WFC3 STAN Issue 35, April 2021

1. New CTE correction in calibration pipeline (calwf3 version 3.6.0) 

J. Anderson, B. Kuhn, S. Baggett, C. Martlin, M. De La Pena

Charge transfer efficiency (CTE), or how effectively a CCD transfers charge from one pixel to another during readout, has been gradually declining on WFC3/UVIS as a result of the on-orbit radiation environment (WFC3 Instrument Handbook). One key strategy to mitigating these losses is to apply a pixel-based CTE correction during calibration processing; such a correction has been available since early 2013. However, with the current state of the CTE losses, the original CTE correction has considerable difficulty treating low-level pixels which adversely impacts (overcorrects) both the image background and faint sources.

The new CTE correction algorithm addresses both these concerns by reducing noise amplification; the results are significantly-improved image backgrounds (Figure 1) and improved flux corrections for bright sources (Figure 2). However, an unavoidable consequence of the new approach to avoid amplifying noise, i.e. to “do no harm”, is that fluxes of faint sources receive only a limited amount of correction. As shown in Figure 2 below, sources with at least ~3000 e- within a 3-pixel radius (S/N > ~50) are corrected to better than ~5%; faint sources receive only a marginal correction ("Updating the WFC3/UVIS CTE Model and Mitigation Strategies"  by J. Anderson et al., ISR 2021-09 ;  and "WFC3/UVIS: New FLC External CTE Monitoring 2009 - 2020" by B. Kuhn et al., ISR 2021-06 ).

The new CTE correction (calwf3 3.6.0) is in the calibration pipeline as of Apr 21, 2021; data taken after this date will automatically have the new correction applied; the software version used in the pipeline is recorded in the CAL_VER science image header keyword. In parallel with the new software, new daily super darks for 2009-2021 have been delivered to the pipeline. These files have been generated using the same CTE correction software in order to improve the darks and provide optimum calibration for the science images. To apply the new correction to data processed with earlier versions of calwf3, observers may re-request the data from the MAST Portal. For observers who may wish to remain with the old CTE correction, data may be reprocessed with the old calibration following these instructions (Github repo with Jupyter notebook).  Please feel free to contact the HST helpdesk (https://stsci.service-now.com/hst) for any questions.

Figure 1.  A 100x100 pixel image subsection of a 900 sec WFC3/UVIS dark frame shown in an inverted greyscale. From left to right are: raw data without any correction, raw data with the original CTE correction applied, and raw data with the new (2021) CTE correction.

Figure 2. Charge transfer efficiency (CTE) losses measured in NGC 6791 and 47Tuc frames with ~15e- background. Results are shown in delta magnitudes (per 2051 pixels) as a function of log of the flux with fits over-plotted; color-coding denotes data acquisition date. From left to right, panels present the fully-calibrated results for data without a pixel-based CTE correction, with the original pixel-based CTE correction, and with the new (2021) pixel-based CTE correction. Photometry is based on a background-subtracted 3-pixel radius aperture; the x-axis covers sources from ~300e- total (S/N ~10) up to ~30,000 e- total (S/N ~175) in the aperture.

2. Updated DrizzlePac Handbook in HDox

J. Mack, S. L. Hoffmann 

A revised version of the DrizzlePac Handbook is now available, including both updated content and an overall modernization of the handbook format. New content highlights software enhancements implemented since the prior 2012 version of the handbook. Notably, this includes a new section describing improvements in the absolute astrometry for WFC3 and ACS data products achieved by matching sources in the HST images directly to the Gaia DR2 reference catalog.

Text-based tutorials have been replaced with interactive Jupyter notebooks and python code which demonstrate the overall workflow for aligning and combining sample HST datasets. These tutorials are continually kept up-to-date in the spacetelescope / notebooks github repository.

For added convenience, the new handbook has been migrated to the HST Document (HDox) webpage which hosts all other HST User Documentation, including the Instrument Handbooks, Proposal Instructions, and Data Handbooks. 

Citation: Hoffmann, S. L., Mack, J., et al., 2021, “The DrizzlePac Handbook”, Version 2.0, (Baltimore: STScI).

3. hstaxe - An Updated Grism Spectral Extraction Package

N. Nikolov, D. Som. 

From aXe to hstaxe: introducing the successor of the Hubble Space Telescope Grism spectral extraction and analysis package aXe. hstaxe is the newest version of the aXe software, offering the functionality of aXe and no longer requiring Python 2.7 or IRAF/PyRAF to be installed. This update was critical as these latter tools have discontinued support. The code, along with directions for installation are available on github: https://github.com/spacetelescope/hstaxe. The instructions include managing the code within the astroconda environment. A complementary cookbook that uses the new code, an example Jupyter notebook, and sample data are also available as a guide: https://github.com/npirzkal/aXe_WFC3_Cookbook.

Credit: Norbert Pirzkal, Markus Demleitner, Martin Kuemmel, Richard Hook, Howard Bushouse, Megan Sosey.

4. New Documentation 

ISR 2021-03: WFC3/UVIS: External CTE Monitoring 2009 - 2020. B. Kuhn, V. Bajaj.

ISR 2021-04: New time-dependent WFC3 UVIS inverse sensitivities. A. Calamida, J. Mack, J. Medina, C. Shanahan, V. Bajaj, S. Deustua.

The complete WFC3 ISR archive is available here.

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