June 05, 2020J. Anderson
Our understanding of the WFC3/UVIS detector continues to improve, even as the detector’s charge-transfer efficiency continues to degrade. There is a new pixel-based CTE model that will be introduced into the pipeline shortly. The new model is based on detailed calibration and on-sky observations that show us in detail how variations in background affect the detector’s ability to transfer charge efficiently. This will be fully written up in an ISR, but we wanted to bring these considerations to Cy 28 GOs now, so that they can plan their upcoming observations as wisely as possible based on the latest information available.
June 05, 2020J. Anderson
Dithering WFC3/UVIS observations allows users to construct the best possible representation of the astrophysical scene. While most WFC3/UVIS users do dither their observations, the archive shows that there is very little uniformity in the way users dither. This ISR is an effort to provide some uniformity to a subset of the dither cases: the small dithers used for improved sampling and defect mitigation. We study a model of the F606W PSF in order to determine how much dithering is necessary to recover all the structure that is present in a WFC3/UVIS image and determine that from a sampling standpoint, there is no need to construct regular patterns that are denser than 3´3 points (N=9). We therefore construct a set of patterns for N=2 to N=9 points in order to provide the best possible sampling for programs that have various numbers of similar exposures. These dithers must be commanded by the telescope, so we examine the archive of commanded POS-TARGs and achieved dithers to map our dithers into commandable POS-TARGs. Tables of the POS-TARGs are provided for users planning future observations.
May 07, 2020H. Khandrika
This report examines the changes in Charge Transfer Efficiency (CTE) as computed by the Extended Pixel Edge Response (EPER) technique. The data for the study were acquired from Cycle 17 through Cycle 26 (August 2009-present). Over the last 10 years, the CTE has declined below 0.9990 for the lowest signal level (~160 e-). In our analysis and report from 2016 we determined that the rate of decline is no longer well-matched by a linear fit but by a quadratic function instead. In 2016 we noted that this may indicate that the CTE decline is leveling off or reducing with time. Given the 10 years of data collected, we observe the periodic nature of the linear fit residuals and find that it is anti-correlated to solar activity. This is the first time this effect has been observed in practice in the WFC3 EPER data.
April 17, 2020V. Kozhurina-Platais, S. Baggett
The observations of the globular cluster omega Cen taken with the Wide Field Camera 3 Infrared Detector (WFC3/IR) in F160W filter over more than 10 years have been used to examine the secular changes in the detector’s sensitivity and search for its variations with time. The WFC3/IR sensitivity appears to be changing at the level of 0.2% (0.002 mag) per year. There is also possibility of the abrupt changes in the WFC3/IR sensitivity around mid-2011. In addition to the secular changes, there is signficant scatter in the sensitivity variations at the 2% level during each interval of the orbital target visibility, which is consistent with the effect of orbital breathing.
April 16, 2020N. Pirzkal, R. Ryan
Background removal is one of the, if not the most, important step during the extraction and calibration of slitless observations. Any error in the amount of dispersed background light has an immediate effect on the quality of the extracted spectra. We show in this work how we determined three new independent background components (Zodi background, HeI emission, and ``Scatter'' Earth light) which have different spectral signature and therefore contribute differently to the overall background of the final observation. We also introduce a generalized implementation of the variable background subtraction described in ISR 2017-01. This approach allows us to account for a temporal variation of both the HeI and ``Scatter'' light and retain the use of on-the-ramp fitting in CALWF3. We also investigate, using all of the available G102 and G141 archival data, the conditions under which we measure an increased levels of HeI and ``scatter'' light emission.
March 31, 2020J. D. Green, H. Olszewski
``Snowballs'' are a known IR detector artifact, similar to but distinct from cosmic rays, identified in HST, JWST, Euclid, and WFIRST detectors, that are removed via pipeline processing. Unlike cosmic rays, snowballs generally show a symmetric profile that suggests a local origin from radioactive decay products of the detector itself, rather than a cosmic origin. The snowball occurrence rate, intensity, shape, and variation over time provides important constraints on candidate species by their decay rates, and HST's WFC3/IR has by far the longest time baseline for analysis. We examine WFC3/IR data from June 2014 through December 2019 and identify a conservative and a broader set of criteria to identify snowballs in this dataset. By cross-checking our analysis against datasets from earlier years published in a previous ISR, we find our criteria consistently bracket the previous reported counts, suggesting that the uncertainty in snowball count is captured between our conservative and broader criteria. We report 5261 new snowball detections under the conservative criteria, and 7545 new snowballs under the broader criteria. Added to the 7291 previously identified between 2009 and 2014, this yields a total of 12552 - 14836 snowballs over an 11 year baseline of observations. Given the quantified uncertainties in identification, we find no evidence that the rate of snowball occurrence has changed significantly in WFC3/IR over this period. This constant occurrence rate over the extended time baseline strongly rules out Th-228 radioactive decay as a major contributor. The moderately higher occurrence rate in WFC3/IR compared with most other IR detectors in shielded laboratory environments may suggest the U-238 decay origin as the most likely cause of the snowballs, but does not rule out other effects.
March 04, 2020J. V. Medina, S. Baggett, & The Quicklook Team
The WFC3 Quicklook anomalies database is introduced. While previously available only for internal (STScI) users, the contents of the table are being made available to external users. This ISR describes the database, its structure, how to access it, and caveats regarding its completeness.
February 18, 2020L. Strolger, A. Rest, O. Fox, A. Calamida, R. Ryan & N. Reid
This document presents general guidelines to investigators proposing ultra-rapid target of opportunity (ToO) observations with the Hubble Space Telescope (HST). Establishing clear plans in advance and communicating with STScI staff, particularly the Program Coordinator, are crucial to minimizing the time between triggering a ToO and executing the observations.
September 30, 2019K. B. Stevenson & W. Eck
Spatial scan observations using WFC3's IR channel exhibit time-dependent systematics (in the form of a ramp or hook) that have been attributed to the effects of persistence. The amplitude of these systematics is often two orders of magnitude larger than the signal sizes of interest and, therefore, must be carefully modelled and removed. The goal of this calibration program (CAL-15400) is to mitigate these systematics by continuously illuminating the detector while repeatedly reading it out during Earth occultation (termed preflashing). Compared to standard observations, we are able to reduce the amplitude of the systematic effect by a factor of ~7 (from 1.30% to -0.19%), thus confirming our hypothesis that the detector more quickly reaches an equilibrium state when subjected to higher flux levels. Compared to the latest modeling techniques (Zhou et al., 2017), we achieve a marginal improvement in the white light curve precision (delta rms = -8±9 ppm); therefore, preflashing is an equally effective means to mitigate WFC3's instrument systematics. We conclude that preflashing does not warrant future consideration due to the increase in the number of channel select mechanism (CSM) motions, effort required to implement, and equivalent ability to model instrument systematics with current techniques.
September 30, 2019K. B. Stevenson & J. Fowler
HST/WFC3's spatial scan monitor automatically reduces and analyzes time-series data taken in spatial scan mode with the IR grisms. Here we describe the spatial scan monitor pipeline and present results derived from eight years of transiting exoplanet data. Our goal is to monitor the quality of the data and make recommendations to users that will enhance future observations. We find that a typical observation achieves a white light curve precision that is 1.07 times the photon-limit (which is slightly better than expectations) and that the pointing drift is relatively stable during times of normal telescope operations. We note that observations cannot achieve the optimal precision when the drift along the dispersion direction (X axis) exceeds 15 mas (~0.11 pixels). Based on our sample, 77.1% of observations are ``successful'' (<15 mas rms drift), 12.0% are ``marginal'' (15 -- 135 mas), and 10.8% of observations have ``failed'' (>135 mas or >1 pixel), meaning they do not achieve the necessary pointing stability to achieve the optimal spectroscopic precision. In comparing the observed versus calculated maximum pixel fluence, we find that the J band is a better predictor of fluence than the H band. Using this information, we derive an updated, empirical relation for scan rate that also accounts for the J-H color of the host star. We implement this relation and other improvements in version 1.4 of PandExo and version 0.5 of ExoCTK. Finally, we make recommendations on how to plan future observations with increased precision.
August 19, 2019B. Kuhn, H. Khandrika
Here we discuss the 2018 superbias reference file for the Wide Field Camera 3 (WFC3) UVIS channel. We analyze the 2018 superbias for the average pixel value per chip and and compare it to the 2017 superbias. We analyze all 11 superbias files from 2009-2018 for temporal variations in the average bias level across chip 1 and 2. We investigate charge transfer efficiency effects by calculating the average value for 200 rows of pixels closest and furthest from the CCD amplifiers. We have determined the average 2018 superbias level is 0.29 ± 0.65 e−, which is 0.030 ± 0.91 e− higher compared to the 2017 superbias. Our analysis of all 11 superbias files shows the average pixel level has been increasing since 2009 at a rate of 0.014 ± 0.001 e−/year and 0.033 ± 0.002 e−/year for chip 1 and 2 respectively. We have determined that the 2018 average superbias level is 0.21 ± 0.69 e− above the 2009 level. The 2018 superbias has been delivered to the Calibration Reference Data System and observers with science images taken after 2018-01-01 may request the improved products of their data through the Mikulski Archive for Space Telescopes.
August 16, 2019J. V. Medina, M. Bourque, S. Baggett
In late 2012 when the post-flash capability became available on-orbit, the WFC3/UVIS daily monitoring program was modified to acquire flashed dark observations in order to mitigate CTE loss and improve the hot pixel maps generated from the superdarks. To determine the efficacy of flashing the UVIS darks, we performed a comparative analysis using the un-flashed equivalent of the post-flashed reference files. The overall behavior of the hot pixel and median dark current evolution shows that post-flashed superdarks contain at least ≅ 50% more hot pixels, and had ≅ 2 electrons/hr lower dark current for each anneal cycle. A study of the hot pixel maps of several post-flashed and un-flashed superdarks shows that flashing the detector does have its limitations, as a ≅ 55% efficiency drop-off was found between the first few and last few rows for the post-flashed superdark (for each chip). Application of the CTE correction step in calwf3 can mitigate the CTE losses further. Finally, we note that flashing the detector has proven to be most efficient at preserving the hot pixels in the middle of the chip.
July 26, 2019H. G. Khandrika, T. D. Desjardins
We report the results of a study of the Wide Field Camera 3 (WFC3) UVIS detector bias pre-scan level fluctuations since installation in 2009. The study was performed by extracting the bias level from the header information of 27,069 calibrated full-frame UVIS images of various targets. We look for variability in the data by computing the $\chi^2$ statistic and we look for periodic signals in the data by performing Lomb-Scargle periodogram analysis and phase dispersion minimization. We find an overall decrease in the bias pre-scan levels of 1.2 to 2.2 electrons over 10 years depending on quadrant. We find long-term periodicity of 0.5 year cycles and short-term periodicity for both chips at 54.7 day cycles. We compare our results with findings from the Advanced Camera for Surveys Wide Field Channel instrument, which observed similar variations and periodicity, but with more structure and larger overall bias level losses. While the fluctuations do not affect science data, they serve as a litmus test for the behavior of the detector, ancillary components, and sensors that may not be able to detect subtle changes over time.
July 03, 2019C.Martlin, V. Bajaj, V. Kozhurina-Platais
The central region of the globular cluster Omega Centauri has been monitored over the last 10 years using the WFC3 UVIS and IR instruments for the purposes of calibrating and monitoring the detectors' geometric distortion. In the current study, the DrizzlePac/TweakReg software was used to investigate the accuracy of the current WFC3/UVIS geometric distortion solutions over time by comparing it against the Gaia DR2 catalog. Using the TweakReg output parameters, which are calculated with allowance for linear terms in the transformations between the coordinate systems, we are able to verify the stability of the WFC3/UVIS geometric distortions over time in the range of << 0.001 pixels and find no discernible temporal variation. We are able to conclude that, because of the extremely crowded field in the central regions of omega Cen, the proper motions in the Gaia DR2 catalog have RMS errors that are 10 times larger than expected. This means that Gaia DR2 cannot currently be useful as an absolute reference system for refining the geometric solutions for WFC3/UVIS, at least not with this much-observed field.
June 07, 2019V. Bajaj
The infrared channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) is frequently used to obtain precision photometric measurements. We investigate the repeatability of WFC3 infrared (WFC3/IR) photometry by analyzing repeated observations of local stellar clusters and star forming regions. In general, the 1σ repeatability for aperture photometry is ±1.5%, even with (included) Poisson noise well under 1%. The repeatability seems to be affected by persistence of previous exposures, which can mostly be alleviated by dithering to previously unused positions, with a spacing of at least 10 pixels between positions. Observations taken this way have a much better 1σ repeatability limit of 0.5% (when Poisson noise is much smaller than 1%). The repeated measurements of standard white dwarf stars used in the absolute photometric calibration show a spread of σ = 1.5%, (Poisson noise is approximately 0.5%, indicating the repeatability is also affected by persistence) so the calibration will be improved by taking observations with a more advantageous dither strategy.