFC Observations of Unresolved Sources

January 04, 2020N. Miles, N. A. Grogin, and A. Bellini
Throughout the almost 18 year lifetime of the Advanced Camera for Surveys Wide Field Channel (ACS/WFC), the charge transfer efficiency (CTE) of its two CCDs has been steadily declining as a result of radiation damage. One way to minimize the effects of an imperfect CTE is by increasing the background level. This is done by either increasing the exposure time or by using the ACS/WFC LED to post-flash the observation. By increasing the background level, many of the charge traps responsible for the imperfect CTE become filled prior to readout. As of late 2019, the ACS Team generally cautions against the use of post-flash because of the additional noise imparted by the post-flash correction. Here we present the results of an analysis into the effectiveness of post-flashing observations of unresolved sources as a means to mitigate the effects of imperfect CTE. We generate a series of images containing artificial stars of varying magnitudes and different amounts of post-flash background. We use the CTE forward modeler available in ACSTOOLS to simulate the effects of readout. Finally, for each source we compare the recovered signal to its known value to examine CTE losses as a function of both S/N and background level when post-flashing an observation of unresolved sources.

ACS Solar Blind Channel Absolute Flux Calibration

January 04, 2020R. J. Avila, R. Bohlin, S. Lockwood, P. L. Lim, and M. De La Peña
The throughput curves for the imaging modes of the Advanced Camera for Surveys' Solar Blind Channel (SBC) have been updated to correct for a 15% - 30% error in the absolute flux calibration. The offset is removed by adjusting throughput curves of various components of the different observing modes, and bringing synthetic photometry into agreement with observed photometry. The resulting curves show that the detector is more sensitive than previously estimated. The practical result of these changes is that the new zeropoints are fainter than before. In other words, until now, the observed astrophysical fluxes of sources have been overestimated. The zeropoints for F122M and F165LP have accuracies of ~4.5%, while the other filters have accuracies better than ~1.7%. New throughput curves and other necessary support files have been delivered to the calibration pipeline so that, from now on, SBC images downloaded from MAST contain the appropriate zeropoints.

CALACS: The Seventeen Year Evolution of a Space Telescope Data Pipeline

October 06, 2019T. D. Desjardins and The ACS Team
For the first time since HST Servicing Mission 4 (SM4) in May 2009, we present an analysis of the amplifier gains of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) CCDs. Using a series of in-flight flat-field exposures taken in November 2017 with a tungsten calibration lamp, we utilize the photon transfer method to estimate the gains of the WFC1 and WFC2 CCD amplifiers. We find evidence that the gains of the four readout amplifiers have changed by a small, but statistically significant, 1-2% since SM4. We further present a study of historical ACS/WFC observations of the globular cluster NGC 104 (47 Tuc) in an attempt to estimate the time dependence of the gains.

Trace and Wavelength Calibrations of the HST/ACS G800L Grism

June 09, 2019N. Hathi et al.
The HST/ACS G800L grism, an optical slitless spectroscopy mode on ACS, has been very stable since its inception in 2002, but no major effort to improve its trace and wavelength calibrations has been attempted since 2005. The accurate calibration of the spectral trace and the dispersion solution are crucial to locate the spectrum in the grism image as well as to precisely identify spectral features in a spectrum. We obtained new observations of an emission line Wolf-Rayet star (WR96) in the HST Cycle 25 (PID: 15401) to undertake a thorough analysis of verifying and improving the ACS grism calibrations. To account for the field dependence, we observed WR96 at 3 different observing positions over the ACS field of view, but that is inadequate coverage to properly sample the entire ACS field of view. To account for that, we are uniformly reprocessing all the archival ACS grism data of WR96. By combining all of the available data we can now derive a more finely sampled field dependence of the grism dispersion solutions. These data, combined with a new approach to solving for the polynomial coefficients of the field dependence of both the trace and wavelength calibrations, allow us to improve the accuracy of the grism calibration. We will present the latest results from this analysis.

Seventeen Years of HST’s Advanced Camera for Surveys: Calibration Update

June 09, 2019N. A. Grogin and The ACS Team
The Advanced Camera for Surveys (ACS) has been a workhorse Hubble Space Telescope (HST) imager for over seventeen years, subsequent to its Servicing Mission 3B installation in 2002. The once defunct ACS Wide Field Channel (WFC) has now been operating over twice as long (>10yrs) since its Servicing Mission 4 (SM4) repair than it had originally operated prior to its 2007 failure. Despite the accumulating radiation damage to the WFC CCDs during their long stay in low Earth orbit, ACS continues to be heavily exploited by the HST community as both a prime and a parallel detector. We present results from long-term monitoring of WFC dark current and readout noise, results from new studies of detector performance for both WFC and the ACS Solar Blind Channel (SBC), and updated ACS software tools for the user community. Highlights include: 1) a WFC readout simulator tool that accurately reproduces the effects of degraded WFC charge transfer effeciency (CTE); 2) color-dependent aperture corrections for SBC point-source photometry; and 3) a refined WFC geometric distortion solution, exploiting precise astrometry of the ACS astrometric calibration field (globular cluster 47 Tucanae) provided by GAIA DR2.

Focus-Diverse, Empirical PSF models for the Hubble Space Telescope’s ACS/WFC

June 09, 2019A. Bellini et al.
Accurate Point-Spread Function (PSF) models are critical in a large variety of science investigations, from stellar photometry and astrometry to galaxy deconvolution. Focus variations, primarily due to uneven Sun and Earth heating of the Hubble Space Telescope but also to outgassing of the metering truss, have a significant impact on the shape of the ACS/WFC PSFs. These variations have been largely overlooked since the installment of the ACS in 2002. Now that thousands of images have been collected by the ACS/WFC over the past 17 years in many filters, we can analyze them in a self-consistent way and derive focus-diverse, empirical PSF models that we show to be superior to any prior library PSF models. These new PSF models will be soon made publicly available to the astronomical community through easy-to-use Python tools within the STScI astropy/photutils package.

Calibration of the HST ACS/WFC Linear Polarization Filters

June 09, 2019T. D. Desjardins
We present the calibration of the Hubble Space Telescope (HST) linear polarization filters for the Advanced Camera for Surveys Wide Field Channel (ACS/WFC). Using observations of the bright (V 11.7 mag), unpolarized white dwarf G191-B2B and a polarized standard GSC 08169-01120 (V 13.4 mag), we re-estimate the coefficients required for transforming ACS/WFC images using the POLV0, POLV60, and POLV120 filters into Stokes I, Q, and U images, along with extracting the polarization fraction and angle. We further discuss the science use-cases for the ACS polarization filters in the era of joint HST and James Webb Space Telescope observations.

An Assessment of ACS/WFC Relative Photometry on Saturated Sources

June 09, 2019M. Olaes et al.
Upon full-well saturation, the pixels on the ACS/WFC CCDs will bleed excess charge onto adjacent pixels along their column. For these saturated sources, aperture photometry may report a lower flux than expected. However, this affect can be mitigated by defining an extraction aperture which encompasses all of the pixels which contain the full-well bleed. Here we present a functional assessment of relative photometry of saturated sources from observations of globular cluster 47 Tuc. Given a successful identification of pixels which contain the lost flux, we demonstrate an alternate method of aperture photometry where >90% accurate photometry of saturated stars can still be obtained out to 2 magnitudes brighter than the standard method.

Updating the HST/ACS G800L Grism Calibration

June 03, 2018N. Hathi
We present results from our ongoing work on obtaining newly derived trace and wavelength calibrations of the HST/ACS G800L grism and comparing them to previous set of calibrations. Past calibration efforts were based on 2003 observations. New observations of an emission line Wolf-Rayet star (WR96) were recently taken in HST Cycle 25 (PID: 15401). These observations are used to analyze and measure various grism properties, including wavelength calibration, spectral trace/tilt, length/size of grism orders, and spacing between various grism orders. To account for the field dependence, we observe WR96 at 3 different observing positions over the HST/ACS field of view. The three locations are the center of chip 1, the center of chip 2, and the center of the WFC1A-2K subarray (center of WFC Amp A on chip 1). This new data will help us to evaluate any differences in the G800L grism properties compared to previous calibration data, and to apply improved data analysis techniques to update these old measurements.

Sixteen Years of the HST Advanced Camera for Surveys : Calibration Update

June 03, 2018N. A. Grogin and The ACS Team
The Advanced Camera for Surveys (ACS) has been a workhorse HST imager for over sixteen years, subsequent to its Servicing Mission 3B installation in 2002. The once defunct ACS Wide Field Channel (WFC) has now been operating nearly twice as long (>9yrs) since its Servicing Mission 4 (SM4) repair than it had originally operated prior to its 2007 failure. Despite the accumulating radiation damage to the WFC CCDs during their long stay in low Earth orbit, ACS continues to be heavily exploited by the HST community as both a prime and a parallel detector.During past year, there have been two new releases of the CALACS image reduction pipeline that have incorporated several recent advancements in ACS calibration capabilities. We review these updates, along with the enhanced calibration reference files (superbiases, superdarks, etc.) associated with these CALACS releases. We also present results from long-term monitoring of WFC dark current and readout noise, and from new studies of detector performance from both WFC and the ACS Solar Blind Channel (SBC). Highlights include: 1) improved characterization of WFC post-flash LED illumination, including a low-level annual modulation of LED intensity; 2) comprehensive assessment of SBC dark current as a function of detector operating temperature, and of SBC operating temperature versus duration of use; and 3) an update to the WFC bad-pixel table resulting from a minor particulate-contamination event in May 2017.

Replacing the IRAF/PyRAF Code-case at STScI: The Advanced Camera for Surveys

June 03, 2018R. A. Lucas
IRAF/PyRAF are no longer viable on the latest hardware often used by HST observers, therefore STScI no longer actively supports IRAF or PyRAF for most purposes. STScI instrument teams are in the process of converting all of our data processing and analysis code from IRAF/PyRAF to Python, including our calibration reference file pipelines and data reduction software. This is exemplified by our latest ACS Data Handbook, version 9.0, which was recently published in February 2018. Examples of IRAF and PyRAF commands have now been replaced by code blocks in Python, with references linked to documentation on how to download and install the latest Python software via Conda and AstroConda. With the temporary exception of the ACS slitless spectroscopy tool aXe, all ACS-related software is now independent of IRAF/PyRAF. A concerted effort has been made across STScI divisions to help the astronomical community transition from IRAF/PyRAF to Python, with tools such as Python Jupyter notebooks being made to give users workable examples. In addition to our code changes, the new ACS data handbook discusses the latest developments in charge transfer efficiency (CTE) correction, bias de-striping, and updates to the creation and format of calibration reference files among other topics.

Re-visiting the Amplifier Gains of the HST/ACS Wide Field Channel CCDs

June 03, 2018T. D. Desjardins & N. A. Grogin
For the first time since HSTServicing Mission 4 (SM4) in May 2009, we present an analysis of the amplifier gains of the Advanced Camera for Surveys (ACS)Wide Field Channel (WFC) CCDs. Using a series of in-flight flat-field exposures taken in November 2017 with a tungsten calibration lamp, we utilize the photon transfer method to estimate the gains of the WFC1 and WFC2 CCD amplifiers. We find evidence that the gains of the four readout amplifiers have changed by a small, but statistically significant, 1–3% since SM4. We further discuss how the flat-fields will be used to determine the relative gains and update the gain values used in the CALACS calibration pipeline.

Accounting for Dark Current Accumulated During Readout of Hubble's ACS/WFC Detectors

June 03, 2018J. Ryon et al.
We investigate the properties of excess dark current accumulated during the 100-second full-frame readout of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) detectors. This excess dark current, called "readout dark", gives rise to ambient background gradients and hot columns in each ACS/WFC image. While readout dark signal is removed from science images during the bias correction step in CALACS, the additional noise from the readout dark is currently not taken into account. We develop a method to estimate the readout dark noise properties in ACS/WFC observations. We update the error (ERR) extensions of superbias images to include the appropriate noise from the ambient readout dark gradient and stable hot columns. In recent data, this amounts to about 5 e-/pixel added variance in the rows farthest from the WFC serial registers, and about 7 to 30 e-/pixel added variance along the stable hot columns. We also flag unstable hot columns in the superbias data quality (DQ) extensions. The new reference file pipeline for ACS/WFC implements these updates to our superbias creation process.

Astrometric Calibrations of HST Images in the Era of Gaia

June 01, 2018V. Kozhurina-Platais, N. Grogin, and E. Sabbi
It is well-known that HST images, taken with ACS/WFC and WFC3/UVIS, have substantial geometric distortion. Over the years our knowledge about this distortion has been vastly improved. Nevertheless, in certain applications it may not be good enough. Preliminary results of comparison state-of-the-art HST astrometric standards and the Gaia DR1 indicate significant scale difference, global rotation, and edge effects in the HST data. However, in terms of positional precision the HST standards are not surpassed yet. The next release of Gaia data DR2 were used to finalize and improve the HST astrometric calibrations down to 0.5 mas or better.

Tiny Tim vs ePSF: A Study of PSF Models for Hubble's Advanced Camera for Surveys

January 08, 2018S. L. Hoffmann and J. Anderson
This study compares the Tiny Tim PSF against an empirically-derived or effective PSF (ePSF; Anderson & King 2006) for the HST Advanced Camera for Surveys/Wide Field Channel imaging. We manipulate the Tiny Tim PSF FITS files into a format that can be utilized by the ePSF FORTRAN photometry code. Then we perform PSF photometry on globular cluster NGC 6397 and analyze the photometry and astrometry results. We find that the ePSF models outperform the Tiny Tim PSFs in every measurement of stellar sources in this field. See the full text of ACS ISR 2017-08.
LAST UPDATED: 05/20/2020

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