NIRCam Persistence

Bright sources may yield faint residual images that persist in subsequent exposures. This persistence is generally low for typical NIRCam observations.

On this page

Image persistence, or latency, represents one of the main anomalies of near-IR detectors. Pixels subjected to strong illumination produce a faint residual image in the following integrations. Even if the effect is more visible for pixels exposed to bright sources, any amount of illumination can cause latent images. Persistence may last for several minutes after the end of an exposure and is not restricted to pixels that have reached saturation; nor does it disappear with detector reset.

Persistence is generally low for typical NIRCam exposures and dithers, but it can be noticeable depending on target brightness and exposure setup. Removing or mitigating persistence during data post-processing is among the objectives of the JWST calibration pipeline. This article provides information to estimate the amount of persistence one may expect in an exposure before attempting a software correction.


Estimating persistence 

ETC persistence estimates

Using the Exposure Time Calculator (version 1.5, for the example outlined below), along with Figures 5, 6, and 7, it is possible to get an idea of the amount of persistence one may encounter in a NIRCam imaging observation.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

In this example, consider a star of magnitude K = 16, with a flat continuum spectrum and medium background, is observed in the F200W filter, Subarray = FULL, Readout Pattern = SHALLOW2, Groups/Int (groups per integrations) = 5, Integrations/Exp (integrations per exposure) = 1. The Total Exposure Time (calculated by APT) is 236.12 s.

The primarily concern is the brightest pixel at the center of the point spread function. To estimate the counts, an extraction aperture, corresponding to the area of a pixel, needs to be set up. Since observations are being done with the F200W filter, the short wavelength channel with a pixel size of 31 mas is being used, an area of about 1/1000 of a square arcsec. The radius of the circular aperture with the same area is 17.5 mas.

In the Strategy tab of the ETC, with Aperture radius = 0.0175" and selection of the noiseless sky background button, a flux of 1,238 e–/s is obtained, having added the small contribution from the background. The total flux is therefore 1,238 e–/s × 236.12 s = 292,317 e–, which is a well fill fraction of about 2.4 (saturation occurs at about 120,000 e–). Using the central panel of Figure 6, the next dithered exposure of the same length and in the same filter will show about 75 e– of persistence at the peak position of the source. A check with the ETC shows that this is comparable to the signal produced, with the same instrument configuration, by a K ~ 25 magnitude source that would be detected with SNR = 4.7.

Note that Figure 6 represents a mean persistence map across the SW detectors. In-flight measurements show higher persistence signals in detectors A3, B2, B3, and B4. Detector A3 shows a ~50% increase in persistence levels relative to estimates from ground testing. Effectively, that means a source ~0.5 mag fainter will result in persistence levels predicted in Figure 6.


In-flight persistence measurements

Persistence levels in the NIRCam detectors were measured in Cycle 1 as part of program 1478. This program created an extreme case for persistence. In these observations, bright stars were used to place more than 10 times the full-well signal onto the detector. In subsequent dark current observations, the signal rate in the previously saturated pixels was monitored over time, and compared to the signal rate in areas that had not been previously saturated. Results are shown in Figures 1 through 4. Detectors A3, B3, and B4 show the longest setting times.

Figure 1. SW A module measured persistence

Measured persistence signal rates for sources on the A3 and BLONG detectors. Values refer to total counts in a 3-pixel radius aperture. Time is relative to the end of the illuminated exposure and denotes the midpoint of each subsequent integration. The illuminated exposure results in >=10x saturation for the sources plotted as dots, while Xs refer to sky areas in the illuminated exposure. Numbers in the legend refer to the (x, y) pixel position of the source on the detector.

Figure 2. Persistence signal rates in the SW A module detectors

Similar to Figure 1, but zoomed in to show the measured persistence signal rate from ~33 to ~56 minutes after illumination. Rates are highest on the A3 detector. 

Figure 3. Persistence in the SW B module detectors

Similar to Figure 2, but for the B module detectors. Here, detector B4 exhibits the longest decay time for the persistence signal. There are also several locations on B3 with high persistence signal rates 30–60 minutes after illumination.

Figure 4. Measured persistence in the LW detectors

Similar to Figures 1-3. Persistence signal rate on the 2 longwave detectors.

Ground-based persistence modeling

Theory

Smith et al. (2008) have proposed a model explaining the origin of persistence. The detector lattice contains impurities that act as charge traps. When the detector is initially reset, a strong electric field creates a charge-free region within the material; under its influence any photo-generated free charge is immediately swept away and collected, thus no trap capture can occur. As the signal accumulates, the electric field decreases and the charge-free region shrinks, leaving behind an increasingly wider neutral region. Photo-generated free charges will slowly diffuse across the increasingly thicker neutral region to reach the residual charge-free region and be collected. In the process some of them may be captured by the traps, with probability and timescales that depend on the properties of the trap impurities. Eventually, the captured charges will be released back and collected, generating a spurious time-delayed signal (persistence).

Detector-based modeling

The capture and release process can be characterized by a combination of exponential functions with timescales associated to the different trap populations. Leisenring et al. (2016) have analyzed the persistence of NIRCam's flight detectors (or sensor chip assemblies, SCAs) with respect to source flux, rate of discharge of the charge-free region, and over-fill levels, crafting a semi-empirical model of NIRCam persistence.

The data indicate that the average rate of charge releases are consistent within a factor of 2 for the 8 short wavelength (SW) detectors. For the 2 long wavelength (LW) detectors, the average rate of charge release are strongly similar (see Figure 5). The average results for the 2 types of detectors are presented below. Certain detectors show some significant spatial variation; therefore, individual maps to illustrate the regions more affected are also provided. These maps may allow observers to place the brightest offending sources in the less affected areas of the focal plane.

Figure 5. Median persistence of NIRCam detectors measured in ground testing

Median persistence behavior for all of NIRCam's SCAs after fully saturating the detectors for approximately 7 minutes. The left image shows the time-dependent latent accumulation, while the right image plots the binned persistence rate. The dashed lines indicate NIRCam's typical quiescent dark current rates as measured during ISIM testing, while the dotted lines show representative zodiacal light estimates for filters F200W and F356W. © Leisenring et al. (2016).

Persistence diagrams

Figures 2 and 3 present the results obtained using the Leisenring et al. (2016) model for the NIRCam short and long wavelength channels, respectively. Each plot shows the accumulated persistence (total released charge, in electrons) after a bright illumination as a function of the fill level of the saturation level ("well fill fraction," vertical axis) and integration time (horizontal axis). The well fill fraction ranges from 0.1 to 5 times the saturation level (about 120,000 e–) whereas the integration times ranges from 0 s to 1,400 s. The results are color-coded according to the color bar at the bottom of each plot. The plots are relative to the following cases:
 

  1. Left plot: integration starting immediately after the illumination exposure, without a break between them. This case is representative of the persistence measured in coronagraphic or in time-series observations, and in general, for all cases of exposures containing multiple integrations. The maximum latent signal, about 200 e– at 5× saturation (light blue at the top-right corner), is much smaller than the shot noise of the source, i.e., SQRT(5*120,000) = 774 e–. However, this effect is systematically raising the measured signal in a ramp, so it must be taken into account, for example, to correct for absolute photometry.
     
  2. Middle plot: in this case the integration starts 45 s after the previous offending integration. This is about the minimum time needed to restart an exposure after a dither move, excluding overheads (detector setup, wait for next readout cycle, reset all pixels, cleanup after the exposure) that may add about 70 s. The time delay allows trapped charges to be released and swept away by the strong electric field set by the interim reset frames. Since a ~50% fraction of the traps has a short decay timescale, the time delay required by a dither move cuts down significantly the total amount of released charge. Still, the systematic effect may become visible over the sky background (TBD).
     
  3. Right plot: in this case, the integration starts after 1,800 s, which is the nominal time (including overheads) charged to slew to a new target. In this case, the release of charges is in practice negligible, reaching 21 e– at the end of a 1,400 s integration following a 5× illumination. 

Figure 6. Short wavelength channel persistence measured in ground testing

Map of the persistence level accumulated at the end of a dark exposure (total latent emission, in electrons) as a function of the illumination level in a previous exposure, in units of the saturation threshold (well fill fractions) for the NIRCam SW SCAs. The 3 figures, left to right, illustrate the case of a dark exposure starting immediately after, 45 s after, and 1,800 s after the bright exposure, respectively.
Figure 7. Long wavelength channel persistence measured in ground testing

Same as Figure 6, for the NIRCam LW SCAs.

Persistence maps

Figures 8 to 11 show the persistence distribution across each SCA. Figures 8 and 9 refer to the SW and LW channels of module A, while Figures 10 and 11 are relative to module B. These maps were derived from extremely super-saturated data held at saturation for a long time, and therefore, they are representative of the total trap populations. The SW maps tend to show considerable structure, apparently correlated with the dark current distribution. The LW detectors, however, are pretty flat. A value of 1.0 corresponds to the median persistence of all SW or LW detectors. Therefore, one can use this information to tie back to the average contour plots presented in Figures 6 and 7, getting estimates at specific locations. This information may be useful if one has bright sources in the target field, as a guide to avoid certain areas. In particular, grism time-series observations use A5, A1, and A3, and this last SCA is affected by non-uniform persistence. Time-series imaging observations (using primarily B1 and B5) and NIRCam coronagraphy (using A2, A4, and A5) are marginally affected by field dependence of persistence. 

Figure 8. Maximum persistence module A (SW)

Spatial map of the persistence level accumulated after a very bright light exposure, for the 4 SW SCAs of NIRCam module A. In Figures 8–11, the red squares indicate the first pixels to be read out from each detector.
Figure 9. Maximum persistence module A (LW)

Same as Figure 8 for the LW SCA A5 (ALONG) of NIRCam module A.
Figure 10. Maximum persistence module B (SW)

Same as Figure 8 for the 4 SW SCAs of NIRCam module B.
Figure 11. Maximum persistence module B (LW)

Same as Figure 8 for the LW SCA B5 (BLONG) of NIRCam module B.

References

Leisenring, J. M., et al. 2016, Proc. SPIE 9915
Characterizing persistence in JWST NIRCam flight detectors

Smith, R. M., et al. 2008, Proc. SPIE 7021
A theory for image persistence in HgCdTephotodiodes



Latest updates

  • Added in-flight persistence plots and info. Reorganized to put in-flight updates at the top

  • updated to mention persistence is somewhat worse than expected (details forthcoming)

  •  
    updated ETC calculation in the 'Estimating Persistence' section & removed old text.
Originally published