NIRCam Persistence
Highly exposed sources can cause faint residual images that persist in subsequent exposures. Persistence is typically inconsequential for NIRCam observations, but instances of long-term persistence have occurred and can be mitigated.
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Image persistence, or latency, is a common characteristic of near-IR detectors. Each pixel retains a small fraction of the charge accumulated during the integration period, causing a faint residual image to linger for a few seconds or several hours depending on the fluence of the source and the amount of accumulated charge. Although the effect is more obvious for pixels exposed to bright sources, any amount of illumination can cause latent images. Persistence is not limited to saturated pixels, and it is not eliminated by simply resetting the detector.
Persistence is not usually a problem for dithered NIRCam observations, but some instances of long-term persistence from highly exposed sources have detrimentally affected subsequent NIRCam observations to an extent that they had to be repeated. STScI is currently obtaining data to calibrate the strength and longevity of latent signal for a wide range of fluence and integrated signal, so that a pixel-based correction can be implemented in the future. In the meantime, STScI has inaugurated an operational strategy for mitigating persistence that involves the insertion of gaps in the on-board observing plan between NIRCam observations that are likely to yield significant long-term persistence and subsequent NIRCam observations.
This article contains information about the observational and analytical strategies for mitigating the effects of persistence that can be employed by users of NIRCam in lieu of the planned pixel-based correction.
Observational strategy for persistence mitigation
A preemptive method for mitigating the impact of persistence from highly-exposed sources on subsequent NIRCam images has been initiated by STScI for Cycle 3 observations. This method features the insertion of a temporal buffer between consecutively scheduled NIRCam observations to allow persistent signal to decay to levels acceptable for most background-limited science programs. These scheduling buffers affect only NIRCam observations; they do not apply to observations using the other JWST science instruments. The buffers will be inserted by STScI staff (as part of the review process) after NIRCam observations containing highly-exposed images that are expected to generate significant latent signal lasting several hours. Such cases typically include, but are not limited to, observations of major Solar System objects, nearby star-forming regions, and bright crowded stellar fields (Figure 1).
Figure 1. Example of Long-Term Persistence in a NIRCam Deep-Field Image
A sequence of NIRCam module B F220W deep field images from GTO program 1305 recorded 40.5 min to 3 hr after an overexposed image of Jupiter from program ERS program 1373 in July 2022. A latent image of Jupiter's disk centered near the upper right corner of detector B4 is visible above the sky background throughout the sequence. Latent scattered light from Jupiter is also seen along the left edge of B4, where there is a higher density of charge traps in the pixels.
Each SPR in an observing program is flagged for persistence mitigation by the NIRCam instrument scientist (IS) assigned to that program. The program coordinator (PC) inserts a scheduling buffer into the APT file immediately following each set of contiguously scheduled SPRs. A dark observation terminates the scheduling buffer and provides a reference image for any residual persistence prior to the next NIRCam science observation. Note that NIRCam is placed in the dark configuration only at the end of the buffer; its detectors are otherwise unshuttered as the latent signal decays.
Because the mitigation of SPR is beneficial to all NIRCam observations, the overhead associated with the added scheduling buffers is absorbed into the budget for general JWST operations. Although the time allocated to an observing program that requires SPR mitigation must be increased to accommodate the added dark observation(s), this extra time will not be drawn from the time originally awarded to the program by the TAC. In short, observers of very bright targets or targets that require very lengthy exposures will not be penalized for the persistence generated by their observations, nor should they be discouraged from submitting proposals that require such observations to achieve their scientific objectives.
Analytical strategy for persistence mitigation
Whether or not an observation is preceded by a scheduling buffer for preemptive persistence mitigation, the amount of latent signal in a NIRCam image from a preceding image can be determined analytically from the level of overexposure (measured directly from the preceding image or estimated from the Exposure Time Calculator) and calibrated models of NIRCam persistence.
Ground-based persistence modeling
Smith et al. (2008) 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).
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 (Figure 2). 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 2. 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).
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.
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.
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 3. 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 4. Long wavelength channel persistence measured in ground testing
Same as Figure 3, for the NIRCam LW SCAs.
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 5–8. Detectors A3, B3, and B4 show the longest setting times.
Figure 5. Measured persistence in SW and LW detectors
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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 >=10× 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 6. Measured persistence in the module A SW detectors
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Similar to Figure 5, 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 7. Measured persistence in the module B SW detectors
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Similar to Figure 6, 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 8. Measured persistence in the LW detectors
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Similar to Figures 5–7. Persistence signal rate on the 2 LW detectors.
Persistence maps
Figures 9 and 10 show the persistence distribution across each SCA of NIRCam module A and B, respectively. These maps were derived from extremely 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 3 and 4, 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 9. Maximum persistence maps for module A detectors
Spatial map of the persistence level accumulated after a very bright light exposure, for the SW and LW SCAs of NIRCam module A. The red squares indicate the first pixels to be read out from each detector.
Figure 10. Maximum persistence maps for module B detectors
Spatial map of the persistence level accumulated after a very bright light exposure, for the SW and LW SCAs of NIRCam module B. The red squares indicate the first pixels to be read out from each detector.
Estimating persistence with the ETC
Using the Exposure Time Calculator (version 3.2, for the example outlined below), along with Figures 2–4, 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.
The primary 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 1,593 e–/s is obtained, having added the small contribution from the background. The total flux is therefore 1,593 e–/s × 236.21 s = 376,283 e–, which is a well fill fraction of about 3.1 (saturation occurs at about 120,000 e–). Using the central panel of Figure 3, the next dithered exposure of the same length and in the same filter will show about 90 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 = 5.4.
Figure 3 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 3.
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