Filter WFC3 ISRs
January 11, 2021C. Martlin, R. O’Brien, I. Momcheva, M. Gennaro
DASH mode enables large, shallow WFC3/IR mosaics within one orbit by dropping to gyros for guiding on all exposures after the first exposure (see Section 6.12.5 in the Instrument Handbook). Due to dropping to the gyros, the FLT/FLC pipeline output images created by the calibration pipeline are unusable for typical science as all sources will be ”smeared” across the detector. While the general process of how to derive science-ready data from DASH observations is described in the paper that introduces DASH (Momcheva, et. al 2016), the resources found at https://github.com/spacetelescope/wfc3_dash are an interactive and adaptable version of instructions and scripted workflow. In this report we provide a detailed walkthrough of the wfc3_dash package and the suggested DASH processing workflow described in a Jupyter Notebook. Each step in the workflow has parameters that can be adjusted within the package. Further, individual steps can be substituted out if the user has a different method in mind for that portion of processing. Between the Jupyter Notebook workflow and the detailed information below a user will be able to create a customized pipeline to create individual science ready images from each DASH exposure image.
ISR 2021-01: WFC3/IR Filter-Dependent Sky Flats
January 11, 2021J. Mack, H. Olszewksi, N. Pirzkal
New ‘pixel-to-pixel’ P-flats have been derived from deep images of the IR sky background, computed by stacking high signal-to-noise observations of sparse fields acquired over the lifetime of WFC3. The new sky flats correct for wavelength-dependent residuals of ± 0.5% in the central 800x800 pixel region of the detector and up to 2% near the detector edges. As of October 2020, these replace the prior 2011 set of P-flats, which were based on ground test data multiplied by a smoothed ‘grey’ (filter-independent) correction derived from sky flats using the first 18 months of in-flight data. An accompanying set of ‘delta’ D-flats now correct for 148 catalogued ‘blobs’ in six IR filters as a function of the epoch of observation. These were computed by stacking the same set of sky flat observations, but only after the appearance of each new blob.
ISR 2020-10: Updated WFC3/IR Photometric Calibration
December 03, 2020V. Bajaj, A. Calamida, J. Mack
We present the continued analysis of photometric measurements of the CALSPEC standard stars over the last 11 years in all of the 15 WFC3/IR filters. In general, the photometry (countrate) is consistent with the 2012 calibration to 1% or better. However, new models for the CALSPEC primary white dwarfs changed the HST photometric flux reference system, thus changing the inverse sensitivities and zeropoints. This change is less than 0.5% on average for the wide filters, but increases to just under 2% for the reddest medium and narrow filters. No discernible changes in sensitivity over time are detected in the measurements, but this is partially due to a lack of precision likely caused by persistence of previous observations, as well as other effects that are not currently well understood. The new zeropoint tables are presented on the website
ISR 2020-09: Updated Calibration of the UVIS G280 Grism
November 19, 2020N. Pirzkal
We present the result of a full calibration effort of the WFC3 UVIS G280 slitless spectroscopic mode. We have combined all of the G280 calibration data (nearly 600 datasets) obtained over the years to determine the traces, wavelength calibration, and flux calibration across the entire field of view of both UVIS detectors, modelling the large amount of field dependence of the G280 grism. This was done for the ±1, ±2,±3, and ±4 orders, and we have calibrated the position of the 0th order as well.
ISR 2020-08: Strategies for Mitigation of CTE Losses in WFC3/UVIS
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.
ISR 2020-07: Supplemental Dither Patterns for WFC3/UVIS
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.
ISR 2020-06: WFC3/UVIS EPER CTE 2009 - 2020
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.
ISR 2020-05: WFC3 IR Sensitivity over Time
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.
ISR 2020-04: The Dispersed infrared background in WFC3 G102 and G141 observations
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.
ISR 2020-03: IR `Snowball' Occurrences in WFC3/IR: 2009-2019 Snowballs Database
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.