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Results: 31

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

January 21, 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 21, 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.

Trace and Wavelength Calibrations of the HST/ACS G800L Grism

June 01, 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 01, 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 01, 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 01, 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 01, 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.

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

January 01, 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.

Calibration Improvements for the HST ACS/WFC: Post-Flash and Commanding Overheads

January 01, 2018N.D. Miles
The Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) post-flash calibration reference file currently suffers from an improper dark subtraction resulting in a variety of image artifacts. In order to cure these artifacts, a new technique has been implemented where the total sum of the exposure time and flash duration for each image is held constant. The flash duration and exposure time are varied to produce two sets of images that are differenced to produce the new post-flash reference file. The first set all have long exposure times and short flash durations, while the second set has exactly the opposite. Next, using the newly generated post-flash reference file we derive the commanding overheads associated with any ACS/WFC post-flashed observation. Whenever ACS/WFC receives commands it takes a finite amount of time for the instrument to execute them, when commands are executed while the instrument is in ACCUM mode additional dark current builds up and is added to the exposure. This additional dark current is not accounted for in the EXPTIME header keyword and therefore is not removed during the DARKCORR processing step in CALACS. By leveraging the stability of hot-stable pixels and the new post-flash reference file, we analyze 1,273 post-flashed darks and extract the commanding overheads associated with ACS/WFC post-flashed data.

Unlocking the Full Astrometric Potential of the HST Imaging Instruments

June 01, 2017V. Kozhurina-Platais et al.
The Hubble Space Telescope can yield sub-milli-arc-second accuracy in differential astrometry and 0.1% precision in photometry. To achieve such high accuracy, a number of instrumental issues and systematics must be understood and accounted for. The most important are: stability of the geometric distortion with time and across the filters, understanding small pixel-grid irregularities at various scales, and the effects of Charge-Transfer Inefficiency. These systematics may affect the measurements of apparent magnitudes, positions, parallaxes, and proper motions. In addition, they worsen the appearance of combined dithered images and the alignment of cross- instrument frames. A deep understanding of the current HST imaging instruments is beneficial to the future space missions such as JWST and WFIRST.

The Hubble Space Telescope “Program of Last Resort” (A Lowest-Priority ACS/WFC SuperSNAP NGC/IC Survey)

June 01, 2017A. Bellini et al.
Every year, the Institute allocates over 3000 orbits of Hubble time to approved General Observer, Snapshot (SNAP), and Director's Discretionary programs. The many targets among all these programs are not distributed uniformly around the celestial sphere, and most targets have observational constraints that limit their schedulability to something less than the entire year. Despite the best efforts of the Hubble schedulers to allocate every last orbit, a small but persistent fraction (∼2– 3%) of the orbits go unused. Salvaging this unused observing time presents an opportunity for the Institute to benefit the astronomy community. The Institute's Hubble Mission Office has initiated a pilot, ultra-low priority SNAP program (14840, PI: Bellini) in Cycle 24, with the goal of taking useful data in Hubble orbits that absolutely no other program is able to use.

Pixel Stability in HST Advanced Camera for Survey Images.

June 01, 2017D. Borncamp et al.
Excess thermal energy present in a Charged Coupled Device (CCD) can result in additional electrical current that is propagated into individual pixels in an exposure. This excess signal from the CCD itself can be persistently existent through multiple exposures and can have an adverse effect on the detectors science performance unless properly flagged and corrected for. The traditional way to correct for this extra charge is to take occasional long-exposure images with the camera shutter closed to map the location of these pixels. These images allow for the measurement of the thermal-electron contamination present in each pixel of the CCD lattice. It can be subtracted from the science images by re-scaling the dark to the science exposure times. Pixels that have signal above a certain threshold are traditionally marked as "hot" and flagged in the data quality array. These pixels may not be "bad" in the traditional sense that they cannot be reliably dark-subtracted. If these pixels are shown to be stable over an anneal period, the charge can be properly subtracted and the extra Poisson noise from this hot pixel's dark current can be taken into account. Here we present the results of a pixel history study that analyzes every individual pixel of the Hubble Space Telescope's (HST) Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) CCDs over time and allows pixels that were previously marked as bad to be brought back into the science image as a reliable pixel. These pixels have been marked as bit flag 32 in the ACS/WFC Data Quality arrays.

Improving Image Drizzling in the HST Archive: Advanced Camera for Surveys

June 01, 2017S. L. Hoffmann et al.
The Mikulski Archive for Space Telescopes (MAST) pipeline delivers products with geometric distortion corrections, associated image combinations, and cosmic ray rejections applied by AstroDrizzle. The MDRIZTAB reference table contains a list of relevant parameters that control this program. This poster details our photometric analysis of Advanced Camera for Surveys Wide Field Channel (ACS/WFC) data processed by AstroDrizzle. Based on this analysis, we updated the MDRIZTAB table to improve the quality of the drizzled products delivered by MAST.

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

June 01, 2017N.A. Grogin and the ACS Team
On orbit for fifteen years, the Advanced Camera for Surveys (ACS) remains a crucial workhorse imager for HST. The Wide Field Channel (WFC) has now been operating almost twice as long (>8yrs) since its 2009 repair than it had before its 2007 failure. The ACS Team at STScI has been exploiting this long history to characterize instrument performance better than ever, resulting in excellent calibrations for the ACS users. Examples shown here include: ultradeep exposure-stacking to refine the Solar Blind Channel (SBC) point-spread function at large radii; and trending in WFC read noise and dark current. As of the current HST observing cycle (Cycle 24), the ACS Team has changed the readout timings of the WFC subarray modes. The subarrays now match identically the WFC full-frame readout timing, except only one amplifier is used instead of all four, and the row readout may be truncated after 512 or 1024 (in addition to the full 2048 rows). All subarrays now return the full 2048 columns of the science area, plus pre-scan. This change has eliminated the significant mismatch in bias structure between full-frame and subarray readouts introduced by the SM4 replacement electronics. This change has also resulted in CTE deferred-charge profiles now matching identically between full-frame and subarray readouts. Cumulative radiation damage to the WFC CCDs steadily worsens their charge transfer efficiency (CTE). After fifteen years, the WFC CTE has become problematic for faint targets against low WFC backgrounds. The ACS Team provides both corrections and mitigations of this effect, including a pixel- based correction, post-readout. The CALACS pipeline pixel-based CTE correction, originally added in 2012, has been extensively updated for Summer 2017 release. Figures below show the pixel-based correction as applied to a stellar field, the uncorrected vs. corrected WFC hot-pixel charge-trails and 47 Tucanae stellar photometry, and a comparison of pixel-based versus catalog-based corrections to stellar photometry during Cycle 23.N

Extreme Charge-Trapping by Pixels in Hubble’s ACS/WFC Detectors

June 01, 2017J. E. Ryon and N. A. Grogin
We investigate the properties of sink pixels in the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) detector. These pixels likely contain extra charge traps and therefore appear anomalously low in images with relatively high backgrounds. We identify sink pixels in the average short dark image from each monthly anneal cycle, which, since January 2015, have been post-flashed to a background of about 60 e-. Sink pixels can affect pixels immediately above and below them in the column, resulting in high downstream pixels and low trails of upstream pixels. We determine typical trail lengths for sink pixels of various depths and background levels. We create a reference image, one for each anneal cycle since January 2015, that will be used by CALACS to flag sink pixels and adjacent affected pixels in science images.
LAST UPDATED: 06/17/2019

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