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CTE Information

Charge Transfer Efficiency (CTE) News

 

CTE Corrections

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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 2016. 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. See ISR 2021-09: "Updating the WFC3/UVIS CTE Model and Mitigation Strategies" (J. Anderson et al.) and ISR 2021-06: "WFC3/UVIS: New FLC external CTE Monitoring 2009 – 2020" (B. Kuhn et al.) for more details.

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 contact the HST Help Desk with any questions.

A triptych of square grayscale plots arranged in a row. Each plot has a lighter gray background with dark gray to black spots. In the first image, the spots have significant streaks downward. In the second plot, the spots have a couple of streaks downward. In the third plot, only one spot has a slight streak downward. This shows the improvement of the CTE correction from uncorrected (first plot), version 1 correction (second plot), and new 2021 correction (third plot).
Figure 1: A 100x100 pixel image subsection of a 900 sec WFC3/UVIS dark frame (iegj2hxaq) shown in inverted grey scale. From left to right are: raw data without any correction, raw data with the version 1 CTE correction applied, and raw data with the version 2 (2021) CTE correction.​​​​​
CTELossOmegaCen
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 CTE correction, with the original CTE correction, and with the new (2021) CTE correction. Photometry is based on a 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.

 

CTE Flux Loss Trends as a Function of Background Level

As mentioned in ISR 2024-04, the WFC3/UVIS External CTE Calibration Program employs different post-flash levels to simulate various sky backgrounds. The various post-flashes result in sky background levels of: 7-10, 13-15, 20-25, 30-35, 40-45, 60-65, 90-95, and 120-125 e- in ~6-7 minute F502N exposures of NGC 104, NGC 6791, and Omega Centauri. The PDF below hosts plots of CTE losses [∆mag/2051 pixels] as a function of flux bin [e-] for each background level. Additionally, we have included plots of the CTE loss evolution (CTE loss [∆mag/2051 pixels] vs year) for three major flux bins probed in the analysis. Each background level has two pairs of figures, one generated using FLT data and the other with FLC data. FLC data are files that have been processed through the pixel-based CTE algorithm within the calibration pipeline calwf3 (version>=3.6.0). The title of every plot will note which file type was used as well as the background level. See ISR 2024-04 for details regarding the analysis and how these plots were created. The ISR includes plot results for the 1 and 20 e-/pixel level; results for these levels, as well as every other flash level, are available in the attached PDF.

Spatial Dependence of the Flash

The post-flash illumination pattern of WFC3 varies by about +/-20% across the full field of view (ISR 2013-12). As seen in Figure 3, the flux is lowest in the lower left and upper left corners (C and A amps, respectively) and highest on the right side (B and D amps). In Figure 4, the normalized pattern for shutter blade A is shown; differences between the shutter blades are less than a few percent.

 

The post-flash illumination pattern for WFC3 is plotted with the format of two long rectangles stacked on top of each other, representing UVIS 1 (top) and UVIS 2 (bottom). The plot is in a rainbow colormap, and shows values from about 0.80 to 1.15. The ratios are largest on the right side of the detector, near the boundary between UVIS CCDs. The ratios decrease with radial distance from this point such that the lowest values are all on the left side of the detector.
Figure 3: WFC3 post-flash illumination pattern; Amps A and C (corresponding to the left side of UVIS 1 and UVIS 2 respectively) have lower flux fraction than Amps B and D (corresponding to the right side of UVIS 1 and UVIS 2 respectively). There is significant variance in the flux across the detector, from about -20% to +15%. This image is also available as a FITS file. 
The post-flash illumination pattern, normalized to Shutter A, is plotted with the format of two long rectangles stacked on top of each other, representing UVIS 1 (top) and UVIS 2 (bottom). The plot is in a rainbow colormap, and shows values from about 0.97 to 1.03. The ratios are largest on the top left side of the detector (UVIS 1) and decrease gradually across the diagonal to the lower right side of the detector (UVIS 2).
Figure 4: Normalized post-flash illumination pattern for Shutter A. The ratio between the shutter blades is only a few percent, ranging from about -3% to +3%. 

 

Coefficients for Aperture Photometry

The following text files contain coefficients to be used to correct flux loss due to CTE degradation in point-source photometry. Their columns are sorted by background level. Please refer to Equations 1-3 in ISR 2017-09 for the flux correction formula and note that the expansion in Equation 2 was misprinted in pre-2017 instrument science reports. Observers should try to use coefficients derived from conditions that most closely match their data, with background levels being the most important consideration. If observers have pre-2016 data with faint sources, they may consider using coefficients from earlier epochs for those sources. However, observers are cautioned against applying the coefficients forward in time.

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Relevant Documentation

Last Updated: 05/01/2024

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