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2017 ADASS, IAU, and AAS Posters
Fifteen Years of the HST Advanced Camera for Surveys : Calibration Update 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.
Unlocking the full astrometric potential of the HST imaging instruments 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.
New Time Dependent Sensitivity Corrections and Updated Flat Fields for the ACS/SBC The sensitivity of the Solar Blind Channel on the Advanced Camera for Surveys changes with time. Until now there was no correction available for this effect. Observations that date back to installation, more than 15 years worth of data, were used to derive corrections. The sensitivity can now be continuously monitored and the correction updated as necessary. Here we present the first correction for the one of the filters, F125LP. The correction achieves the promised photometric accuracy of 1%. Additionally, we present new low and high frequency flat fields. These are necessary because previous studies showed that the low frequency flats are too lumpy, and because a change in the high frequency flats was observed in 2007.
Extreme Charge-Trapping by Pixels in Hubble’s ACS/WFC Detectors 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.
Pixel Stability in HST Advanced Camera for Survey Images. 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 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.
The Hubble Space Telescope “Program of Last Resort” (A Lowest-Priority ACS/WFC SuperSNAP NGC/IC Survey) 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.
2016 ADASS, IAU and AAS Posters
New Sky Flats for HST’s ACS/WFC We have begun experiments to make new sky flat files for HST's ACS/WFC. Sky flats can be especially useful for deep imaging in such programs as deep, extragalactic survey programs because they can help to better deal with noise at low levels. Although we also hope to make similar sky flats for some other popular filters including F606W and F814W, we are beginning this experiment with the F435W filter on the ACS/WFC since it is a popular filter in use in many deep extragalactic surveys, and since the bluer filters such as F435W generally have lower throughput and images in that filter are typically noisier than others at some longer mid-optical wavelengths. Initially, although sources will be masked in these images, etc. we are endeavoring to use just post-SM4 F435W images of duration equal to or greater than 800 seconds and which are free of bright stars in order to try and avoid scattered light and sky background color issues as much as possible, although the sky in different images taken at different times and in different directions will likely have some different background levels and color terms in any event. However, our hope is that the final sky flats will be of sufficient S/N to be good calibrators for deep survey programs.
ACS/WFC Pixel History– Bringing the Pixels Back to Science Electrical current that has been trapped within the lattice structure of a Charged Coupled Device (CCD) can be present through multiple exposures, which will have an adverse effect on its science performance. The traditional way to correct for this extra charge is to take an image periodically with the camera shutter closed throughout the lifetime of the instrument. These images, generally referred to as dark images, allow for the characterization of the extra charge that is trapped within the CCD at the time of observation. This extra current can then be subtracted out of science images to correct for the extra charge that was there at the time of exposure. Pixels that have a charge above a certain threshold of are traditionally marked as “hot” and flagged in the data quality array. Many users will discard these pixels as being bad. However, 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 dark current can be taken into account. Here we present the results of a pixel history study that analyzes every individual pixel of ACS/WFC over time and allows pixels that were previously marked as bad to be brought back into the science image as a reliable pixel
Satellite Detection in ACS/HST Images Satellites are a problematic transient event that occurs in astronomical images which will need to be identified and taken care of before any data can be properly used for science. However many people with small programs or doing archival research may not have the resources to manually inspect every images they are using. This necessitates automation of the detection and masking. Though it can be applied to any image, here we show the process by which satellite trails can be identified and properly masked in Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images with a very high completeness and low false positive rate. We also provide the Python source code as a stand-alone package to be used by users (Borncamp & Lim, 2016).
Photometric Aperture Corrections for the ACS/SBC We present aperture correction tables for the Advanced Camera for Surveys/Solar Blind Channel (ACS/SBC). As part of a campaign to improve the instrument calibrations, we observed the white dwarf J132811.4+463050 using three filters (F125LP, F140LP, F150LP). The observed point spread functions (PSFs) contain more flux in the wings than Tiny Tim models, which can underestimate aperture corrections by as much as ∼ 9%, when compared with the observed fluxes. The updated aperture correction tables will be provided to the ReDCaT team so that they can be used in pysynphot and HST’s Exposure Time Calculator.
SBC Internal P-flat Monitoring We report on a Cycle 23 calibration program to monitor the status of the SBC P-flat. We find random pixel to pixel fluctuations to be small, with only ∼2% of pixels having changed by more than 3σ. There are coherent structures that we measure to be above the poisson errors, in some regions as high as 4% peak to peak. We recommend that the ACS team obtain new observations in order to create a new P-flat. We also measured the degradation of the deuterium lamp used to create internal flats. The brightness of the lamp is currently ∼65% of its initial level, the degradation being dependent on lifetime usage. For full details see ISR ACS 2016-02.
2015 AAS and IAU Posters
Toward 1% Polarimetry with the Hubble Space Telescope Advanced Camera for Surveys Abstract: Polarimetry on Advanced Camera for Surveys (ACS) is conducted only by changing filters that are linear polarized at 0, 60 and 120 degrees. This combination of filters allows for the determination of the Stokes parameters as discussed by Sparks & Axon (1999). Here we present the results of a study of the low frequency flat fields for the ACS Wide Field Channel (WFC) polarizing filters. This study aims to enhance the current flat field calibrations by making improvements to the low frequency component which currently only accounts for the broadband filter and does not include the low frequency variations. These variations in the flat can reach 6% across the field of view so it is important that they be removed.
ACS/WFC Geometric Distortions and Its Linear Time Dependency Abstract: We present a revised time dependency for the ACS/WFC geometric distortion model and a new standard astrometric calibration catalog for 47Tuc which includes proper motions of cluster and background stars. The primary goals of this study are two fold; to derive the principle X and Y components of the time dependence in the distortion model for superior alignment and combination of WFC images, and create a new astrometric catalog with proper motions to remove its effect in the distortion model. Accurate calibration of the distortion & its time-dependency will allow DrizzlePac software (Gonzaga, et al, 2012) to align and combine ACS images with any HST images with high accuracy and precession. DrizzlePac has been modified to properly handle the new implementation of the time-dependency distortion (Avila, et. al., 2014)
2014 AAS Posters
ACS/WFC Geometric Distortions and Its Linear Time Dependency Abstract: We present a new geometric distortion model and its time dependency for the Wide Field Camera (WFC) of the HST Advanced Camera for Surveys (ACS). The primary goal of this study is to derive a new Instrument Distortion Coefficient (IDC) table for superior alignment and combination of WFC images. This will allow the STSDAS and DrizzlePac software to accurately align and combine ACS images with any HST images. The calibration observations of globular cluster 47Tuc taken through the ACS/WFC F435W, F606W and F814W filters before and after Servicing Mission 4 over a wide range of the HST roll angles were used for astrometric calibration. DrizzlePac had to be modified to properly handle the new implementation of the time-dependency. !
The HST Frontier Fields: DrizzlePac Workflow Abstract: We demonstrate the power and usability of the DrizzlePac image processing tools developed at the Space Telescope Science Institute. These tools are available to the astronomical community, to align, distortion-correct, and combine stacks of images such as the Frontier Fields mosaics. Using 'cosmic-ray cleaned' images, we test various techniques for producing source catalogs to refine the image alignment. We present methodology for aligning images across visits, across filters, across detectors, and finally to an absolute reference catalog. The alignment solutions, or 'headerlet' files, will be made available to community as 'High Level Science Products' which may be applied to archival data in order to reduce the amount of work needed to re-process the Frontier Fields dataset. We also describe methodology for optimizing the drizzling 'pixfrac' (or drop size) of the final image for any given plate scale in order to provide the best signal-to-noise trade-off between pixel sampling and background noise.
ACS/WFC Geometric Distortion: Time Dependency Study Abstract: We re-visit the issue of the time-dependency variation of the linear terms in the ACS/WFC geometric distortion. We performed a detailed photometric/astrometric study using F606W FLT and FLC images from the calibration field near globular cluster 47 Tucanae. We analyzed the time dependency of the linear terms by comparing individual observations with a standard catalog. A previous calibration of these drifts proved to be able to restore positions to the milli-arcsecond level for pre-SM4 data. We confirm this previously existing solution and we provide new and simple corrections for both FLT and FLC images that will allow observers to perform global astrometric studies with 0.02 WFC pixel precision using both pre- and post- SM4 images.

2013 AAS Posters
The Evolution of the ACS/SBC Sensitivity Abstract: The Solar Blind Channel on the Hubble Space Telescope has been in orbit for over 11 years and is one of the oldest far ultraviolet imagers on the telescope. Here we present the first study of the evolution of the sensitivity of the camera. A long baseline has been established by observing a calibration field (NGC6681) every year since launch in all six filters (five long- and one medium-pass). From these observations we derive the sensitivity curves from launch to present.
Mitigation of CTE Losses in ACS/WFC: Overview of Methods Abstract: The charge transfer efficiency (CTE) of the ACS/WFC CCD detectors is declining with time due to the cumulative effects of radiation damage. The methods that have recently become available to observers to mitigate CTE losses are reviewed. These include post-observation corrections, adjusting the observing strategy to minimize CTE losses, and the use of a short post-flash to increase the background.
Mitigation of CTE Losses in ACS/WFC: Observed Sky Backgrounds Abstract: Over time, exposure to the harsh radiation environment of space has diminished the charge transfer efficiencies (CTE) of the ACS CCDs4. While post-processing techniques1 can combat this loss of charge, it is also possible to ameliorate CTE losses in advance: by observing with a natural background high enough (~20 e-) to “pave over” charge traps. HST observers can use the estimates provided here to anticipate the natural background that should be present in their exposures, and can then determine whether they want to supplement that background to improve CTE by lengthening their exposures or through using a post-flash.
Mitigation of CTE Losses in ACS/WFC: Post-Flash Capabilities Abstract: The charge transfer efficiency (CTE) correction that is currently being applied in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) pipeline effectively corrects lost charge for most cases. However, when the background level is low a large percent of the original signal is lost during read out and cannot be recovered by the CTE correction algorithm.To address these cases, the ACS team is investigating the post-flash capabilities of ACS. Here we present results from our initial analysis of the repeatability, count rate, and gradient across the CCDs.
Mitigation of CTE Losses in ACS/WFC: Optimal Background Parameters From Simulated Images Abstract: The Advanced Camera for Surveys (ACS) team has been exploring ways to further mitigate the effects of charge transfer inefficiency (CTI) on the wide field channel (WFC), in particular at low background levels where losses are so large that the current methods cannot recover the original signal. Using post flash increases the background levels and mitigates losses, although this also increases the effective background noise. We simulated images to represent typical astronomical scenes with various levels of post flash and explored the relationship between signal preserved and noise added. From this we present guidelines for guest observers on how best to optimize signal to noise ratios in their observations.
Using SExtractor With the Drizzlepac Tweakreg Software for Aligning and Combining Sources in HST/ACS Images Abstract: The Tweakreg software (which is part of the new STScI Drizzlepac software for aligning and combining images) has initially relied upon a daofind-like algorithm which is tuned to finding stellar sources to use for image alignment. This works well when there are enough stellar sources present in the images. However, a significant number of images are of higher-latitude extragalactic fields with few if any notable stellar images useful for alignment. In such cases, the use of software such as SExtractor can be useful for the generation of object catalogs which can be used for image alignment. In this poster, we explore the use of SExtractor with Tweakreg for such cases. This poster is the second of two in a series by the first author. The first is currently available on-line at http://www.stsci.edu/hst/HST_overview/drizzlepac/resources/ral_aas-anchorage_2012_template-v7_Lucas.pdf and was presented at the 220th AAS Meeting in Anchorage, Alaska in June 2012. It is part of a group of posters on Astrodrizzle currently available at http://www.stsci.edu/hst/HST_overview/drizzlepac/resources/ as part of the Astrodrizzle documentation for users. This poster will be on-line in the same location.