AAS Posters, January 2014
| 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. |
AAS Posters, January 2013
| 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. |
