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WFC3 STAN Issue 26, December 2017

WFC3 Space Telescope Analysis Newsletter - Issue 26, December 2017

Late-breaking news about WFC3 is posted here.

Previous issues of the WFC3 STAN are archived here.


  1. STScI at the AAS 231: January 2018, Washington, D.C.
  2. WFC3 Help Desk Changes - New Web Portal Live
  3. Geometric Distortion Solution Updates for Medium Band UVIS Filters
  4. LINEAR: A Novel Algorithm for Reconstructing Slitless Spectroscopy from HST/WFC3
  5. Aligning HST Images to Gaia Catalogs
  6. Photometric Repeatability of Spatial Scans with WFC3/UVIS
  7. New Documentation

1. STScI at the AAS 231: January 2018, Washington, D.C.

As stated in the STScI Newsletter, Space Telescope Science Institute will have an exhibit booth and several events at the 231st AAS meeting in Washington D.C. There will be presentations in instrument sessions, a variety of science presentations and press releases, as well as time to talk with Hubble (and many other) experts throughout. For more information on the exhibit booth and on the offers feel free to browse the STScI Newsletter (linked previously).

2. WFC3 Help Desk Changes - New Web Portal Live

C. Shanahan, C. Martlin.

On December 8th 2017 the new HST Help Desk Portal powered by ServiceNow went live. Users should now visit the help desk portal at to submit questions to the appropriate team. On this portal users will soon be able to read Knowledge Base articles, answers to FAQs, and view HST announcements - these features are in the process of being created and will be updated as they are completed and tested. As before, questions may still be submitted directly to Replies to responses from STScI staff can be done via the web portal or by replying directly to response emails from

3. Geometric Distortion Solution Updates for Medium Band UVIS Filters

C. Martlin, V. Kozhurina-Platais, M. McKay.

The Instrument Distortion Coefficients Table (IDCTAB) and filter-dependent distortion reference files (NPOLFILEs) are used in the HST calibration pipeline to correct the WFC3/UVIS images for geometric distortion.

The IDCTAB reference file has been updated recently with newly obtained unique polynomial coefficients for the medium band UVIS filters F390M, F546M, F763M and F845M, based on observations of the globular cluster Omega Cen (calibration program CAL-14393).

The solutions for the geometric distortion coefficients were derived for each medium band filter by comparing the raw X and Y positions from the median images to the X and Y positions from the standard astrometric catalog OMCCOOLR_BR.RIGID.XYVI in the vicinity Omega Cen (Anderson, 2010), where the raw positions have had the lithographic mask pattern correction applied (WFC3 ISR 2014-012). The previous distortion corrections for the medium band filters were based on the F606W solution; the newly-derived solutions improve the medium band astrometric correction on the level of 2 milli-arcseconds.

The NPOLFILE reference files (2-D look-up tables of filter-dependent low scale distortion) are also now newly available for the medium filters F390M, F410M, F467M, F547M, F621M, F689M, F763M and F845M. Previously, there were no specified NPOLFILE solutions available for these medium filters thus the updates will help reduce the positional systematic errors due to the filter-dependent distortions.

Note that the updates discussed here only affect the medium band filters listed above. Observers with UVIS data taken prior to December 21st, 2017 with any of the listed medium filters and who have used packages that utilize the IDCTAB and/or the NPOLFILE should re-request their data from MAST (Mikulski Archive for Space Telescopes) to ensure they have the most up-to-date calibration.

For more specific information on the IDCTAB and NPOLFILE reference files and how they are used please visit the Calibration Database System (scroll to the bottom) or the WFC3 Data Handbook for more details.

4. LINEAR: A Novel Algorithm for Reconstructing Slitless Spectroscopy from HST/WFC3

R. Ryan.

We present a new algorithm for reconstructing optimal, one-dimensional spectra for G102 and G141 when multiple orients and dither positions are available. In brief, the algorithm enumerates all of the transformations from an existing direct image and all of the two-dimensional grism images, which establishes a large system of sparse linear equations, the solution of which are the spectra for all of the sources present in the data. This approach obviates the need for contamination corrections, as the contribution for each source is naturally accounted for in the linear system. The paper describing the algorithm has been accepted to PASP (LINEAR: A Novel Algorithm for Reconstructing Slitless Spectroscopy from HST/WFC3 by R. Ryan, S. Casertano, & N. Pirzkal), and details several key advantages to this approach (improved spectral resolution and mitigated contamination), and demonstrates the sensitivity of the algorithm with respect to the calibrations and observations. The software is written entirely in IDL and C, and will be made public very soon, however beta-versions can be obtained by emailing the authors ( We will be presenting this work in a poster at the AAS in January.

5. Aligning HST Images to Gaia Catalogs


We present a workflow for aligning images to the first data release (DR1) of the Gaia space mission catalogs (Gaia Collaboration et al., 2016a; Gaia Collaboration et al. 2016b). By providing high accuracy astrometry for over 1.1 billion sources, the Gaia all-sky catalogs enable image registration to sub-HST-pixel precision (down to a few milliarcseconds) using the DrizzlePac software TweakReg. This is especially useful when combining HST dithered images into mosaics, and/or when the input images have very little overlap, as the images would already be aligned to each other as a result of being aligned to Gaia. Furthermore, when making large mosaics, the small errors in the HST distortion solutions can compound to a significant astrometric error and prevent successful alignment and combination of mosaics with many tiles if the tiles are iteratively aligned to each other. However, much of this residual distortion is removed when the Gaia astrometry is provided in conjunction with TweakReg's six-parameter, general fitting geometry, and so the images can then be aligned and combined (provided there is a high enough source density). This method has proven to be much easier than the conventional method of relatively aligning mosaic tiles to each other, and thus can significantly reduce the time required to make mosaics.

An example workflow is presented in a Jupyter notebook here and is briefly discussed in ISR-2017-19. The example details how to perform all the tasks required to align a set of HST images, including preprocessing to determine the region of interest, querying the Gaia DR1 catalog, and aligning the images to the catalog using TweakReg to the catalog - all in Python.

6. Photometric Repeatability of Spatial Scans with WFC3/UVIS

C. Shanahan, P. McCullough, S. Baggett.

The Cycle 24 calibration program 14878 observed white dwarfs GD153 and GRW+70 in a subset of UVIS filters in spatial scan mode. The primary purpose of this program was to compare the photometric repeatability in UVIS filters obtained with spatial scans to that obtained with (traditional) staring mode. The promising results of this program were used to inform the decision to continue the comparison with program 15398 in Cycle 25.

WFC3 ISR 2017-21 discusses program 14878, its design, photometric analysis technique, as well as the results from the first identical visit pair – two identically structured visits of GD153. Since that publication, the program completed with five identical visits in total: three visits of GD 153 (making two unique pairings) and two visits of GRW+70 (making one unique pairing).

We obtained the goal of ~0.1% repeatability between nearly all identically structured visit pairs. Figure 1 shows the ratio of identical visits of GRW+70. The left panel shows the ratio of fluxes measured on UVIS chip 1 while the right panel shows the same for chip 2. Twice the amount of data was taken on chip 2 to experiment with post and non-post flashing the data as well as alternating the scan direction. Figure 2 is similar, but for GD153.

A follow up ISR (in preparation) will discuss this program in its entirety, with a more detailed noise model for the observations, and more sophisticated methods of photometry to obtain somewhat better repeatability.

Visit12/Visit11_1 Visit12/Visit11_2

Firgure 1: Figure 1: Flux ratios of two identical visits of GRW+70. The x-axis is filter, the y-axis is the flux ratio to visit 11. Aperture ‘alpha’ refers to the size of the photometric aperture used (see ISR 2017-21).

ratio_1 ratio_2

Figure 2: Flux ratios of the three identical visits of GD153.

7. New Documentation

The archive of all WFC3 Instrument Science Reports (ISRs) is here.

The most recent available version of the WFC3 Data Handbook can be found here.

Need help? Start here.

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