2016
SBC Internal Lamp P-flat Monitoring (ISR 16-02)
We report on a Cycle 23 calibration program to monitor the status of the SBC P-flat. We find random pixel to pixel changes to be small, with only 2% of pixels having changed by more than 3. There are coherent changes 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.
R.J. Avila et al. Mar 2016
Satellite Detection in Advanced Camera for Surveys/Wide Field Channel Images (ISR 16-01)
This document explains the process by which satellite trails can be found within individual chips of an Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) image. Since satellites are transient and sporadic events, we used the Hubble Frontier Fields (HFF) dataset which is manually checked for satellite trails has been used as a truth set to verify that the method in this document does a complete job without a high false positive rate. This document also details the process of producing a mask that will update data quality information to inform users where the trail traverses the image and properly account for the affected pixels. Along with this document, the Python source code used to detect and mask satellite trails will be released to users with as a stand-alone product within the STSDAS acstools package [1].
D. Borncamp and P. Lim Jan 2016
2015
Flat Field Determinations Using an Isolated Point Source (ISR 15-07)
The traditional method of measuring ACS flat fields (FF) involves a complicated analysis of multiple observations of a region of the 47 Tuc globular cluster at overlapping field positions. The analysis of the dithered 47 Tuc images suffers from source crowding and possible systematics related to the CTE correction and the high density of sources. New programs 13167 and 13602 avoid these problems by observing a single bright star at several locations around the field of view (FOV) in F435W and F814W. A discrepancy of ~3% with a 10σ level of significance exists between the two FF measurement techniques and is currently unexplained.
R. C. Bohlin et al. Aug 2015
ACS/WFC Revised Geometric Distortion for DrizzlePac (ISR 15-06)
The goal of the ACS/WFC astrometric calibration for DrizzlePac is to provide a coordinate system free of distortion to a precision level of ∼0.1 pixels (∼ 5mas). The astrometric calibration of ACS/WFC is based on the astrometric standard field in the vicinity of globular cluster 47 Tuc. We used a polynomial model to derive the geometric distortion in the WFC channel relative to the distortion–free coordinates, which now accounts for proper motions of stars in the astrometric field. A new and straightforward representation of time-dependent distortion in the linear terms is now implemented in the IDCTAB reference file and in the STScI software DrizzlePac to obtain simultaneously the ACS/WFC geometric distortion and its time-dependent correction. As a result, the geometric distortion can be corrected down to a precision level of 0.02 pix (1mas), which allows now for improvement of the alignment and registration of the ACS/WFC images with accuracy of ∼0.05 pix (2.5mas) or better.
V. Kozhurina-Platais et al. 22 Jun 2015
Basic Use of SExtractor Catalogs With TweakReg - II (ISR 15-05)
The TweakReg task in the DrizzlePac software package for aligning and drizzling images is an effective tool for the initial processing of HST images. However, the internal object-finding software which it uses to generate catalogs to be used for object matching and alignment, ImageFindPars, is based on the well-known stellar-oriented daofind routine, and though it is appropriate for most images, there are some for which the task is not well-suited, such as some extragalactic fields which have lots of galaxies but almost no stars, and in some cases, if taken over long time-baselines, even the few stars present may have moved due to proper motion, thereby making them unsuitable for use in image alignment. In that case, other object-detection programs such as SExtractor can be used to generate external catalogs of primarily galaxies which can then be fed to the TweakReg task. Even multiwavelength morphological differences may make some galaxies less suitable than others for use in image alignment and registration. The basic use of simple external SExtractor catalogs with the DrizzlePac TweakReg task is described here, using ACS/WFC data as an example. This includes updating geometric distortion files, initial drizzling to make cosmic ray-cleaned images, the set-up and use of required SExtractor files to generate SExtractor catalogs, basic manipulation of those catalogs, and their use with the TweakReg task in the DrizzlePac package. More options are explored for iteratively improving the astrometric solution than in Paper I (ACS ISR 2015-04) but this still represents an intermediate level of sophistication in methods. This paper introduces several methods but the ultimate best methods can vary with the nature of the objects and the data, etc. and are for the user to explore. Also, the use of crclean.fits images may be more suited to ACS/WFC and WFC3/UVIS than WFC3/IR.
R. Lucas 27 May 2015
Basic Use of SExtractor Catalogs With TweakReg - I (ISR 15-04)
We describe using external SExtractor (v2.8.6) catalogs from crclean.fits images to align ACS/WFC images with DrizzlePac/TweakReg. Note that this example was originally created before a more recent update to ACS/WFC geometric distortion files. At the time of this writing, one must follow the advice on the ACS Geometric Distortion web page as the first step in the process. By late 2015, as part of OPUS 2015.3, this part will be included by default in the standard pipeline processing and this will no longer need to be manually done by the user. We describe the rest of the process of preparing images for SExtractor, running SExtractor, and using the ouput catalogs to feed to the TweakReg task for alignment, and show that reasonably good first-cut results can be obtained with mostly default parameters in SExtractor and TweakReg. Better results may be possible with more exacting methods. This describes a method for quick alignment, not the ultimate best alignment. Note also that the use of crclean.fits images may be more suited to provide better results for ACS/WFC and WFC3/UVIS than for WFC3/IR.
R. Lucas et al. 27 May 2015
We present a summary and analysis of the changes made to the ACS/WFC dark reference files. As of January 15, 2015 the ACS team has begun to produce post- flashed dark reference files for the Wide Field Channel (WFC). This change was made to combat the charge transfer efficiency (CTE) losses caused by radiation damage that the two WFC CCDs have suffered since being put into orbit by artificially increasing the background in the dark images. This has resulted in several changes to the reference file pipeline, and an improved calibration dark.
S. Ogaz et al. 23 Jun 2015
Results of the Updated ACS/WFC Distortion Correction (ISR 15-02)
We present the results of testing an updated, interim, geometric distortion correction forthe Advanced Camera for Surveys (ACS) Wide Field Channel (WFC). This testing includes not only the updated distortion correction, but also a more robust implementation of the time dependent distortion. The updated geometric distortion correction including this time dependency can greatly improve the accuracy of the image alignment and provides a better representation of the undistorted image by as much as 0.15 pixels at the edge of the chips.
D. Borncamp et al. 12 Mar 2015
Optimizing pixfrac in Astrodrizzle: An example from the Hubble Frontier Fields (ISR 15-01)
We present the results of pixfrac tests conducted for the Hubble Frontier Fields and describe the methodology for optimizing this parameter for any given plate scale. These tests are the final step in the drizzling process and they provide the information necessary for reaching the best possible image resolution using the AstroDrizzle task. They are presented as an example for users to follow. Even though most users don’t have an exquisite data set like the HFF program, which consists of a large number of optimally dithered frames, these guidelines are still applicable to single visit programs that make use of sub-sampling dither patterns. We also provide code that cycles through some of the relevant parameter space and provides useful statistical analysis for inspection.
R. Avila et al. 31 Mar 2015
2014
Local Pixel Bundles: Bringing the Pixels to the People (ISR 14-04)
The automated galaxy-based alignment software package developed for the Frontier Fields program (hst2galign, see Anderson & Ogaz 2014 and http://www.stsci.edu/hst/campaigns/frontier-fields/) produces a direct mapping from the pixels of the flt frame of each science exposure into a common master frame. We can use these mappings to extract the flt-pixels in the vicinity of a source of interest and package them into a convenient “bundle”. In addition to the pixels, this data bundle can also contain “meta” information that will allow users to transform positions from the flt pixels to the reference frame and vice-versa. Since the un-resampled pixels in the flt frames are the only true constraints we have on the astronomical scene, the ability to inter-relate these pixels will enable many high-precision studies, such as: point-source-fitting and deconvolution with accurate PSFs, easy exploration of different image-combining algorithms, and accurate faint-source finding and photometry.
The data products introduced in this ISR are a very early attempt to provide the flt-level pixel constraints in a package that is accessible to more than the handful of experts in HST astrometry. The hope is that users in the community might begin using them and will provide feedback as to what information they might want to see in the bundles and what general analysis packages they might find useful. For that reason, this document is somewhat informally written, since I know that it will be modified and updated as the products and tools are optimized.
J. Anderson 09 Dec 2014
hst2galign: an Automated Galaxy-based Alignment Routine (ISR 14-03)
This brief document describes the procedure by which the individual images in the Frontier Fields program have been aligned in order to enable the self-calibration procedure (which will be described in a separate document). Along with this document, we will release the FORTRAN source code (hst2galign) that accomplishes the alignment. The source code is provided as-is, with no guarantee that it will work on any particular data set. However, it should work “out of the box” on datasets that are similar to the Frontier Fields.
J. Anderson & S. Ogaz 10 Oct 2014
Readout Dark: Dark Current Accumulation During ACS/WFC Readout (ISR 14-02)
The Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) takes about 100 seconds to read out all of the pixel data from the two CCDs. During this time, dark current continues to contribute to the image noise. We call this contribution "readout dark". Readout dark increases across the detector; pixels furthest from the amplifiers (last to be read out) are subject to the most readout dark. This additional source of noise is relatively small but has yet to be quantified in detail. We find readout dark has increased over the lifetime of ACS as radiation damage has produced more warm and hot pixels (above average sources of dark current). In recent ACS/WFC fullchip images, readout dark contributes about 0.5 electrons of noise to pixels furthest from the readout amplifiers. That is, we measure read noise to be ~4.0 electrons closest to the amplifiers and ~4.5 electrons (including readout dark) furthest from them (varying somewhat for each amplifier) in recent images. This detailed understanding of the ACS/WFC readout dark should improve algorithms for badpixel masking and for pixelbased correction of CTE/CTI (charge transfer inefficiency), which is also most significant for pixels furthest from the amplifiers.
D. Coe & N. Grogin 08 Dec 2014
Post-Flash Capabilities of the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) (ISR 14-01)
As a possible method to decrease CTE losses, the ACS/WFC post-flash capabilities have been tested and a reference file has been created. The flash level is highly varied across both WFC CCDs, with a factor of two difference in signal level between the brightest and the darkest parts of the flash. The direction of the variation is such that the post-flash is brightest far from the readout amplifiers, where the CTE trailing is stronger. The added noise and uneven correction of the post-flash, along with the success of the pixel-based and photometric CTE corrections already in place, result in a limited set of cases where post-flash may be helpful.
S. Ogaz et al. 07 Apr 2014
2013
ACS/WFC Geometric Distortion: a Time Dependency Study (ISR 13-03)
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 analized 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