Filter WFC3 ISRs
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.
ISR 2020-02: WFC3 Quicklook Anomalies Database
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.
ISR 2020-01: Recommendations for Optimizing Rapid Ultraviolet HST Observations of Gravitational Wave Optical Counterparts
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.
ISR 2019-13: Pre-Flashing WFC3/IR Time-Series, Spatial Scan 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.
ISR 2019-12: Analyzing Eight Years of Transiting Exoplanet Observations Using WFC3's Spatial Scan Monitor
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.
ISR 2019-11: WFC3/UVIS: 2018 Superbias Reference File
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.
ISR 2019-10: WFC3/UVIS CTE Monitor: Efficacy of Post-Flash in the UVIS Darks
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.
ISR 2019-08: Periodicity in the WFC3/UVIS Bias Pre-Scan
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.
ISR 2019-09: Comparison of WFC3/UVIS Geometric Distortions Solutions to Gaia Data Release 2
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.
ISR 2019-07: WFC3/IR Photometric Repeatability
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.
ISR 2019-06: Monitoring of the Internal Flat Fields for WFC3/IR
May 22, 2019R. E. Ryan
We analyze 10 cycles of WFC3/IR internal flat field images taken with the tungsten lamp, which includes two additional years of data since ISR WFC3 2015-11. We follow past analyses to mask short- and long-term persistence, determine robust image statistics, and create stacked subsets of the data. We show that the mode count-rate of each filter is decreasing with time and is strongly correlated with wavelength, such that the data from the red filters (e.g. F153M and F160W) show very little variation (~0.05 %/yr), while the blue filters (e.g. F098M) decrease by typically ~0.35 %/yr. We present "master" and "cycle" stacks, where we combine the normalized exposures for all and a given cycle, respectively. From inspection of ratio images (cycle-to-master), we do not find strong evidence for changes in the pixel-to-pixel sensitivities as a function of time for most filters and cycles. The largest deviations are for F125W between cycles 17 and 18, where the wagon wheel changes considerably. Combined with the photometric stability, our results imply that the tungsten lamp is effectively becoming redder with time, which is consistent with the filament-vaporization hypothesis Baggett (2009), whereby the tungsten filament is vaporizing over time and coats the inner surface of the lamp and acting as an additional filtering surface.
ISR 2019-05: Improved Drizzled Data Products for the WFC3/IR Detector
May 06, 2019J. Mack, V. Bajaj
ISR 2019-04: Time-dependent WFC3/IR Superdarks
May 06, 2019B. Sunnquist, M. Mckay, S. Baggett
ISR 2019-03: Time-dependent WFC3/IR Bad Pixel Tables
May 06, 2019B. Sunnquist, G. Brammer, S. Baggett
ISR 2019-02: A characterization of persistence at short times in the WFC3/IR detector. II
April 24, 2019M. Gennaro, S. Baggett, V. Bajaj