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July 7, 2021

About This Article

1. Cycle 29 Tips and Tricks 

J. Green. 

The deadline for Cycle 29 GO program Phase II's is July 29, 2021. We encourage PIs using the WFC3 to read our Phase II Proposal Roadmap for a step by step guide in preparing Phase II proposals. 

We have introduced additional UVIS dither patterns. The WFC3 Instrument Handbook (Appendix C) displays the definitions of the 2 point line dither, a 3 point line dither, and a 4 point box dither, which have been implemented in APT. In some cases there is also a need for more points in a pattern (e.g., 6 full frame exposures fit in an orbit). J. Anderson (2020a) provides supplemental dither patterns for sets of more exposures - unique n > 4 dither line patterns. These new patterns are available in the form of prescribed POS TARGs for Phase II Cycle 29 preparations.

We remind the user that IR dither patterns should be at least 10 pixels to avoid self-persistence for sources that are larger than 5 pixels in size. The standard box pattern is significantly smaller than this. For more information, see the ISR by V. Bajaj (2019).  J. Anderson (2016) provides supplemental dither patterns for sub-pixel dithers for the3 (under-sampled) IR case.

Note that the optimum background level for WFC3/UVIS CTE mitigation in Cycle 29 is 20 e-/pix total (sky + dark + post-flash if needed). J. Anderson (2020b) provides more detailed advice for how best to mitigate charge transfer efficiency (CTE) losses. When beginning your Phase II's, please check the updated recommendations.

2. Drizzlepac Troubleshooting Tips

M. Cara, S. L. Hoffmann 

Collected here are issues recently experienced by DrizzlePac users in the HST astronomy community. Below we provide both an explanation of what is going wrong and information about how to fix or workaround each problem. For any additional questions about these tips or help troubleshooting DrizzlePac, please contact the HST Help Desk at

1) KeyError: "Extension ('', 1) not found."

In previous versions of DrizzlePac (before 3.2.1), AstroDrizzle may have produced drizzled images whose primary header contained the keyword EXTVER in the primary header. When TweakReg uses a reference image with this keyword, it can result in a crash with the following signature: KeyError: "Extension ('', 1) not found."

This can be prevented either by updating to the latest DrizzlePac version (3.2.1) and reprocessing with AstroDrizzle to create a new drizzled image without the EXTVER header keyword or by deleting EXTVER from the primary header of the reference image with the code below.

>>> from import fits
>>> fits.delval('filename_drc_sci.fits', keyword='extver', ext=0)

2) updatehdr=False causes ValueError: 'fname' must be a file object opened in 'update' mode.

Setting parameter updatehdr to False may crash TweakReg with the following error: ValueError: 'fname' must be a file object opened in 'update' mode.

This issue has been fixed in the latest DrizzlePac version. Please update to version 3.2.1. More details on this issue are located at Github here:

3) AttributeError: module 'photutils' has no attribute 'findstars'

Attempting to run DrizzlePac version 3.2.1 or earlier with any photutils version greater than 1.0.2 results in a crash with the following signature: AttributeError: module 'photutils' has no attribute 'findstars'

Recent changes to the photutils package have made versions after 1.0.2 incompatible with DrizzlePac. Until this is corrected in the next release of DrizzlePac, please downgrade photutils to version 1.0.2. Three possible commands to do this are given below with options for both conda and pip installation.

Downgrade photutils in an existing conda environment

$ conda install -n env_name -c photutils=1.0.2

or create a new conda environment

$ conda create -n env_name -c stsci python=3.7 photutils=1.0.2

or downgrade using pip:

$ pip install --upgrade photutils==1.0.2

4) TweakBack crashes with default parameters

In DrizzlePac version 3.1.8, several changes were made to improve how TweakReg modifies the primary WCS in image headers when updatehdr is set to True. These changes included:
- No longer producing duplicate copies of the same WCS in subsequent runs.
- Only archiving the primary WCS when necessary (with reusename=True, for example).
- Implementing additional protections to prevent overwriting primary or archived WCS.

Unfortunately, these changes are incompatible with the *default* settings in TweakBack. With the default parameters, TweakBack expects at least two archived WCS. DrizzlePac version 3.1.8 and newer, however, often creates a single archived WCS of the original, unaligned WCS, while the updated WCS is saved in the primary header. This inability to find a duplicate, archived copy of the updated WCS leads to problems with TweakBack. It could crash attempting to retrieve the original WCS in the drizzled image or attempting to overwrite the primary WCS in the FLT/FLC images, or it could use incorrect WCS from the drizzled image.

The best approach to address this issue is currently being investigated. Meanwhile, TweakBack users will need to explicitly specify *all* WCS name parameters (newname, origwcs, and wcsname) instead of depending on any default behavior.

3. Updating the WFC3/UVIS CTE Model and Mitigation Strategies

J. Anderson. 

A new Instrument Report (ISR 2021-09) has been published. From the abstract: The pixel-based charge transfer efficiency (CTE) correction was last updated for WFC3/UVIS in 2016. Since the strength of CTE generally increases linearly with time, as does the population of warm pixels, the efficiency losses are almost twice as large after five additional years of HST operations — and there are twice as many warm pixels available to use in deriving the correction. Unfortunately, the new model confirms that charge-transfer losses have continued to increase steadily since WFC3/UVIS was installed. Now, even with a background of 20 electrons, CTE losses are almost 50% for sources near the chip-gap (i.e., sources that undergo the maximum number of parallel transfers), compared to about 30% in 2016. The pixel-based algorithm works well only when the correction is small enough to be considered a perturbation on the signal received. If the correction is too large, then the algorithm tends to amplify noise, which causes more harm to the image than benefit. For this reason, the default pipeline setting is now designed to suppress the correction for faint sources in order to avoid noise amplification. In general, for observers with relatively bright targets (S/N > ~30) on image backgrounds of at least 20 e- per pixel, the new pixel-based CTE correction works well, correcting such targets to within 5%. Observers with fainter targets, which the new algorithm treats minimally in order to avoid noise amplification, will need to make additional adjustments. We provide advice to users about how to plan observations and how to carry out reductions in order to get the most out of WFC3/UVIS observations. The two figures below highlight some of the results.


Figure 1. The noise measured in flt images (green points, no corrections), v1.0 CTE-corrected flc images (blue points), v2.0 CTE-corrected flc images (red points); theoretical level (Poisson+read noise) is the black line. Symbol size denotes the percentile region within which sigma is measured: from the inner 68% (large) and by the inner 95% (small). From Figure 13, ISR 2021-09


Figure 2: Average empirical CTE losses (black) compared with the CTE correction model v1.0 and 2.0 (blue, red, respectively) as a function of distance from the readout amplifier. Background sky levels are noted in each panel. From Figure 4, ISR 2021-09.

4. New Documentation 

ISR 2021-05: Photometric Repeatability and Sensitivity Evolution of WFC3/IR. D. Som, V. Bajaj, J. Mack, A. Calamida.

ISR 2021-06: WFC3/UVIS: New FLC External CTE Monitoring 2009 - 2020. B. Kuhn, J. Anderson.

ISR 2021-07: Accuracy of the HST/WFC3 Standard Astrometric Catalog w.r.t Gaia EDR3. V.Kozhurina-Platais, C.Martlin.

ISR 2021-08: WFC3 IR Blob Classification with Machine Learning. F. Dauphin, J. V. Medina, P. R. McCullough.

ISR 2021-09: Updating the WFC3/UVIS CTE model and Mitigation Strategies. J. Anderson, S. Baggett, B. Kuhn.

The complete WFC3 ISR archive is available here.

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