The pixel-based CTE correction was last constructed for ACS/WFC in 2010 and for WFC3/UVIS in 2013. Each of these instruments has now been in orbit for about twice as long as when the model was last constructed. Since the strength of CTE generally increases linearly with time, similar to the population of warm pixels, it makes sense to revisit the correction now that the effect is twice as strong and there exist twice as many pixels to measure it with. This ISR will demonstrate how we constructed a new model for ACS/WFC and will evaluate the model with on-sky data.
J. Anderson and J.E. Ryon, 29 August 2018
Here we present our investigation of three image artifacts that recently appeared in images from Hubble's Advanced Camera for Surveys (ACS) Wide Field Channel (WFC). We discovered one of the artifacts through visual inspection of a WFC image during an unrelated task. A search of routine calibration data revealed that it, and two additional artifacts, appeared on May 5th, 2017. We named the three image artifacts "flecks" because they looked to be small flecks of material sitting on the surface of the WFC detector in our initial examination. We characterized the flecks by analyzing them in routine calibration images. Due to their small size, we do not believe the flecks will have an impact on science use of ACS/WFC. Finally, while we believe the appearance of the flecks to be an isolated incident, we will continue to monitor ACS/WFC images for new flecks in the future.
S.L. Hoffman et al, 25 May 2018
This report presents a new technique for generating the post-flash calibration reference file for the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC). The new method substantially reduces, if not, eliminates all together the presence of dark current artifacts arising from improper dark subtraction, while simultaneously preserving flat-field artifacts. The stability of the post-flash calibration reference file over time is measured using data taken yearly since 2012 and no statistically significant deviations are found. An analysis of all short-flashed darks taken every two days since January 2015 reveals a periodic modulation of the LED intensity on timescales of about one year. This effect is most readily explained by changes to the local temperature in the area surrounding the LED. However, a slight offset between the periods of the temperature and LED modulations lends to the possibility that the effect is a chance observation of the two sinusoids at an unfortunate point in their beat cycle.
N. Miles, 29 March 2018
The goal of astrometric calibration of the HST ACS/WFC and WFC3/UVIS imaging instruments is to provide a coordinate system free of distortion to the precision level of 0.1 pixel ( 4-5mas) or better. This astrometric calibration is based on two HST astrometricstandard elds in the vicinity of the globular clusters, 47 Tuc and ω Cen, respectively. The derived calibration of the geometric distortion is assumed to be accurate down to 2-3 mas. Is this accuracy in agreement with the true value? Now, with the access to globally accurate positions from the rst Gaia data release (DR1), we found that there are measurable osets, rotation, scale and other deviations of distortion parameters in two HST standard astrometric catalogs. These deviations from the distortion-free and properly aligned coordinate system should be accounted and corrected for, so that the high precision HST positions are free of any systematic errors. We also found that the precision of the HST pixel coordinates is substantially better than the accuracy listed in the Gaia DR1. Therefore, in order to nalize the components of distortion in the HST standard catalogs, the next release of Gaia data is needed.
V. Kozhurina-Platais et al., 16 February 2018
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. We present an analysis of simulated readout dark images developed in ACS ISR 2014-02, and find that the results from the simulated data cannot be fully reconciled with the results from observed bias images. We develop a new method to estimate the readout dark noise properties in ACS/WFC observations instead of using simulated images. 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 will update the ERR extensions of superbias images to include the appropriate noise from the ambient readout dark gradient and stable hot columns. We will also flag unstable hot columns in the DQ extensions of the superbiases. A new reference file pipeline for ACS/WFC that will implement these changes is currently under construction.
J.E. Ryon et al., 22 December 2017
Every year, the Space Telescope Science Institute allocates over 3000 orbits of Hubble time to approved Guest Observer, Snapshot, 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. The initial target list comprises ~500 moderately large, bright NGC/IC galaxies that were not priviously imaged by HST in V -like filters. As of Septemer 2017, over 100 galaxies have been observed as part of this program (≳ 2 galaxies per week). This document focuses on the data quality of the first 10 months of observations. All data taken through the SNAP-14840 program are intended for legacy science only, and STScI strongly encourage the astronomical community to use these data for science purposes.
A. Bellini et al., 11 September 2017
A study was undertaken using synthetic photometry of CALSPEC stars to compare the ACS Wide Field Channel (WFC) photometry to the WFC3 UVIS imaging channel in eight similarly named passbands corresponding to the broadband filters F435W (ACS/WFC) F438W (WFC3/UVIS) and F475W, F555W, F606W, F625W, F775W, F814W and F850LP (both ACS/WFC and WFC3/UVIS). The uncertainty of the photometric calibration of ACS/WFC and WFC3/UVIS with respect to the white dwarf standard stars is within +/- 0.5% for F814W, F775W, F606W and F475W, and within +/-1% for F625W and F850LP. For F555W the apparent difference in the calibration is 2% for F555W and 6% for UVIS/F438W and ACS/F435W due to inherent differences in the filter passbands. Comparing the ACS/WFC to WFC3/UVIS mean flux for stars having a range of spectral types shows a color dependence. The WFC to UVIS F814W color dependence is +/- 0.02 mags for F814W, F775W, F475W and F606W. For the other filters the range is -0.06 to +0.02 mags. Aperture photometry of the 47 Tucanae cluster confirm the results from using synthetic photometry of CALSPEC stars.
S.E. Deustua and J Mack, 12 March 2018
Parallel imaging data from the HST Frontier Fields campaign (Lotz et al. 2017) have been used to compute sky flats for the ACS/WFC detector in order to verify the accuracy of the current set of flat field reference files. By masking sources and then co-adding many deep frames, the F606W and F814W filters have enough combined background signal that errors from Poisson statistics are <1% per pixel. In these two filters, the sky flats show spatial residuals ~1% or less. These residuals are similar in shape to the WFC flat field ‘donut’ pattern, in which the detector quantum efficiency tracks the thickness of the two WFC chips. Observations of blue and red calibration standards measured at various positions on the detector (Bohlin et al. 2017) confirm the fidelity of the F814W flat, with aperture photometry consistent to ~1% across the FOV, regardless of spectral type. At bluer wavelengths, the total sky background is substantially lower, and the F435W sky flat shows a combination of both flat errors and detector artifacts. Aperture photometry of the red standard star shows a maximum deviation of 1.4% across the array in this filter. Larger residuals up to 2.5% are found for the blue standard, suggesting that the spatial sensitivity in F435W depends on spectral type.
J. Mack, R.A. Lucas, et al
This study compares PSFs generated with Tiny Tim against an empirically-derived, effective PSF (Anderson & King, 2006) for the Hubble Space Telescope 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 effective PSF FORTRAN photometry code. Then we perform PSF photometry on globular cluster NGC 6397 and analyze the photometry and astrometry results. We measure a value of 0.227 ± 0.032 for a quality-of-fit metric of the Tiny Tim PSF and a corresponding 0.117 ± 0.021 for the effective PSFs, an improvement of a factor of approximately two. We find that the effective PSF models outperform the Tiny Tim PSFs in every measurement of stellar sources in this field.
S.L Hoffmann and J. Anderson, 26 October 2017
We made use of the new ACS/WFC subarray images of CAL-14410, taken taken with a large range of flash exposure times (0.1–30 seconds), to probe the temporal stability of the reference flash file and to validate the current post-flash correction pipeline of CALACS and ACS DESTRIPE PLUS on the new subarray modes. No statistically-significant deviations are found between the new post-flashed subarray exposures and the flash reference file, indicating that the LED lamp used to post-flash ACS images has been stable over several years. The current calibration pipelines (both CALACS and ACS DESTRIPE PLUS can be successfully used with the new subarray modes.
A. Bellini et al., 19 May 2017
Excess thermal energy present in a Charged Coupled Device (CCD) can result in additional electrical current. This excess charge is trapped within the silicon lattice structure of the CCD electronics. It can persist through multiple exposures and have an adverse effect on science performance of the detectors 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. These images, generally referred to as “dark” images, allow for the measurement of the thermal-electron contamination present in each pixel of the CCD lattice. This so-called “dark current” can then be subtracted from the science images by re-scaling the dark to the corresponding exposure times. Pixels that have signal above a certain threshold are traditionally marked as “hot” and flagged in the data quality array. Many users will discard these because of the extra current. However, these pixels may not be unusable because of an unreliable dark subtraction; if we find these pixels to be stable over an anneal period, we can properly subtract the charge and the extra Poisson noise from this dark current will be propagated into the error arrays. 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 flagged as unuseable to be brought back into the science image as a reliable pixel.
D. Borncamp et al., 16 June 2017
The results of dark rate monitoring programs for the ACS/SBC are presented here. The dark rate has a very low and stable value of 8.11 × 106 cts/pix/s when the instrument is 25°C. In a 1000s exposure, less than 1% of pixels will have 1 count. As the instrument warms up, the overall dark rate increases due to an elevated dark rate in the central region of the detector. Recommendations are made regarding observation planning and data analysis.
R.J. Avila, 03 May 2017
At the start of Cycle 24, the original CCD-readout timing patterns used to generate ACS Wide Field Channel (WFC) subarray images were replaced with new patterns adapted from the four-quadrant readout pattern used to generate full-frame WFC images. The primary motivation for this replacement was a substantial reduction of observatory and staff resources needed to support WFC subarray bias calibration, which became a new and challenging obligation after the installation of the ACS CCD Electronics Box Replacement during Servicing Mission 4. The new readout patterns also improve the overall efficiency of observing with WFC subarrays and enable the processing of subarray images through stages of the ACS data calibration pipeline (calacs) that were previously restricted to full-frame WFC images. The new readout patterns replace the original 512×512, 1024×1024, and 2048×2046-pixel subarrays with subarrays having 2048 columns and 512, 1024, and 2048 rows, respectively. Whereas the original square subarrays were limited to certain WFC quadrants, the new rectangular subarrays are available in all four quadrants. The underlying bias structure of the new subarrays now conforms with those of the corresponding regions of the full-frame image, which allows raw frames in all image formats to be calibrated using one contemporaneous full-frame “superbias” reference image. The original subarrays remain available for scientific use, but calibration of these image formats is no longer supported by STScI.
D. Golimowski et al., 17 April 2017
The Mikulski Archive for Space Telescopes (MAST) pipeline performs geometric distortion corrections, associated image combinations, and cosmic ray rejections with AstroDrizzle. The MDRIZTAB reference table contains a list of relevant parameters that controls this program. This document details our photometric analysis of Advanced Camera for Surveys Wide Field Channel (ACS/WFC) data processed by AstroDrizzle. Based on this analysis, we update the MDRIZTAB table to improve the quality of the drizzled products delivered by MAST.
S.L. Hoffmann and R.J. Avila, 06 March 2017
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 (0.5-second) 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 the pixels immediately above and below them in the same column, resulting in high downstream pixels and low trails of upstream pixels. We determine typical trail lengths for sink pixels of different depths at various background levels. We create a reference image, one for each anneal cycle since January 2015, that will be used to flag sink pixels and the adjacent affected pixels in science images.
J.E. Ryon and N.A. Grogin, 24 Feb 2017
We present a study characterizing scattered light anomalies that occur near the edges of Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) images. We inspected all 8,573 full-frame ACS/WFC raw images with exposure times longer than 350 seconds obtained in the F606W and F814W filters from 2002 to October 2013. We visually identified two particular scattered light artifacts known as “dragon’s breath” and edge glow. Using the 2MASS point source catalog and Hubble Guide Star Catalog (GSC II), we identified the stars that caused these artifacts. The stars are all located in narrow bands (~3" across) just outside the ACS/WFC field of view (2" - 16" away). We provide a map of these risky areas around the ACS/WFC detectors – users should avoid positioning bright stars in these regions when designing ACS/WFC imaging observations. We also provide interactive webpages which display all the image artifacts we identified, allowing users to see examples of the severity of artifacts they might expect for a given stellar magnitude at a given position relative to the ACS/WFC field of view. On average, 10th (18th) magnitude stars produce artifacts about 1,000 (100) pixels long. But the severity of these artifacts can vary strongly with small positional shifts (∼ 1′′). The results are similar for both filters (F606W and F814W) when expressed in total fluence, or flux multiplied by exposure time.
B. Porterfield et al. 01 Nov 2016
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.
R.J. Avila and M. Chiaberge, 30 Sept 2016
Newly acquired data and improved data reduction algorithms mandate a fresh look at the absolute flux calibration of the CCD cameras on the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The goals are to achieve a 1% accuracy and to make this calibration more accessible to the HST guest investigator. Absolute fluxes from the CALSPEC1 database for three primary hot 30,000–60,000K WDs define the sensitivity calibrations for the WFC and HRC filters. The external uncertainty for the absolute flux is ∼1%, while the internal consistency of the sensitivities in the broadband ACS filters is ∼0.3% among the three primary WD flux standards. For stars as cool as K type, the agreement with the CALSPEC standards is within 1% at the WFC1- 1K subarray position, which achieves the 1% precision goal for the first time. After making a small adjustment to the filter bandpass for F814W, the 1% precision goal is achieved over the full F814W WFC field of view for stars of K type and hotter. New encircled energies and absolute sensitivities replace the seminal results of Sirianni et al. that were published in 2005. After implementing the throughput updates, synthetic predictions of the WFC and HRC count rates for the average of the three primary WD standard stars agree with the observations to 0.1%.
Ralph C. Bohlin 18 August 2016
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
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 .
D. Borncamp and P. Lim Jan 2016
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
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
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
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
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
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
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
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
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
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
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 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.
L. Ubeda & V. Kozhurina-Platais 26 Aug 2013
In 2010 Anderson and Bedin devised a pixel-based algorithm to correct the charge transfer ef- ficiency (CTE) losses in the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS). There have been several improvements to the CTE code throughout its development. In this ISR we will discuss the modification made to the CTE code to account for the column to column variation in the number of Y charge traps. For WFC we have found that 81% of columns fall within 10% of the average, and 96% fall within 20%. These column-specific measurements have been incorporated into the new version of CALACS.
S. Ogaz, et al. 02 Jul 2013
We present a study of the sensitivity loss of the ACS/WFC CCDs for one medium-band, eight broad-band, and three narrow-band filters. This study was done using a calibration field lo- cated 6.7 arcmin West of the center of globular cluster 47 Tucanae. For pre-SM4 images, a comparison of the sensitivity loss rates found in this research with those calculated using stan- dard white dwarf stars by Bohlin, R. et al. (ISR ACS 2011-03) shows excellent agreement within the uncertainties of the two methods. We found that the sensitivity losses are less than ∼0.0004 mag/year. We also have a baseline of at least three years of post-SM4 observations of the 47 Tucanae calibration field. Our study shows that, on average, the sensitivity loss post- SM4 is negligible. This is a remarkable result considering that ACS is an instrument that has been in space for over ten years and subject to contamination.
L. Ubeda & J. Anderson 28 Jan 2013
We present a new CTE photometric correction formula based on observation of 47Tuc obtained during Cycles 17, 18 and 19. Images were taken with two filters and different exposure times, in order to sample a wide range of background levels. In addition, the Cycle 19 program included imaging of a denser field near the center of 47Tuc with the F502N filter. Thanks to the increased number of stars available for the analysis, we are able to characterize CTE losses down to the lowest background levels (down to ~0.2e-) without significant loss of accuracy with respect to higher sky levels. The data from these three Cycles allow us to derive a new form of the correction formula that is significantly more accurate that those previously published. The formula may be used to correct stellar photometry for CTE losses on drizzled images taken after SM4. We compare the results of our new CTE correction to previous versions of the correction formula for ACS/WFC, and with the pixel-based CTE correction that is currently available as part of CALACS. The formula presented in this ISR and the pixel-based correction are in substantial agreement at high stellar fluxes and for relatively high background levels. However, the former is significantly more accurate than the latter for faint stars superimposed to a low sky background.
M. Chiaberge 03 Oct 2012
This report compares the on-orbit sky background levels present in Cycle 18 ACS/WFC full- frame images against estimates provided by the Exposure Time Calculator (ETC). Backgrounds of over ~20 e- can alleviate charge-transfer efficiency (CTE) losses, which especially affect low S/N sources. HST observers can use these estimates to anticipate the natural background that should be present in their exposures, and can then determine whether they want to supplement that background with a post-flash to improve the CTE – at the cost of more background noise.
J. Sokol et al. 31 Jul 2012
We perform a comprehensive and detailed study of the evolution of the effect of charge transfer efficiency (CTE) of the Wide-Field Channel of the Advanced Camera for Surveys (ACS). The study is based on the profiles of warm pixels in all the dark frames ever produced by ACS between 2002 and 2011. We apply the pixel-based empirical approach by Anderson&Bedin (2010, PASP, 122, 1035) which restores flux, position and shape of sources in the original images. We demonstrate that this image-restoration process properly accounts for the time and temperature dependence for CTE in ACS, and that it works for all epochs: the original setting when the camera was operated at -77C and also on the post-SM4 data obtained with the current temperature set at -81C. We also demonstrate that the code has been successfully integrated in the ACS calibration and reduction pipeline CALACS.
L. Ubeda & J. Anderson 12 Mar 2012
Since the installation of the CCD Electronic Box Replacement (CEB-R) during Servicing Mission 4, the ACS Wide Field Channel has exhibited two bias anomalies that have been linked to the CCDs’ external preamplifiers and the CEB-R’s dual-slope integrators. One anomaly is a temporally stable bias gradient of 5-10 DN across each quadrant; the other anomaly is a local bias shift that depends on the pixel signal and has an e-folding time comparable to the serial transfer of several hundred pixels. Although the bias shift is relatively small (0.02−0.3% of the pixel signal), it can be an impediment to high-contrast science observations and to the removal of other electronic anomalies (e.g., 1/f noise). We have developed a pixel-based algorithm for correcting the signal-dependent bias shift in full-frame WFC images. We describe the calibration and tuning of this algorithm for each WFC quadrant, and we discuss the implementation of this algorithm in the standard CALACS image processing pipeline. (Revised: July 10, 2013)
D. Golimowski et al. 10 Jul 2013
ISRs I-III in this series define the charge transfer efficiency (CTE) for the CCD detectors, the encircled energy fractions, and the optical throughput degradation. This fourth ISR es- tablishes the absolute flux calibration for the photometry, as corrected with the algorithms specified in I-III. Synthetic photometry from the SEDs of three primary hot 30,000-60,000K WDs define these sensitivity calibrations for the standard HRC and WFC filters. Even though the external uncertainty for the absolute flux of the three primary WD flux standards is ~1%, the internal consistency of the sensitivities is ~0.3%. However, when these WD sensitivities are applied to the cooler F, G, and K stars, there are internal inconsistencies of 1-2% for the F775W, F814W, and F850LP filters. For the same cooler stars, the other broadband filters are internally consistent to better than the 1% ACS flux calibration goal after applying the corrections and calibrations specified here. One filter F435W requires a small shift of the long wavelength edge of the bandpass by 18Å toward longer wavelengths in order to reduce the cool star residuals from as much as 2% to <0.5%; but wavelength shifts cannot remedy the flux discrepancies for the F, G, and K stars in the three long wavelength filters.
For HRC, the sensitivities change by 0.5-2% as a monotonic function of wavelength because of improvements in data processing. Additional residual corrections for individual filters are typically <0.5% for wide filters in both cameras. For the medium and narrow filters, these residuals are as large as ~3% for the WFC F550M and ~4% for HRC F344N. After implementing these throughput updates, the synthetic predictions of the WFC and HRC count rates for the average of the three primary WD standard stars agree with the observations to 0.1% for every filter.
Bohlin, Ralph C. 06 Feb 2012
Images taken with the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) after Servicing Mission 4 newly exhibit a row-correlated noise imparted by the CCD Electronics Box Replacement (CEB-R). We characterize the noise component and assess algorithms for software mitigation.
N. A. Grogin et al. 26 Jul 2011
The ACS CCD Electronics Box Replacement (CEB-R) installed during SM4 features a Teledyne SIDECAR ASIC that permits optimization of the WFC via adjustment of CCD clock voltages, bias voltages, and pixel transmission timing. A built-in oscilloscope mode allows sensing of the analog signal from each output amplifier. An on-orbit campaign to optimize the performance of the WFC was undertaken at the start of the SMOV period. Initial tests with pre-SM4 default voltages and timing patterns showed that WFC’s performance matches or exceeds its pre-failure levels, notwithstanding the expected increases in dark current and hot pixels and the decline in charge-transfer efficiency due to prolonged exposure to HST’s radiation environment. The WFC2 CCD exhibited anomalous behavior when operated with nondefault settings of its amplifiers’ reset-drain voltage (VOD). The CCD again displayed normal behavior when VOD was restored to its default setting. Consequently, the Optimization Campaign was truncated after two iterations, and ACS science operations commenced with the pre-SM4 default configuration.
D. Golimowski, et al. 14 Dec 2011
The flux calibration of HST instruments is normally specified after removal of artifacts such as a decline in charge transfer efficiency (CTE) for CCD detectors and optical throughput degradation. This ACS ISR deals with the HRC and WFC losses in sensitivity from polymerization of contaminants on the optical surfaces. Prior to the demise of the ACS CCD channels on 2007 Jan. 27, the losses are less than ~0.003 mag/year, except for the two short wavelength HRC filters F220W and F250W. The measurements of the sensitivity loss rates using a set of observations of WD flux standards has a precision of ~0.0008 mag/year, while the sensitivity loss rates using repeated observations of the globular cluster 47 Tuc are probably consistent within their currently lower precision. Following the revival of ACS WFC during the Servicing Mission 4 (SM4) in 2009 May, the gain of the new electronics was set so that the measured signal in electrons s^-1 matched the signal for the same 47 Tuc field as measured in 2002 with the F606W filter. However, a longer time baseline is required to reliably determine the post-SM4 loss rates.
R. Bohlin et al. 02 Jun 2011
In order to convert a point source flux calibration into a surface brightness calibration, the total response to a point source in an infinite aperture is required. In practice, infinite is defined as an aperture with a radius of 5".5. However, aperture photometry for such a large radius is exquisitely sensitive to the measured sky background level. In order to minimize uncertainties, corrections from one arcsec to infinity (5".5) are derived from averages over as many heavily exposed, isolated stellar images as possible. Calibrations, such as the change in sensitivity with time or flux calibrations from specific standard stars, utilize the low noise photometry for one arcsec radius; and the average correction to infinite aperture is used only as required. This ISR deals with the ACS encircled energy for one arcsec relative to infinity for the HRC and WFC.
R. C. Bohlin 29 Apr 2011
The flux calibration of HST instruments is normally specifed after removal of artifacts such as a decline in charge transfer efficiency (CTE) for CCD detectors and optical throughput degradation. This ISR deals with ACS/WFC CTE losses, which had been considered negligible for bright stars prior to the demise of the ACS CCD channels on 2007 Jan. 27. Following the revival of ACS WFC during the Servicing Mission 4 (SM4) in 2009 May, CTE corrections are now typically several tenths of a percent and should be included, even for our bright standard star observations that utilize a standard reference point which is only 512 rows from the CCD amplifier B readout corner. For such bright standard stars with negligible background signal, a simple correction algorithm with an accuracy of better than 0.1% is derived, which eliminates the need to execute the CTE correction code for the complete image.
R. Bohlin & J. Anderson 07 Jan 2011
We use an empirical approach to characterize the effect of charge-transfer efficiency (CTE) losses in images taken with the Wide-Field Channel of the Advanced Camera for Surveys (ACS). The study is based on profiles of warm pixels in 168 dark exposures taken between 2009 September and October. The dark exposures allow us to explore charge traps that affect electrons when the background is extremely low. We develop a model for the readout process that reproduces the observed trails out to 70 pixels. We then invert the model to convert the observed pixel values in an image into an estimate of the original pixel values. We find that when we apply this image-restoration process to science images with a variety of stars on a variety of background levels, it restores flux, position, and shape. This means that the observed trails contain essentially all of the flux lost to inefficient CTE. The Space Telescope Science Institute is currently evaluating this algorithm with the aim of optimizing it and eventually providing enhanced data products. The empirical procedure presented here should also work for other epochs (e.g., pre-SM4), though the parameters may have to be recomputed for the time when ACS was operated at a higher temperature than the current -81Â°C. Finally, this empirical approach may also hold promise for other instruments, such as WFPC2, STIS, the ACS's HRC, and even WFC3/UVIS.
J. Anderson & L. Bedin Sep 2010
The ACS/WFC detector consists of two CCDs, each of which is read out through two amplifiers. While reading each quadrant of the detector, the electronic crosstalk between the amplifiers induces faint, typically negative, mirror-symmetric ghost images on the other three quadrants. The effect is strongest for high-signal offending (source) pixels. Analysis of pre-SM4 crosstalk showed that its impact on ACS/WFC science is not significant and can be ignored in most science applications. In this report, we analyze crosstalk after SM4. Crosstalk due to low-signal offenders is much weaker than before SM4 and does not produce ghosts similar to those seen in pre-SM4 images. For high-signal offending pixels, we find substantial differences between the gain=1 eˉ/DN and gain=2 eˉ/DN cases. For the default gain setting of 2, the crosstalk is similar to what it was before the SM4, up to 5–8 eˉ per pixel on the same CCD. For gain=1, the crosstalk is ~100 eˉ per pixel for saturated offending pixels on the same CCD, which is more than an order of magnitude above the pre-SM4 level. The crosstalk from saturated pixels is ~20–30 eˉ per pixel on the other CCD, which is also much higher than it was before SM4.
A. Suchkov, et al. 10 Mar 2010
Charge Transfer Inefficiency (CTI) due to radiation damage above the Earth's atmosphere creates spurious trailing in Hubble Space Telescope (HST) images. Radiation damage also creates unrelated warm pixels - but these happen to be perfect for measuring CTI. We model CTI in the Advanced Camera for Surveys (ACS)/Wide Field Channel and construct a physically motivated correction scheme. This operates on raw data, rather than secondary science products, by returning individual electrons to pixels from which they were unintentionally dragged during readout. We apply our correction to images from the HST Cosmic Evolution Survey (COSMOS), successfully reducing the CTI trails by a factor of #30 everywhere in the CCD and at all flux levels. We quantify changes in galaxy photometry, astrometry and shape. The remarkable 97 per cent level of correction is more than sufficient to enable a (forthcoming) reanalysis of downstream science products and the collection of larger surveys.
Richard Massey, Chris Stoughton, Alexie Leauthaud, Jason Rhodes, Anton Koekemoer, Richard Ellis, and Edgar Shaghoulian 26 Jan 2010
For the default setting of gain=2, the individual gain values of the four WFC amplifiers are determined from internal flat field observations. The average absolute gain remains unchanged, but matching the flat fields at the boundaries of the four quadrants provides a more accurate determinations of the relative gains among the four separate amplifiers.
R. C. Bohlin et al. 08 Oct 2009
New measurements were made of the SBC dark rate in December 2008. As a function of temperature, the dark rate was found not to have changed over the two year period. A few images were found to have abnormally high count rates. The effect was traced to a flight passage within the outer edges of the South Atlantic Anomaly. The effect of detector temperature on the dark rate is discussed.
C. Cox 28 Apr 2009
Observations for the ACS external CTE monitoring program were performed in Cycles 11 through 14 using both WFC and HRC. The aim of the program is to monitor the change in CTE in both cameras, and provide correction formulae for stellar photometry. Here we present the results of the data analysis, and we provide correction formulae for photometry on drizzled images for both HRC and WFC. The correction formulae we present here are significantly more accurate than those previously published, both because of the larger amount of data available and because of a more advanced analysis strategy. Observers are encouraged to use the new formulae to correct photometry, especially in presence of faint stars on a low sky background.
M. Chiaberge et al. 06 Apr 2009
The Hubble Legacy Archive (HLA) was created to make high-quality calibrated HST image products easily available to the astronomy research community. In its first public release, the HLA database has been populated with calibrated images from the Advanced Camera for Surveys (ACS). This report serves as a guide to identify ACS image anomalies that cannot currently be corrected in the HLA calibration pipeline.
M. Stankiewicz et al. 05 Dec 2008
One of the main advantages of space observatories is the quality and stability of the point spread function that allows programs not feasible from the ground. However, when pushed to the limits, even the Hubble Space Telescope exhibits variations in the PSF that can be problematic for studies like weak lensing or identification of the host halos of bright quasars at high redshift. These variations are primarily due to small displacements in the focus of the telescope, which to a first approximation can be ascribed to temperature variations. The aim of this report is to characterize the variation of the focus position for HST in terms of the average temperature sensor values of the telescope. We propose a comprehensive temperature-focus model able to predict the position of the focus at the micron level over a dynamic range that extends from sub-orbital variations (< 1 hour) to seasonal and yearly variations. This allows us to predict the focus position significantly more accurately than using interpolation of the monthly direct measurements. Our model is also at least as accurate as the previously proposed breathing model for sub-orbital variations and it is the first one that describes longer term variations, potentially helping the determination of the model point spread function for observations lacking reference point sources.
D. Di Nino et al. 07 May 2008
We have used ACS/SBC observations of two UV astrometric fields derived from ACS/HRC data to create a new geometric distortion solution for the Solar Blind Channel. The new solution consists of three components: (a) a filter-dependent linear part that takes into account the existence of three epoch ranges in the SBC alignment; (b) the remaining 24 higher-order components of a 4th degree polynomial; and (c) a fine-correction look-up table. Some of the previous solutions introduced possible errors of several pixels when producing mosaics due to an incorrect orientation. That issue has been solved and the accuracy due to the solution itself (excluding rotation) has been improved by an additional factor of 1.5-2.0. As a result, it is now possible to measure positions of medium to high S/N stars with a relative astrometric accuracy of 3-4 mas.
J. M. Apellaniz 31 Mar 2008
A revised flux calibration is presented for the G800L grism with the ACS Wide Field Channel.
H. Kuntschner, et al. 25 Jan 2008
We have used the HST ACS/WFC observations of a Galactic bulge field taken over a continuous interval of 7 days (Prop 9750) to investigate the possible dependence of the ACS focus with the external temperatures. This dataset allows us to investigate possible focus variations over timescales of a few hours to a few days. The engineering data related to the external temperatures for this duration show that the maximum temperature change occurred over the first 1.5 days. Among all the different temperatures recorded, the truss diametric differential and the truss axial temperatures are the only two temperatures which have the same timescale of variation as the PSFwidth variations. The PSF-widths also strongly correlate with these two temperatures during this time interval. We empirically fit the PSF-width variations with these 2 temperature sensor values. This suggests that the focus has a similar dependence, and we recommend that this finding be followed up with the determination of actual focus values to check if the focus values indeed have the same correlation. If so, the temperature data can be useful in estimating the focus values, which can then be used to predict the PSFs to a first order.
K.C.Sahu et al. 18 Sep 2007
Ramp filters provide ACS users with the equivalent of tunable narrow and medium-width passband filters. Here we analyze images taken under calibration programs CAL 9671 and CAL 10742 to measure the wavelength calibration of the ACS ramp filters by observing standard stars with the ramp filters crossed with the ACS G800L grism. We find that the tested HRC ramp filters are relatively well centered on the requested wavelengths. A number of the WFC filters, however, display significant o sets from the desired central wavelength. While the WFC direct images show large image o sets (wedges) due to the crossed filters, we have attempted to remove these o sets. Our results suggest we have reduced these o sets to one pixel or less both in images taken with the ramps crossed with wideband filters and in the spatial direction of images taken with the ramps crossed with the grism. O sets in the spectral direction of the grism images cannot be distinguished from a wavelength miscalibration. Data taken of line emission from an astrophysical source through the FR782N filter and compared to ground-based imaging suggest that this filter is well calibrated, in contrast to the result of the crossed filter observations reported here. Ray tracing simulations, deeper second order grism spectra, and further observations of astrophysical targets with known strong spectral features should be able to determine the extent to which the wavelength o sets reported here are real or an artifact of the calibration method.
A.S.Fruchter & N. Pirzkal Sep 2007
A subset of the polarized images from calibration proposals 9586, 9661, and 10055 were analyzed to help determine the polarization calibration accuracy level of the ACS camera. The polarization values found here are shown to be accurate to better than 1%. Differences in Multidrizzle weighting schemes are examined.
M. Cracraft & B. Sparks 20 Aug 2007
I present the data for two fields that can be used to obtain accurate astrometric calibrations in the UV. The two fields are located in NGC 604 (a Scaled OB Association in M33) and NGC 6681 (a Galactic globular cluster). The coordinates are derived from multiple ACS/HRC exposures, use the Anderson and King (2004) geometric distortion solution, have typical relative uncertainties of 1 mas, and can be used to derive geometric distortion solutions for detectors with a field of view smaller than 1′. In the process of generating the astrometric fields, the long-term stability of the HRC geometric distortion solution has been successfully tested. In two future ISRs these results will be used to derive new geometric distortion solutions for the STIS NUV- and FUV-MAMA and for the ACS SBC(Revised on 4 Oct 2007).
J. M. Apellaniz 04 Oct 2007
The linear skew terms have changed monotonically since ACS was installed 2002. These skew terms cancel out when relating data sets taken at the same epoch and same orientation, but they must be accounted for when dealing with observations taken at different roll angles.
J. Anderson Jul 2007
Astrometric calibrations of JWST will use observations of a reference field in the Large Magellanic Cloud. This field will itself be astrometrically calibrated using observations with ACS/WFC on HST. The understanding of the ACS/WFC scale and rotation obtained through these analyses is sufficiently accurate to meet the JWST astrometric requirements.
R. P. van der Marel, et al. 02 Jul 2007
The absolute flux calibration of the standard WFC and HRC filters is derived from the available constraining observations of spectrophotometric standard stars. Values for the encircled energy (EE) of one arcsec radius relative to an infinite aperture radius are derived for hot stars and compared to the EE for cooler stars. The sensitivity degradation for five year ACS lifetime is defined and used to correct the ACS photometry before deriving revised quantum efficiency (QE) curves for the CCD detectors. Broad band EQ changes with a maximum of 2.3% for WFC are also included in the revised QE curves for both CCD cameras. Revisions of the average filter transmissions of up to 4% are required to bring both broad and narrow band photometry into exact agreement with synthetic photometry from the primary white dwarfs (WD) stars.
R.C. Bohlin 12 Jun 2007
We report the detection of an optical ghost in the Advanced Camera for Surveys (ACS) Solar Blind Channel (SBC). The ghost was first detected in deep imagery of two Herbig Ae stars, HD 169142 and HD 100453, using filter F122M in April and June of 2006, respectively.
K.A. Collins et. al. 04 Jun 2007
We present an analysis of the CTE correction for a science project using high-precision photometry in a crowded field derived with the ``effective PSF'' (ePSF) method of Anderson & King (2006) on ACS/WFC non-drizzled images. We present a CTE correction technique which can be used for data sets where images of a given field with different exposure times are compared or combined. The CTE-induced photometric losses and centroid shifts are parameterized in terms of the location of the source on the ACS/WFC CCD chips and the magnitudes of sources.
V.Kozhurina-Platais et al. 25 May 2007
We present results of the observations of the star 15 Mon, obtained with the aim of checking the impact of red leaks in the UV (and U-band) and two narrow-band filters on ACS CCDs. We derive updated passbands for the three filters and we present the updated correction table for different spectral types.
M. Chiaberge & M. Sirianni 01 May 2007
Following the recovery of ACS with the side-2 electronics in July 2006, the temperature of the WFC detector was lowered from -77C to -81C in order to mitigate CTE and hot pixels. A revised detector QE curve and a new set of photometric zeropoints have been computed for all WFC observations obtained at the new operating temperature. These zeropoints must be applied manually until the new QE curves are implemented in SYNPHOT.
J. Mack et al. 02 Oct 2007
The pixel-to-pixel flat field changes noted by Bohlin and Mack (2005) for the WFC are further quantified. During each period between anneals, a population of pixels with lowered sensitivity develops which is largely reset by the next anneal.The sensitivity deficits are twice as large in the blue as in the red.The low QE pixels recover 90% of their losses on a time scale of a few monthly anneals, but never return fully.
R. Gilliland & R. Bohlin Jan 2007
The internal deuterium lamp was used to illuminate the SBC detector through the PR110L and PR130L prisms for 12.2 hours each to produce a total of ~12,000 counts/pixel. This illumination does not simulate the OTA optics and, therefore, is not suitable for the production of a low frequency L-flat. However, the pixel-to-pixel P-flat is an improvement over the laboratory SBC P-flat currently used in the ACS pipeline for the two dispersing modes. In addition, short exposure internal lamp flats were obtained in the standard imaging filters. These flats have sufficient signal to define the low frequency L-flat field for five filters relative to the high signal F125LP flat, assuming that the relative lamp illumination does not vary with wavelength. These five ratio L-flats are smoother than the ratios of the current pipeline L-flats; but there is evidence for variation of the internal lamp illumination with wavelength. Thus, the current SBC L-flats may have some errors of a few percent due to local inappropriate lumpiness; but the alternative flats defined by the internal illumination may also have errors.
R.C. Bohlin & J. Mack Dec 2006
The ACS CCDs are equipped with LEDs that can illuminate the chips with a controllable short exposure added to an image. The purpose of this is to counteract the loss of efficiency due to charge traps which develop from exposure to ionizing radiation causing a readout loss or redistribution of charge in science images. The LED post-flash exposure fills these traps but adds statistical noise. Mainly because of this noise, the mechanism has not been used yet on science data, but might come into play as radiation damage accumulates. Once a year the procedure has been tested to confirm that it is in working order and to measure its stability. Over a four year period the mechanism has continued to function and shows no variation in output.
C. Cox 24 Oct 2006
Following the recovery of ACS with Side 2 electronics, the temperature setpoint for WFC was lowered from -77C to -81C. By comparing internal tungsten exposures taken before and after cooldown, spatial changes in the WFC sensitivity (L-flats) have been computed for all filters with a unique useafter of July 4, 2006.
Gilliland, Bohlin, & Mack Oct 2006
The log files from APSIS, the ACS science team's image processing pipeline, have been analyzed to determine the relative astrometric scatter among ACS images observed within a single visit.
R. White 07 Aug 2006
The policy and procedure are described for the implementation of MAMA (currently, ACS/SBC) observations of targets subject to infrequent and unpredictable large outbursts, that would exceed the countrate limits should they occur during the observations.
N. Walborn et al. Mar 2006
Calibrations derived from cycle 13 observations for the PR200L prism are available as configuation files to aXe software.
S.S.Larsen, J. Walsh & M. Kummel Mar 2006
Calibrations derived from cycle 13 observations are available as configuation files to aXe software.
S. Larsen Feb 2006
The effective PSF for 6 filters is presented.
J. Anderson & I. King Feb 2006
Corrections to the SBC flatfields are described as well as the time-dependent component. Six new flats were delivered to the pipeline, the resulting photometric accuracy is now +/-1% for F115LP F122M, F125LP, and F140LP and +/-2% for F150LP and F165LP.
J. Mack et al. 17 Nov 2005
These HRC earth flats are not available in the routine pipeline, but may be more appropriate than the regular pipeline flats for observations of some large, diffuse objects such as the Moon, Jupiter, or the Orion Nebula, for example.
R. Bohlin et al. 25 Oct 2005
There is no ISR 05-11
The goal of polarimetry calibrations for the ACS/HRC polarizer filters is to obtain photometric accuracy from polarized images at the level of 1%. So far such calibration has been done only for the standard wide-band filters. Thus, observations of the globular cluster 47 Tuc exposured through the filters F220W, F250W, F330W, F435W crossed with three blue-optimized UV polarizers, and F475W, F606W, F658N, F775W crossed with three visible-light-optimized polarizers have been used to examine how the sensitivity varies across the detector.
V. Kozhurina-Platais & J.Biretta 13 Aug 2005
The internal flat field lamp has been used since launch to monitor the stability of the ACS HRC and WFC flat fields. The only ubiquitous change observed in these flat fields is an excess of pixel responses that are low. This excess of values that are low by more than 3 ? varies from factors of two to several over what is expected from the tail of the Gaussian distribution of Poisson statistics. Occasionally, a pattern resembling the growth rings of a tree are seen on the WFC with an amplitude of ~1%; but this anomaly is sufficiently rare and short lived, so that the monitoring frequency can be decreased.
R. Bohlin & J. Mack 18 Jul 2005
A revised wavelength calibration is presented for the G800L grism used with the ACS Wide Field Channel.
S. S. Larsen & J. R. Walsh Jul 2005
We present the results of the pointing stability tests for HST as measured with the ACS/HRC during the two-gyro test program conducted in February 2005.
A. Koekemoer et al. 29 Jun 2005
We demonstrate a technique that can significantly improve the absolute astrometric accuracy of HST data.
A. Koekemoer et al. 28 Jun 2005
An analysis of the coronographic data quality during the February 2005 two-gyro test.
C. Cox & J. Biretta 28 Jun 2005
The internal deuterium lamp was used to illuminate the SBC detector through the F125LP filter. This illumination does not simulate the OTA optics and, therefore, is not suitable for the production of a low frequency L-flat. However, the pixel-to-pixel P-flat is an improvement over the laboratory SBC flat currently used in the ACS pipeline for the six SBC imaging filters.
R. C. Bohlin & J. Mack May 2005
We present the results of over two years of inflight charge transfer efficiency (CTE) monitoring of the CCDs in the Advanced Camera for Surveys (ACS), based on two internal tests: Extended Pixel Edge Response (EPER), and First Pixel Response (FPR). In general, we find that CTE losses are worst at the lowest signal levels, and at each signal level, CTE declines linearly over time, at a rate which is consistent with results from external photometric tests (Riess, 2004). We compare our inflight results to similar pre-flight baseline data, and to predictions for inflight performance, which were based on radiation tests.
M. Mutchler & M. Sirianni 06 Apr 2005
The flat-fielding of the ACS WFC and HRC for the G800L grism is described. The efficacy of this flat-field cube has been investigated from observations of a spectrophotometric standard at different positions in the field. By requiring the extracted spectra from different positions over the field to match in flux, a smooth function was derived and applied to the flat-field cube. The single sensitivity curve for the whole detector area is then determined from the mean spectrum at the different positions. Flat-fields and sensitivity curves are supplied for routine spectral extraction of ACS slitless spectral data as part of the aXe package.
J. R. Walsh & N. Pirzkal 24 Feb 2005
This ISR describes the steps taken to switch from MEB1 to MEB2 including detector startup procedures and recalibration requirements.
C. Cox et al. 10 Feb 2005
We utilized 10 epochs of 15-tile ACS WFC mosaics imaging the HDFN in the F850lp filter over two years, and originally obtained for the science goal of finding type Ia supernovae at z>1, to examine the photometric stability of the WFC (with at least filter f850lp). Using repeated measurements of 371 stars, we performed a multi-variate linear regression to determine the dependence of photometric variations on the time-dependent components of parallel and serial CTE degradation as well as an overall time-dependence which would indicate a change in the WFC?s sensitivity. Despite important differences between the HDFN scenes and those in the original calibration field of 47 Tuc (i.e., source crowding and sky level), we find the losses due to imperfect CTE to be consistent between the two independent calibrations. Interestingly, we also find a decrease in the overall sensitivity of the ACS WFC at a rate of 0.004 +/- 0.001 magnitudes per year (consistent with findings based on 47 Tuc data from work in progress by Mack et al. 2005, in prep.).
A. Riess 27 Dec 2004
The effects of vignetting by the Lyot stop and the presence of the occulting spots and their time dependent motion on the ACS coronographic flat fields.
J. Krist et al. 30 Aug 2004
Fitting the low frequency and high frequency components of the HRC geometric distortion.
J. Anderson & I. King Aug 2004
SBC dark images have been collected to provide dark current subtraction for science images and to monitor the instrument health and performance. Cumulative images which add counts from the whole history of the detector's use are also generated to anticipate any long-term degradation in performance. Images collected to date show no loss of efficiency or indications of any problem.
Colin Cox 16 Jul 2004
Cross talk is observed when using ACS/WFC. A strategy using GAIN=2 is described to minimize cross talk in ACS.
M. Giavalisco 10 Aug 2004
Images acquired with ACS/WFC are affected by cross talk which is observed as negative ghost images. The phenomenology is described and its effect on photometry.
M. Giavalisco 10 Aug 2004
The goal of the astrometric calibration of the ACS/HRC polarizer filters is to obtain a coordinate system free of distortion to a precision level of 1 mas.
V.Kozhurina-Platais & J. Biretta 01 Jul 2004
Describes the discovery, possible cause and POLV filter geometry of the anisotropy between polarized and non-polarized images.
J. Biretta & V.Kozhurina-Platais 17 Jun 2004
A status review of the ACS polarization calibration begins with a brief description of the instrument and the GO science program, reviews the pre-flight calibration and on-orbit data. Various key parameters are derived and discussed. Closes with a summary of remaining issues, advice for observers, and a summary of future plans.
J. Biretta, et al. 14 Jun 2004
Characterizes the on-orbit sensitivity of the ACS CCDs cameras via observations of spectrophotometric standard stars including updates to SYNPHOT. Photometric zero points are calculated for the WFC and HRC in VEGAmag, STmag and ABmag.
G. De Marchi et al. 11 Jun 2004
Production of the routine superbias and superdark reference files is explained. We describe the primary features contained in these files, and provide some guidance on how ACS users can produce even higher signal-to-noise calibrations for datasets with extraordinary calibration requirements (e.g. deep field observations).
M. Mutchler et al. 21 May 2004
We see evidence for a modest increase in these photometric losses with time. We provide a global fitting formula to correct for CTE losses for all flux levels, sky values, and times. We extrapolate our time-dependent correction formula to the end of the decade and predict that the vast majority of science applications will retain their precision of flux measurements to better than a few percent.
A. Riess & J. Mack 05 May 2004
The UV performance and contamination of the HRC and SBC are discussed.
F. R. Boffi et al. 08 Apr 2004
The ACS charge transfer efficiency (CTE) is analyzed during March 2003 when the elevated temperature program was executed.
M. Mutchler & A. Riess 01 Mar 2004
Without SM4, gyroscope survival is a critical factor for the HST Mission lifetime. A simple Monte-Carlo model is presented to calculate the survival probabilities for various scenarios. Predictions are made for the HST Mission end date (~2007) based on current gyroscope usage. Recommendations are made for alternative usage strategies that will maximize the overall HST mission lifetime. Such strategies have the potential to push forward the HST Mission end date by as much as 10 months.
R. van der Marel 21 Jan 2004
The Lossy Compression of ACS images is analyzed and discussed.
C. Cox 20 Jan 2004
Corrections to the supported gain values, maps of the full well depth saturation and linearity at different levels are presented.
R. L. Gilliland 02 Jan 2004
Modelling of the layer structure of the ACS HRC and WFC CCD's is described. Application of a fringe model to the correction of extracted spectra is outlined.
J. R. Walsh et al. 05 Dec 2003
The ACS filter wheel movements are accurate to one motor step, which leads to errors that exceed one percent in the flat fields over small regions for a few filter combinations. For seven of these filter modes on the WFC and six on HRC with the worst blemishes, flat fields are available as a function of filter wheel offset step; and the pipeline data processing will select the flat corresponding to the offset step of each observation.
R. Bohlin et al. 29 Oct 2003
A method is presented for determination of the low-frequency flat-field (L-flat) structure from photometry of a stellar field that is imaged multiple times with different pointing or roll. A numerical implementation of the algorithm is presented and its accuracy verified using tests with artificially generated data. The software was used to generate the L-flats currently in the ACS pipeline. Newly implemented features in the software should allow further improvements in these L-flats.
R. van der Marel Sep 2003
A preliminary on-orbit calibration of the photometric losses due to imperfect CTE. The characterization of the WFC CTE in this report has been superseded by ACS ISR 04-06.
A. Riess 15 Aug 2003
The verification tests for the Astronomer's Proposal Tool (APT) Exposure Time Calculator (ETC) for the Advanced Camera for Surveys (ACS) are presented.
F.R. Boffi et al. 10 Jul 2003
G800L grism spectra of the Wolf-Rayet star WR45, obtained with the High Resolution Channel (HRC) during the Servicing Mission Orbital Verification (SMOV) tests, are presented. The target has been observed in five different positions across the HRC field of view in order to quantify the field dependence of the grism physical properties and wavelength solution.
A. Pasquali et al. 08 Jul 2003
The ACS point spread function remains fairly stable over its field of view, compared to the PSFs in WFPC2 or STIS. However, ACS/WFC PSF core width and ellipticity variations are large enough to be of concern to those undertaking very small aperture photometry or measuring small, bright-nucleus galaxy ellipticities.
J. Krist 25 Jun 2003
The background contributed by scattered Earth light at various angles is discussed along with flat field artifacts.
John Biretta et al. 23 Jun 2003
We have investigated the dark rates and hot pixel counts by raising the WFC temperature. We find that the predominant effect of a change in temperature is a simple scaling of the dark rate in each pixel. These results are applicable to the effects of the Aft Shroud Cooling System installation in in Servicing Mission 4.
C. Cox et al. 23 Jun 2003
Calibration observations (9662) have been used to quantify precise exposure time values, stability and shading effects down to the shortest allowed exposures (0.1s HRC, 0.5s WFC).
R. L. Gilliland & G. Hartig 03 Jun 2003
Creation of Earth flats for ACS/HRC modes are described including red leak and dust mote information.
R.C. Bohlin & J. Mack 21 Apr 2003
We present the G800L grism spectra of Wolf-Rayet stars acquired with the ACS/WFC during SMOV and early Cycle 11. We discuss the procedure for fitting the dispersion correction of the grism. We also describe the calibration files derived from these data which are used by the ST-ECF extraction package "aXe".
A. Pasquali, et al. 05 Mar 2003
Small time-and-initialization-dependent instabilities in the ACS coronagraph have led to revisions of the coronagraph commanding procedures and to the suggested methods for optimizing coronagraphic observations. An example optimized sequence is given.
J. Krist 18 Dec 2002
We describe the anticipated software tool development requirements for ACS for the period September 2002 to mid-2003, including issues related to image registration and combination.
W. B. Sparks, et. al 16 Dec 2002
The anneal rate of hot pixels on the ACS WFC is discussed. A fitted, successive annealing function is used to project forward in time the expected fractional coverage of the CCD by hot pixels.
A. Riess 12 Dec 2002
The uniformity of the WFC detector response has been assessed by using multiple dithered pointings of dense stellar fields. The original WFC laboratory flat fields produce photometric errors of +-5 to +-9 percent from corner-to-corner.
J. Mack et al. 21 Aug 2002
We have made an initial study of the characteristics of cosmic ray impacts on the two ACS imaging cameras, HRC and WFC. Distributions of sizes and anisotropies are determined for both cameras, characteristics which can be useful for distinguishing cosmic rays from astrophysical sources in a single image.
Riess 06 Jun 2002
We have made an initial study of hot pixels on the ACS CCD s, i.e.those with elevated dark current. The characteristics of these pixels are similar to those seen on previous CCD s flown on HSTand are likely caused by radiation damage.
A. Riess et. al 06 Jun 2002
Description of the patch used to correct the HRC external illuminated LP-flats after the disapperance of a dust-mote during the flight-level acoustics test in March 2001.
J. Mack & R.C. Bohlin 07 May 2002
The flat field correction scheme for the ACS prism (PR200L) and grism (G800L) is discussed.
R. Bohlin & G. Hartig Mar 2002
The relative gain ratios among the four WFC amplifiers are determined from flat field observations for each of the four gain settings.
R. Bohlin et al. Feb 2002
Aperture positions and image distortion coefficients are determined from RAS-HOMS measurements.
C. Cox & D.Lindler 28 Jan 2002
Discusses the ground flats for HRC and WFC ramp filters.
R. Bohlin & G. Hartig Jan 2002
Laboratory flats with simulated sky illumination of the CCD cameras have been obtained for supported and many unsupported ACS modes. All 157 flats are now available as reference files for the pipeline processing of ACS observations from Cycle 11.
Ralph Bohlin et al. Dec 2001
We describe the anticipated software tool development requirements for ACS for the period November 2001 to mid-2002, a period which is expected to include the installation of ACS on HST.
W.B. Sparks et al. 12 Dec 2001
Criteria for selection of wavelength calibration targets for the ACS grism in order to determine its dispersion and zero point are discussed.
A Pasquali et al. 14 Nov 2001
Describes a calibration reference file format and how it will be used to calibrate ACS observations for geometric distortion.
W. Hack & C. Cox 31 Jul 2001
We provide some background on pointing the ACS, and present the dither and mosaic patterns that will be provided as a convenience for HST Cycle 11 Phase II proposal writers.
M. Mutchler & C. Cox 19 Dec 2001
An update of ACS/ISR 2000-02. The initial default non-proprietary pure parallel program is described including the observing sequence and scientific questions that can be addressed. The field of view plot has been updated.
W.B. Sparks et al. 29 Aug 2001
The components of the ACS need to be maintained within certain temperature limits for health and safety considerations and for the optimal scientific use of the cameras. Current planning calls for the NICMOS cryolcooler to be installed at the same time as the ACS but the ASCS to be placed on a later mission. The ACS will operate for at least a year without the cooling benefit of the ASCS.
C. Cox & C. Cottingham 07 Jun 2001
In order to provide geometric correction for single pointing ACS images, and to provide geometric correction together with simple image combination for associations of ACS images, we describe plans to implement the "drizzle" code by means of a python wrapper, and to use this wrapper in calacs.
W. Sparks et al. 25 Apr 2001
We introduce SLIM, a slitless spectroscopy simulator written in Python which can be used to simulate the ACS grism and prism modes. Here, we outline the features of SLIM and present some WFC and HRC grism simulations of emission line objects.
N. Pirzkal et al. 04 Apr 2001
We present SLIM simulations of the WFC and HRC grism, obtained by varying the object size and orientation with respect to the dispersion axis. Our aim is to quantify the extent by which the object extension along the dispersion axis can degrade the nominal spectral resolution of the grism.
A. Pasquali et al 07 Mar 2001
Tables of the essential properties for polarization measurement of all the ACS filters are presented. The tables provide a comparative assessment of the quality of the polarization measurements between the different filters. The data will be compared with ground-based and in-flight calibrations to refine the polarization properties, especially for the most popular filters.
J. R. Walsh 26 Feb 2001
The new reference table, IDCTAB, will support the description of geometric distortion models for instruments. This report describes the columns in the table and how the coefficients in the table can be used.
W. Hack & C. Cox 06 Dec 2000
The ACS WFC pixel-to-pixel P-flats with the build-3 detector are discussed and compared to the HRC build-2 P-flats. The goals for these WFC and HRC ground flats are stated.
R. Bohlin et al. Oct 2000
The space environment is a complex, orbit dependent phenomenon. CCD detectors are particularly vulnerable to damage by ionizing radiation. This document summarizes the modeling and analysis that was performed to determine the appropriate exposure level for ground testing of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) Charge-Coupled Device (CCD) detector.
M. Jones 15 May 2000
Our baseline suite of test cases for the Spectroscopic Exposure Time Calculator are documented for all five spectroscopic modes with a variety of point source flux distributions.
F. R. Boffi et al. 25 Aug 2000
The verification tests for the ACS Ramp Filter Exposure Time Calculator are presented. Our baseline suite of test cases includes one calculation for all filter modes with the same target, plus one subset for all kinds of targets through the same filter.
D. Van Orsow et al. 23 Aug 2000
The verification tests for the Imaging Exposure Time Calculator for the Advanced Camera for Surveys are presented. Our baseline suite of test cases includes one calculation for all filter modes with the same target, plus one subset for all kinds of targets through the same filter.
F. Boffi et al. 04 Aug 2000
The ACS ramp filters are characterized by transmission curves which populate the STScI Synphot database for use in the Exposure Time Calculator.
R. Bohlin & Z. Tsvetanov Jun 2000
The Aberrated Beam Coronograph (ABC) on the Advanced Camera for Surveys (ACS) is modeled to determine what the coronagraph can do and how to get the most out of it.
J. Krist 30 Mar 2000
We describe the issues relating to internal geometrical distortions in the ACS. A software tool development route is outlined and we describe other software tool development activities.
W. Sparks et al. 08 Jun 2000
We describe the initial default non-proprietary pure parallel program for the Advanced Camera for Surveys. We describe the observing sequence and outline scientific questions that may be addressed with the data.
W. Sparks et al. 08 Jun 2000
The three prisms and grism in ACS are characterized in terms of dispersion relations and sensitivity.
R. Bohlin et al. May 2000
This report details the features of the ACS quicklook PDF products produced by the HST data pipeline.
A. Suchkov 16 Dec 1999
We describe the testing that was done to ensure that CALACS properly rejects cosmic ray contamination when combining multiple images.
M. Mutchler et al. 30 Nov 1999
This report describes the reference files used by CALACS to calibrate ACS observations.
W. Hack 11 Nov 1999
Tables of limiting magnitudes for all observation modes of the ACS Solar Blind Channel are calculated for a range of stellar energy distributions, using Kurucz model fluxes and observed standard stars.
F.R. Boffi & R.C. Bohlin 31 Aug 1999
This report details how CALACS produces ERR (error) array output.
D. VanOrsow et al. 17 Aug 1999
The decisions made in the design of the data format and packaging of the ACS science headers.
R. Jedrzejewski 25 Jun 1999
This report describes the basic testing that was done to ensure that CALACS properly reduces raw ACS images and makes the corresponding modifications to their headers.
M. Mutchler et al. 28 May 1999
This report describes the usage and implementation of CALACS. Instructions for using stand-alone tasks and the format of the input data are described in this paper. Furthermore, the processing steps for ACS data and the functional flow of the entire pipeline is outlined, along with descriptions of how the memory model was implemented.
W. Hack 28 May 1999
SBC flats for the six filters and two prisms taken during thermal vacuum testing at GSFC in 1999 March are characterized.
R.C. Bohlin et al. May 1999
The flat field baseline goal is to obtain a complete set of pixel-to-pixel P-flats before launch and to use the onboard lamps to track changes. A second goal of the ground calibration program is to obtain the low fre-quency L-flat variation over the field of view.
R. Bohlin et al Apr 2001
An extensive set of experiments was performed to test the performance of the onboard compression algorithm created for ACS by R. White.
F. Boffi & M. Stiavelli 28 Jan 1999
Bright object protection for the ACS MAMA is similar in concept to that for the STIS FUV MAMA, but several mechanisms are discussed which provide better protection for the ACS MAMA.
C. Cox et al. 17 Aug 1998
We review the motivations for dithering exposures with the ACS, discuss possible strategies for combining dithered exposures automatically and the implementation of dither patterns in RPS2.
M. Stiavelli et al. 02 Nov 1998
This report documents the basic design of those parts of CALSTIS relevant for CALACS.
W.Hack 26 May 1998
The algorithm for on-board compression on the fly of ACS data is briefly reviewed and its benefits discussed.
M. Stiavelli & R. L. White Nov 1997
We describe the expected usage of the ACS during Cycle 9 and estimate ground system requirements for the full complement of HST instruments available for Cycle 9. We recommend that the current ground system throughput capacity be doubled to support an average daily capacity of 12 Gbits/day and a peak capacity of 18 Gbits/day following the third servicing mission.
M. Stiavelli et al. Oct 1997