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WFC3/UVIS: External CTE Monitoring 2009-2024

This report analyzes 14 years of WFC3/UVIS data to track the increasing Charge Transfer Efficiency (CTE) flux losses in the Hubble Space Telescope, providing updated empirical correction coefficients and noting that even with current corrections, significant flux loss and slight over-corrections persist in 2024.

Original authors: Benjamin Kuhn

Published 2026-02-10

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CC BY 4.0

Original authors: Benjamin Kuhn

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Leaky Bucket" Problem: A Simple Guide to the WFC3 Report

Imagine you are trying to collect rainwater in a bucket to measure how much it storms. If your bucket is brand new and perfectly sealed, you can trust exactly how much water is inside. But what if, every year, tiny microscopic cracks appear in the bottom of your bucket? If you don't know how big the cracks are, you’ll always think it rained less than it actually did.

This scientific report is essentially a "leakage report" for one of the most important "buckets" in space: the WFC3 UVIS detector on the Hubble Space Telescope.

1. The Problem: The "Sticky" Pixels

The WFC3 is a camera that uses a sensor made of millions of tiny pixels. When light from a distant star hits a pixel, it creates a tiny electrical charge (like a drop of water). To "read" the image, the camera has to move those charges across the sensor to a readout area, much like a conveyor belt moving boxes from one end of a factory to another.

However, Hubble lives in a harsh environment. It is constantly being pelted by high-energy cosmic radiation. This radiation acts like tiny, invisible hammers, denting the "conveyor belt." These dents create "charge traps"—think of them as little potholes or sticky patches on the belt.

As the electrical charge (the light signal) travels across the sensor, it gets caught in these potholes. Some of the signal gets stuck or delayed, meaning when the camera finally "reads" the data, it thinks the star was much dimmer than it actually was. This is called CTE (Charge Transfer Efficiency) loss.

2. The Trend: The Bucket is Getting Leakier

The report tracks this "leakage" over 14 years (from 2009 to 2024). The bad news? The leaks are getting worse.

As the telescope stays in space, more and more radiation hits it, creating more "potholes." The report notes that for certain faint stars, the "leakage" is increasing at a steady rate every year. If you don't account for this, your measurements of how bright stars are will be wrong, which could lead to incorrect conclusions about the universe.

3. The Solution: "Pre-filling" the Potholes

Scientists have found a clever way to fight this. If you know you have a bucket with tiny holes, you can pour a little bit of "background" water into it before you start measuring the rain.

In space terms, they use something called a "post-flash." They intentionally flood the sensor with a tiny bit of extra light (background noise). This extra light fills up the "potholes" (the charge traps) so that when the actual light from the star comes through, it slides right over the top of the traps instead of getting stuck in them.

The report recommends a "sweet spot" for this background level: about 20–25 electrons per pixel.

Too little background: The star signal gets stuck in the potholes (high leakage).
Too much background: You might actually "over-correct" and end up with data that is slightly too bright (like overfilling the bucket).

4. The "Math Patch": The Empirical Model

Since we can't perfectly stop the leaks, scientists use math to "patch" the data after the photo is taken.

The report provides a set of updated coefficients (basically a specialized math formula). It’s like having a spreadsheet that says: "If your bucket is 5 years old and you had 10% background light, add exactly 3 drops of water back to your measurement to make it accurate."

Summary: The Takeaway

The Issue: Space radiation is damaging Hubble’s camera, creating "sticky" spots that steal light from our images.
The Status: The damage is cumulative; the camera is getting "stickier" every year.
The Fix: We can mitigate this by using a little extra light (post-flash) and using new, highly accurate math formulas to add the "lost" light back into our final pictures.

In short: The camera is aging, but thanks to this report, astronomers have a better "repair kit" to ensure our view of the universe remains crystal clear.

Technical Summary: WFC3/UVIS External CTE Monitoring (2009–2024)

Problem: Charge Transfer Inefficiency (CTE) in Space-Based CCDs

The Wide Field Camera 3 (WFC3) UVIS detector is subject to continuous bombardment by cosmic rays and high-energy radiation while operating aboard the Hubble Space Telescope. This radiation creates "charge traps" within the CCD pixels. During the readout process, these traps capture a fraction of a charge packet and release it with a delay, causing two primary issues:

Photometric Loss: A reduction in the measured flux of a source.
Astrometric/Morphological Distortion: The signal appears "smeared" into trails extending from the source core in the anti-readout direction.

Because the severity of these effects depends on the distance from the readout, the source brightness, and the background level, accurate photometry requires sophisticated, time-dependent corrections.

Methodology

The study utilizes a 14-year baseline of observations (October 2009 to February 2024) to monitor the degradation of CTE.

Observing Strategy: The team employs a "one-chip dither" strategy using star clusters (47 Tuc, NGC 6791, and more recently, Omega Centauri). By taking two exposures where stars land on different CCDs, the team can compare the flux of the same star at varying distances from the readout.
Background Control: To study the mitigation effects of background levels, the team uses "post-flashing" to simulate various total background levels ranging from $\sim$ 0.1 to 120 $e^-$ /pixel.
Data Analysis: The study analyzes both FLT (non-CTE-corrected) and FLC (pixel-based CTE-corrected) files. The researchers apply a bivariate empirical model (based on Noeske et al. 2012) using a second-order polynomial constrained by source flux and epoch to derive correction coefficients.

Key Contributions

Updated Empirical Model Coefficients: The report provides new, updated coefficients (Tables 6–9 in the Appendix) for observers to apply supplemental corrections to both FLT and FLC data.
Long-term Trend Characterization: It establishes the precise rate of CTE degradation over a 14-year period.
Validation of Mitigation Strategies: The report quantifies the effectiveness of post-flashing and the current pixel-based correction algorithms.

Key Results

Degradation Rates: For a 500–2000 $e^-$ source farthest from the readout in FLT data (with a 1–3 $e^-$ /pix background), the flux loss increases at a rate of $\sim$ 0.05 $\Delta$ mag/2051 pix/year (or $\sim$ 3% flux loss/year).
Impact of Background: Increasing the background level significantly mitigates losses. In 2024 FLT data, a 500–2000 $e^-$ source farthest from the readout suffers $\sim$ 23% flux loss at a 20–25 $e^-$ /pix background, compared to much higher losses in lower background environments.
Effectiveness of FLC Data: While FLC (corrected) data provides relief, it is not a perfect solution. In 2024, 500–2000 $e^-$ sources still experience $\sim$ 12% flux loss in 2024 FLC data at a 20 $e^-$ /pix background.
Over-correction Issue: A notable finding is that FLC data exhibits "over-correction" when background levels exceed $\sim$ 40 $e^-$ /pix. For example, 8000–20000 $e^-$ sources in a 90 $e^-$ /pix background are over-corrected by approximately 3%.

Significance

This report is critical for the Hubble Space Telescope community because it acknowledges that there is no perfect correction for CTE. Even with the best current software (FLC files), residual losses persist.

The findings provide actionable guidance for observers:

Minimum Background Recommendation: To minimize photometric errors, observers should aim for a total background level of at least 20–25 $e^-$ /pix.
Supplemental Correction: The provided coefficients allow for a secondary layer of empirical correction, which is essential for high-precision photometry in aging CCD detectors.
Future Calibration: The identification of the "over-correction" at high backgrounds highlights a specific area for future algorithmic refinement by the WFC3 team.