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WFC3 STAN Issue 42, June 2023

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June 1, 2023
WFC3 NEWSLETTERS

About This Article

1. WFC3 at AAS 242!

We invite all interested users at the AAS to stop by the WFC3 iPosters at the upcoming AAS meeting, and to reach out to the presenters directly if unable to attend the poster session. Below, we provide abstracts and session information, and link to the presentation listings in the AAS meeting itinerary. Times are given in Mountain Daylight Time (MDT).

102.02 HST/WFC3: Recent Calibration and Machine Learning Updates for 2023 - I. Rivera, C. Martlin, J. Green, F. Dauphin, S. Baggett, and the WFC3 Team
Monday, June 5, 9:00 - 10:00 AM MDT

The Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST) has enabled high-resolution imaging and low-resolving power slitless grism spectroscopy from the ultraviolet (UV) into the near-infrared (NIR) since it was installed in 2009. The WFC3 team works to provide the latest, up-to-date software for calibrating and analyzing WFC3 images to ensure the highest quality data products possible for observers. This poster provides an overview of the available reference files for calibration (e.g. new UVIS post-flash files) as well as new notebook tutorials for WFC3 analysis and machine learning, which are hosted in our “WFC3Library” and “DeepWFC3" GitHub repositories. In the remainder of the poster, we discuss implementation of “stenv”, a software environment developed by Space Telescope Science Institute to replace Astroconda. Stenv provides a common environment for both the HST and the James Webb Space Telescope (JWST) pipelines. Here we will show how to get started with it and highlight the included packages.

102.06 Updates for Slitless Spectroscopy with HST/WFC3 and ACS - A. Pidgeon, D. Som, B. Kuhn, A. Pagul, D. Nguyen, R. O'Steen, N. Hathi
Monday, June 5, 9:00 - 10:00 AM MDT

Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS) are primarily used as imaging instruments for the Hubble Space Telescope (HST), but are also capable of slitless spectroscopy using grism and prism elements. HSTaXe is the officially supported software for extracting and calibrating slitless spectroscopic data from WFC3 and ACS. We present recent updates to HSTaXe with major bug fixes and enhanced support for all HST slitless spectroscopy modes. Alongside these updates, we showcase several Jupyter Notebook “cookbooks” designed to improve the experience of new HSTaXe users. These cookbooks serve as tutorials on how to extract spectra from WFC3 and ACS grism data, and include several pre-processing steps that allow for additional functionality and improved output quality. We also discuss the creation of a new master sky image for the WFC3/UVIS G280 grism and the development of new user tools for slitless spectroscopy with HST and JWST.

102.08 HST/WFC3 Photometric Calibration: Recent Results and Tools - M. Marinelli, V. Bajaj
Monday, June 5, 9:00 - 10:00 AM MDT

The Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) is a powerful imager with wavelength coverage from the near-UV to the near-IR. Capable of observing in both direct (staring) and scanning modes, and with 61 total filters and 3 grisms, WFC3 has been the “workhorse” of HST since its installation in 2009. We review the status of the instrument, present results from recent photometric calibration programs, and discuss ongoing work to characterize detector sensitivity for both the WFC3/IR and WFC3/UVIS channels. We also introduce new/newly-public tools for photometric analysis in an effort to increase both accessibility and transparency.

2. Alpha Release of the Pandeia-based HST Exposure Time Calculator

J. Ryon, S. Lockwood, and V. Laidler

STScI has released an alpha version of the Pandeia-based Exposure Time Calculator (ETC) for the Hubble Space Telescope (HST) at hst.etc.stsci.edu (link no longer operational). The alpha release is intended for users to familiarize themselves with the new interface and perform example calculations for supported observing modes.

IMPORTANT: The official HST ETC for Cycle 31 proposal submissions is located at etc.stsci.edu. Please do not include results from the Pandeia-based HST ETC in Cycle 31 proposals.

Pandeia is a pixel-based exposure time calculator paired with a modern graphical user interface. While Pandeia was developed for JWST, it is a general framework, data-driven ETC capable of supporting multiple missions, including HST. It includes advanced features that go beyond the capabilities of previous ETCs, such as algorithms that accurately model both data acquisition and post-processing of data, and it provides functionality for users to efficiently explore and compare a large volume of parameter space in their calculations.

The HST implementation of Pandeia supports many HST observing modes. Its graphical user interface allows users to create workbooks to manage related sets of calculations, create complex astronomical scenes with multiple sources, compare the results of multiple calculations, and share their workbooks with other users. Several HST observing modes are not yet available, but will be made available in future releases.

A Migration Guide has been prepared to facilitate the transition to the Pandeia-based ETC for experienced users of the official ETC. Please start there if you are not already experienced with the JWST ETC.

We request feedback from the user community regarding the alpha release via the HST Help Desk, particularly feedback related to the clarity of calculation results. An ETC representative will also attend the summer AAS meeting in Albuquerque to demo the Pandeia-based ETC and answer questions.

A User's Guide for the Pandeia-based HST ETC is available on HDox, and Known Issues are available as a Knowledge Base article on the HST Help Desk.

Planning to attend AAS? Visit the HST table at the STScI booth to see a demonstration of the new ETC. Isaac Spitzer will be presenting an iPoster on the new ETC on Monday June 5, 9:00 - 10:00 AM, and will also be available for further questions and discussions at the STScI booth Tuesday June 6 (3:00 - 4:00 PM and 5:00 - 6:30 PM) and Wednesday June 7 (10:00 AM - 12:00 PM and 4:00 - 5:30 PM). All times are given in Mountain Daylight Time (MDT).

3. New WFC3 Jupyter Notebooks: Correcting for IR Variable Background

A. O'Connor, J. Mack, K. Huynh, and F. Dauphin

The WFC3/IR background is a combination of zodiacal light, helium line emission in the Earth's upper atmosphere, and/or scattered light from observing close to the bright Earth limb.  Both line emission and scattered light can vary significantly within a MULTIACCUM exposure, corrupting calwf3's 'up-the-ramp' fit which is used to identify cosmic-rays. This algorithm assumes that a given pixel sees a constant count rate from both sources and diffuse background (i.e., that the "ramps" are linear).  Strong time-variability may compromise the quality of the calibrated 'FLT' image and require manual reprocessing prior to any photometric analysis.

The WFC3 team has developed a new set of Jupyter notebooks to aid the user in identifying images affected by variable background. New visualization tools now make it easier to inspect individual reads and plot the accumulated signal through the exposure. Each notebook illustrates a unique method for correcting the IR images, including:  1) turning off the ramp fitting step, 2) 'flattening' the ramp by subtracting the excess background per read, or 3) excluding reads impacted by scattered light. The notebooks are available in the WFC3Library GitHub repository and summarized in the table below for various use cases. 

 

Notebook Title Method(s) Usage

IR IMA Visualization Tools with an Example of Time Variable Background

 Inspect the IMA MULTIACCUM images and produce signal ramp plots to determine whether a dataset needs reprocessing.

Any IR imaging filter.
Compares cumulative vs instantaneous signal in ramp.

Manual Recalibration with calwf3: Turning off the IR Linear Ramp Fit

 Use the final read of the IMA MULTIACCUM image, skip the ramp fit, and create an FLT image.

Any IR imaging filter.
Uses Astrodrizzle to combine FLTs and reject cosmic rays.

Correcting for Helium Line Emission Background in WFC3/IR Exposures using the "Flatten-Ramp" Technique

 Subtract the median background per IMA read and re-run the ramp fit with calwf3.

All filters affected by the helium line emission background.
Corrects uncharacteristically 'noisy' FLT images with poor ramp fits.

Correcting for Scattered Light in WFC3/IR Exposures: Using calwf3 to Mask Bad Reads

 Mask specific reads in the RAW image and reprocess with calwf3, including running the ramp fit.

Removes scattered light present in reads at the beginning or end of an exposure.
Uses AstroDrizzle to combine FLTs.

Correcting for Scattered Light in WFC3/IR Exposures: Manually Subtracting Bad Reads

 Subtract specific reads from the IMA MULTIACCUM image, skip the ramp fit, and create an FLT image.

Removes scattered light or anomalies present in any read.
Uses AstroDrizzle to combine FLTs and reject cosmic rays.

 

4. A Novel Method for Aperture-Correcting WFC3/UVIS Spatial Scans

M. Marinelli

Spatial scanning is an observation method in which the telescope moves in a prescribed trajectory after guide star acquisition, trailing flux across the detector. As detailed in WFC3 ISR 2022-04 (Marinelli et al.), spatial scans offer greater photometric precision than standard staring mode observations do. Despite this, spatial scans have never been directly used in the absolute calibration of the WFC3/UVIS detector, since existing software used to generate synthetic observations is unable to model non-circular photometric apertures.

In a new report, we describe our methodology developed to calculate spatial scan aperture corrections, and initial results. As seen in Figure 1, we created scan spread functions (SSFs) to model calibration scans by convolving a point spread function (PSF) with the linear scan trajectory path. Two sets of PSFs were used: one generated from interpolating published encircled energy (EE) curves (top row), and another generated from blending those EE-derived point spread functions (PSFs) with empirically-derived PSFs (bottom row). By applying the same procedure used to calculate the scan photometry to the SSFs, we calculated the fraction of total modeled flux encapsulated within the photometric aperture, and used that as a multiplicative factor to correct synthetic observations.

The ratios of observed-to-synthetic count rates are constant over time (validating the currently-implemented time-dependent zeropoints), but there is a wavelength- and chip-dependent offset, as seen in Figure 2. We also find indications that the UVIS PSF may have 1-2% additional flux beyond a 150-pixel radius, which may be one of the underlying systematic factors contributing to the noted offset. An upcoming calibration program will make deep observations of isolated standard stars to refine our understanding of the UVIS PSF at large radii.

For more details, see WFC3 ISR 2023-02 (Marinelli & Bajaj).

A six-paneled plot (2 rows, 3 columns). The top left panel is titled "Simple PSF" and depicts a 300-pixel-wide radially-symmetric PSF that is bright in the center and darkens with radius. The bottom left panel is called "Blended PSF" and shows a PSF with more spatial variance (not perfectly radially symmetric). The two center column plots is titled "Scan Trajectory", and show identical vertical red lines 192 pixels long. The top right plot is titled "Simple SSF", and shows an oval taller than it is wide that is bright at the center but darkens towards the edges. The bottom right panel shows the SSF derived from the blended PSF.
Figure 1: Demonstration of how a PSF (left column) is convolved with a trajectory line (middle column) to product a SSF (right column). The top row represents the “simple” models, which are derived from published EE curves, while the bottom row represents the “blended” models, created from combining the aforementioned EE curves with an empirical PSF. Duplicated from Figure 3 of WFC3 ISR 2023-02.
A scatter plot with "Pivot Wavelength" in Angstroms on the x-axis and "obs/syn offset (%)" on the y-axis. 7 dark blue diamonds are plotted, representing the mean offset for seven UVIS filters on UVIS 2. 6 green stars are plotted, representing the mean offset for six filters on UVIS 1. One gray "x" is plotted, representing the mean offset for F606W on UVIS 1.
Figure 2: Calculated offset of the observed-to-synthetic count rates, as a percentage, plotted against the pivot wavelength of the filter. UVIS 1 points (with one exception) are marked with green stars and UVIS 2 points are marked with indigo diamonds. Unlike the offsets for the other filter/CCD combinations, the UVIS 1 F606W mean offset is not representative of the entire dataset; accordingly, while we include it here for completeness, it is marked with a gray “x”. Duplicated from Figure 7 of WFC3 ISR 2023-02.

 

5. Procedural Updates to WFC3/UVIS Superbias Reference File Creation

I. Rivera

In a new report, we describe an updated procedure for generating WFC3 UVIS superbias reference files. The 2021 and 2022 superbiases created with the updated procedure have been delivered to CRDS; all UVIS data since 2021 has been reprocessed and is available through MAST.

As part of the vetting process, we took a deep dive into the effects of readout dark current on the rising superbias level from 2009-2022. We found that readout dark contributes an appreciable fraction of the gradual increase in superbias level we have observed over the years. Figure 3 shows how the average residual signal in the superbias level is closely correlated with the estimated readout dark current, from 2009 to present. CTE trails from hot pixels and CRs also artificially increase the measured level in the superbias.

For more details see WFC3 ISR 2023-03 (Rivera & Kuhn).

text
Figure 3: The blue triangles are the average residual pixel values due to readout dark and CTE trails. The black "x"s are the expected readout dark. The observed residual values were found by subtracting the average pixel value of the 200 rows farthest and closest to the amplifiers. The expected median readout dark was calculated by multiplying the median dark current per amp by the detector readout time, 96 s. The observed values are consistently higher than the expected readout dark, indicating that readout dark is only one contribution in the extra signal. The majority of the remaining signal is attributed to CTE loss effects. Duplicated from Figure 13 of WFC3 ISR 2023-03.

6. New Documentation

ISR 2023-01: WFC3/UVIS Post-Flash: Stability of the LED and Creation of Time-Dependent Reference Files - C. Martlin & J. Green

ISR 2023-02: Testing Aperture Corrections for WFC3/UVIS Spatial Scans - M. Marinelli & V. Bajaj

ISR 2023-03 : WFC3/UVIS: 2021 and 2022 Superbias Reference File Procedural Updates - I. Rivera & B. Kuhn

The complete WFC3 ISR archive is available here. Additional information about WFC3 calibration, performance, data analysis, software tools, and more can be found online.

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