The terms “persistent image,” “residual image,” and “latent image” are used interchangeably to describe the contamination of an IRAC image by a bright source from a previous exposure. When a pixel is illuminated, a small fraction of the photoelectrons become trapped. The traps have characteristic decay rates, and can release a hole or electron that accumulates on the integrating node long after the illumination has ceased. The warm mission short-term residual images are different in character than the cryogenic residuals, as the behavior of the trap populations is a function of the impurity type and array temperature. During the cryogenic mission, in all arrays, the longest e-folding decay time is about 1000 seconds. For the warm mission, residuals in channel 1 are < 0.01% of the fluence of the illuminating source after 60 seconds.
For extremely bright sources, residuals are produced even when the source is not imaged on the array. Residuals at 3.6 and 4.5 µm can be produced during slews from one science target to another and from one dither position to the next. These slew residuals appear as linear features streaking across IRAC images. Note that while the pipeline flags persistent images (Section 5.1.22), the pipeline cannot flag slew residuals, as there is no reasonable way of tracking the appearance of bright sources relative to the moving telescope pointing.
Observations contaminated by residual images can often be corrected with the data themselves. If the observations were well dithered, it is likely that the persistent image artifacts will be rejected as outliers when building the mosaic. Examining the median stack images that can be downloaded from the Spitzer Heritage Archive together with the data can often be used to identify pixels that are affected by residual images. Residual images can often be at least partially mitigated by subtracting the normalized median stack image (made with object and outlier rejection).
7.2.8.1 Cryogenic Mission Persistent Images
Tests performed during the In-Orbit Checkout (IOC) revealed that there are both short-term persistent images, with time scales of order minutes and which are present in all four arrays, and longer-term persistent images in channels 1 and 4. The short-term persistent images were known before the launch, and extensive calibrations and data analysis were performed to characterize them. The pipeline produces a mask (bit 10 of the imask) for each image that indicates whether a bright source seen by a previous exposure would have left a persistent image above three times the predicted noise in the present frame. To identify persistent images in your own data, we recommend doing a visual search on a median combined stack.
The longer-term persistent images were discovered in flight. In channel 1, the persistent images are generated by stars as faint as Vega K mag = 13 (in very long stares). They can be generated by any long dwell time with a bright star on the array, whether or not the array is being read out. They were first noticed during a high-gain antenna downlink, when IRAC was left at a fixed position viewing the Galactic plane (by chance) for 45 minutes. The persistent images do not have the same size as a direct point source; they are significantly more diffuse (looking more like the logarithm of the point spread function). The channel 1 long-term persistent images have time scales of order six hours, and they decay gradually. The cause of these persistent images has been identified as a known feature of the flight array (broken clamp) that cannot be fixed. The longer-term persistent images in channel 4 are induced by bright mid-infrared sources or bright stars. The channel 4 persistent images have very unusual properties: they have lasted for as long as two weeks, they can survive instrument power cycles, they do not decay gradually, and they can switch sign, as they decay, from positive to negative. The amplitude and decay rate of long-term persistent images is variable and no secure model exists to remove these artifacts from the data.
Figure 7.9: Median of channel 1 images from a calibration observation performed after observing Polaris. The five bright spots are persistent images from staring at the star while observing, while the set of criss-crossing lines were generated by slews between the various pointings. These observations were taken from PID=19, AORKEY 3835904.
We instituted a proactive and highly successful method of eliminating persistent images in the cryogenic mission. Channels 1 and 4 were temporarily heated, or “annealed” briefly, with a small current running through the detector. The arrays were annealed after every telemetry downlink, which erased any persistent images built up during the downlink or during the previous 12-hour period of autonomous operations (PAO). This strategy, combined with scheduling known bright object observations immediately before downlinks, greatly decreased the possibility that preceding observations produced persistent images.
We have found that stars brighter than about magnitude -1 at 3.6 µm, when observed for more than about six seconds, left a residual image that persisted through an anneal, and even through multiple anneals. These latents from extremely bright objects are seldom visible in a mosaic of a science observation, but they appear in skydarks and other median-filtered stacked images of longer science observations. In channels 2, 3, and 4, all residual images are completely removed by a single anneal, and since January 2006 until the end of the cryogenic mission, we annealed all four arrays every 12 to 24 hours.
Finally, we show an example of persistent images. Note that not all cases will be this obvious. In Figure 7.9 we see not only residual images of the star Polaris, but also residual streaks left by Polaris as the telescope moved between dither positions.
7.2.8.2 Warm Mission Persistent Images
Channel 1 and channel 2 have different persistent image responses in the warm mission data. There are no long-term residual images that last for weeks, such as those seen in channel 4 data during the cryogenic mission. Channel 1 residual images last for minutes to hours, depending on the brightness of the original source and the background levels in the subsequent images. The residual images in channel 1 in warm mission never exceed 1% of the illuminating source in exposures beginning immediately after the illumination of the bright source ends. Figure 7.10 shows this persistent image behavior for a first magnitude star (data taken from PID 1318). The residual image decay in channel 1 is exponential in character, as expected for trapped electron decay rates. The decay rate is constant for all sources, so that while residual images from brighter sources take longer to decay below the background level, all the persistent images decay at the same rate. These rates have been implemented for residual image flagging in the warm mission IRAC pipeline.
A consequence of the intermediate-term (hours) residual images is that it is possible for observations from a previous AOR to produce residual images. The residual image flagging module correspondingly tracks residuals from one AOR to the next. Given the original brightness of the saturation-corrected source, and the decay time calculated with the exponential decay rate, the pipeline flags all residual images until their aperture fluxes are less than three times the background noise in each image. For channel 1, each image in each AOR observed is checked for residual images from all previous observations within the observing campaign, and the residual images are followed across PAOs (downlinks). Using calibration data from bright stars in channel 1 we have empirically modeled the residual flux density (in electrons) decreasing as
(7.1)
where A is –5614.36 + 880.12*log10 F (residual causing source flux in electrons) and ltime is the time in hours since the residual causing source was observed. An example of this function is shown in Figure 7.10. We determine the background in each image by randomly selecting 100 regions in the frame, fitting a Gaussian to the electron number distribution and taking the mean from that fit as the background. If the predicted residual flux in electrons is more than 2σ above the background, the pixel is flagged as containing a residual image. To remove a saturated star from a possible residual image causing source list, its predicted flux in electrons needs to be below the 2σ background level in four consecutive images (which allows two HDR mode long exposure images to not contain a residual).
Residual images in channel 2 decay much faster than those in channel 1. In channel 2 the residual images last less than 10 minutes for even the brightest stars. Therefore, the pipeline flagging for channel 2 does not cross AORs. Channel 2 residuals start out as positive, but then become negative. The timing of the switch from positive to negative depends on the exposure time and brightness of the source. In channel 2 the decay times are much faster (minutes) and a linear function of the flux of the culprit source. The duration of channel 2 pixel flagging after a culprit bright source in warm mission is given in Table 7.3.
Duration of residual image flagging after bright source(seconds)
< 1.96x107
220
< 1.31x108
320
< 3.74x108
360
> 3.74x108
440
Table 7.4: Warm mission residual image durations.
Star K-magnitude
Channel 1 residual duration (hours)
Channel 2 residual duration (minutes!)
1
10
6
2
7
< 6
3
3.5
< 6
4
1.5
< 6
The arrays were not annealed during the warm mission as there was no evidence that annealing removed residual images (the arrays operated at nearly the old annealing temperature), and all residual images decayed in a reasonably short time scale compared to those mitigated by annealing in the cryogenic mission.
Table 7.4 gives a rough idea of warm mission latent durations. Durations should not be taken as exact because they also depend on the background levels in the images that will change from one AOR to the next. This example comes from bright star observations in PID 1318 and starts with 12 second observations of the bright stars.
Residual image flagging is only done for full array frames, because subarray frames do not have saturated stars tracked in the saturated star module of the pipeline, which is a necessary input to the residual image flagging routine. Residual images from non-saturated stars have not been studied.
Figure 7.10: Residual image brightness decay as a function of time interval since exposure to a first magnitude source at 3.6 μm. The residual is compared to three times the noise in the sky background as measured in an equivalent aperture. The fitted exponential decay function is plotted as the dot-dashed line. These curves have been smoothed to mitigate flux jumps due to sources at the position of the original source in subsequent images.
Figure 7.11: Residual image examples: a. channel 1 positive residual images near the center of the array, PID=90175, AORKEY 47943424; b. same as previous but showing the median image of the observation that has also positive slew residuals; c. channel 1 positive slew residuals, PID=80096, AORKEY 45585920; d. channel 2 negative residual images, PID=90109, AORKEY 47828736.
Figure 7.12: More residual image examples; a. channel 1, PID=90124, AORKEY 48337408 (the red arrow is pointing to an image residual); b. channel 2, negative slew residuals and column pull-down residuals PID=70044, AORKEY 40840192; c. channel 1 bright slew residual, PID=61009, AORKEY 35354880; d. channel 1 bright residual image from Reuleaux pattern dithering in center, PID=80015, AORKEY 42191104.
