Overview
Image persistence in the IR array occurs whenever a pixel is exposed to light that exceeds more than about half of the full well of a pixel in the array. Persistence can occur within a single visit, as the different exposures in a visit are dithered. Persistence also occurs from observations in a previous visit of completely different fields.
Image persistence is seen in a small, but non-negligible fraction of WFC3/IR exposures. Its properties are discussed in the WFC3 Instrument Handbook in Section 7.9.4. Persistence is primarily a function of the degree to which a pixel is filled (in electrons) and the time since this occurred.
Two examples of persistence are shown below:
The left panel shows an image obtained with WFC3/IR as a parallel from program 11519, Visit 0V. The primary target was Ton 550 which was observed with COS. The IR exposure is of a nearby field and the image obtained shows a bright diffuse object in the center of the field. About 2 hours earlier, the nearby Sb galaxy NGC 2841 had been observed with WFC3 IR. The bright diffuse region in the center of the image is a persistence after-image.
The right panel shows an image obtained of the gamma ray burst GRB090423 as part of program 11189, visit H2. Several observations of fields containing bright fields from programs 11677 and 11548, visits 19 and AJ, preceded this observation. The dither pattern used in these sets of observations are clearly visible in the image. Such obvious examples of persistence are fairly rare in the HST data; using information in the Phase II proposals, STScI scientists attempt to identify observations that are likely to cause this much persistence. STScI planners inhibit WFC/IR observations for several orbits after observations from these "bad actors", long enough for the afterglow images to fade. However, while this screening process has improved significantly over time, it is not perfect.
Moreover, it does not deal with the most common cases of persistence, which are far less obvious, a few isolated spots in random locations in the image.
Observers need to consider persistence in planning observations and in analyzing data. Strategies to minimize persistence in planning observations are discussed here. Tools provided to help observers account for the effects of persistence in analyzing their data are discussed here.
What causes persistence and what are the characteristics of persistence in the WFC3/IR array?
Persistence is caused by traps that exist in the active regions of diodes that make up the pixels of the detector. When the diodes are exposed to light, voltage levels within the diode change slightly and allow free electrons and holes to reach these traps. When the diode is discharged, the trapped electrons and holes escape the traps slowly over time and cause after images. The greater the saturation of the detector, the greater the number of traps and the greater the afterglow. Smith et al. 2008 (Proc. SPIE, 7021, 70210j) has provided a very clear description of the physics of persistence and the effects in IR arrays.
The characteristics of persistence vary for different devices and device technologies. The figure below shows the characteristic shape of persistence as observed in a series of darks following an image of Omega Cen which had been deliberately exposed to a level that many stars in the image were saturated. Here, stimulus is the depth to which individual pixels were exposed. Note that the persistence is fairly small until the exposure level reaches about half of full well and saturates near full well exposure. The persistence gradually decays with time from the first dark exposure which took place a few minutes after end of the Omega Cen exposure to the last dark which took place about one orbit later.
The next figure shows how persistence decays with time. The different curves here show the decay for different levels of saturation. There are 3 curves for each level corresponding to the three times this experiment was repeated. The differences are partially due to the fact that different pixels were illuminated to different levels each time, but may also indicate some intrinsic variability that is not understood.
To good approximation, the persistence decays as a power law with time (with a suggestion that the decay is faster at lower levels of saturation). For comparison, the dark current is about 0.015 electrons/s.
Based on considerations like those shown above, the WFC3 team has developed a "working model" for persistence in the WFC3/IR array.
\(P_{ij} = N_{ij} \left ( \frac{1}{e^{ \left (x-x_0\right )/\delta x }+1} \right ) \left (\frac{x}{x_0}\right )^\alpha \left ( \frac{t}{1000 s} \right ) ^{-\gamma}\)
Here:
| Parameter | Description |
|---|---|
| \(P_{ij}\) | The persistence in the ijth pixel |
| \(x\) | The maximum depth to which the pixel has been filled |
| \(t\) | The time since the pixel was filled |
| \(N_{ij}\) | The position-dependent normalization factor (at 1000 s) |
| \(x_0\) | The "Fermi energy" definding the midpoint of the region where the persistence is rising rapidly |
| \(\delta x\) | The "Fermi kT" defining the width of the region where the persistence rises rapidly |
| \(\alpha\) | The power law index that captures the slow increase in persistence at high saturation levels |
| \(\gamma\) | The power slope for the decay with time |
Using the model and the exposure history, it is possible to estimate the persistence in image. Tests on individual exposures indicate that it is possible to remove most (about 90%) of the persistence in an image although this requires tuning of the parameters from observation to observation. As is discussed below, we have incorporated this model into software that the WFC3 group is running on all WFC3/IR data. As a result, it is possible for users to obtain an estimate of the amount of persistence in their data.
The original and persistence-subtracted images for the two examples discussed above are shown in the figure above. Both images are highly stretched and presented as histogram equalized images to show the persistence as clearly as possible. There is some residual persistence in both images, but clearly the persistence subtracted ones provide a better representation of the celestial objects in both cases. This is fairly typical of the improvement that can be achieved with our existing model, assuming the parameters are tuned to the individual observation sequence.
Accordion
There are several obvious ways to reduce the effects of persistence on your own observations:
- Do not dither a bright object across the region of your image containing your most important science data. Inspect the dither patterns using the tools provided by APT.
- Dither where possible. Dithering reduces the effects of image persistence within your own exposures and due to exposures from earlier visits. Persistence is a characteristic of an individual pixel whereas MultiDrizzle "averages" various exposures taken at a specific position on the sky.
- If possible, keep exposure levels well below saturation everywhere in the image. Be especially careful in cases where a large area of the detector will be saturated as can happen with very bright HII region or in observations of a nearby elliptical galaxy.
- Also, be careful if you are using very short exposures; the pixels accumulate charge for about 8 seconds more than the nominal exposure time. (See ISR 2011-09 for details.) Similarly, if targets are very bright, consider how long it takes to move from filter position to filter position. Although the detector flushed between exposures, the effective exposure time is about 4 sec. See the Instrument Handbook, for the location of filters in the filter wheel.
- If persistence is going to be a problem in your observation, then to the extent possible take your "thinnest" exposures first. Be careful of over-exposing broad band images simply to fill out an orbit.
All observers should check whether IR data obtained with WFC3 is affected by persistence.
In discussing the effects of persistence on WFC/IR data, it is useful to distinguish between persistence caused within a single visit (hereafter "internal persistence") and persistence arising from earlier visits (hereafter, "external persistence"). Internal persistence is generally not a problem from a scientific perspective, unless an observation involves large dithers, because the persistence appears within the psf of bright objects in the field (which are usually not the science target). External persistence is more of a problem because without all the earlier data, it is difficult to know where persistence will occur in a science image.
One way to eliminate the possibility that early IR data could have affected an observation, is to use the MAST history tool. This allows one to search the archive for WFC3/IR exposures that have preceded the exposure you are worried about (based on the association name, keyword ASN_ID). If there is nothing in the archive in the few hours before your observation, then it is unlikely you have a persistence problem.
However, as noted earlier, the WFC3 team has developed a set of prototype software to predict the amount of persistence in an image based on the earlier history of exposures with the detector. The model is by no means perfect but it is good enough to locate regions of the detector that are affected by persistence and to predict the amount of persistence. Outputs from our software are available through MAST using this search form. The outputs include fits files containing the predicted persistence in each "flt" file as well as various summary files that should enable observers to evaluate whether they should be concerned about the effects of persistence on their science images. Separate fits files are provided for external persistence and for total persistence. A complete description of the search form and how to use it is here.
At present, the persistence outputs are provided to MAST as add-on products. It is not possible to obtain the data from the standard HST retrieval screen. Instead you must use the special persistence interface. This will allow you to check the summary outputs for each flt file and to retrieve the persistence output files on a visit by visit basis.
If you identify image persistence in your images, the first thing you should do is determine whether it affects your ability to extract the science from your program. If the answer is no, then there is no need to worry further about persistence. If small areas of the detector are affected, there are several possibilities:
- A conservative approach is to use the persistence images to augment bad pixel maps so that such regions do not affect down stream analysis. The choice of which pixels to flag will clearly depend on the science goals of the analysis being undertaken.
- A second approach is to actually use the persistence-corrected "flt" files provided by the prototype software; the persistence-corrected flt files will almost certainly produce a better cosmetic image than the uncorrected one. In some cases, a better subtraction will be obtained by scaling the persistence image and subtracting it from the original flt file to produce your own persistence-subtracted images. The persistence model we are using is an approximation of the persistence an overall renormalization of the persistence model often does produce a better result. If that does not work, please alert help@stsci.edu to the problem so that we can advise you on how to proceed.
- Finally, in extreme cases, at least for observers you may consider asking the TTRB to approve a repeat of your visit. Here again, your first step should be to contact help@stsci.edu.