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NICMOS Data Handbook > Chapter 4: Anomalies and Error Sources > 4.8 Effects of Overexposure

4.8 Effects of Overexposure
Because each pixel of the NICMOS detectors is read individually, overexposure does not cause “bleeding” along columns or rows, as is seen with CCDs. Sources that produce extremely large numbers of electrons in the detector, however, can result in two other sorts of NICMOS artifacts: persistent images, and electronic ghosts known colloquially as the “Mr. Staypuft” anomaly.
4.8.1
HgCdTe detector arrays are subject to image persistence. When pixels collect a large amount of charge, the photoelectrons are incompletely removed by reading the allay and the remaining charge results in a signal on subsequent reads. This persistent signal decays exponentially with a time scale of about 160 60 seconds, but there is also a long, roughly linear tail to the decay such that persistence from very bright sources remains detectable as much as 30 to 40 minutes after the initial exposure. Exposures of bright astronomical targets therefore can leave afterimages which appear in subsequent exposures taken within the same orbit, or even into the next (Figure 4.10). If you are analyzing dithered exposures of a bright target, you may wish to carefully inspect each image for residual ghosts from the previous exposure(s), and perhaps mask them out when co-adding the dithered images. It appears that all sources of photoelectrons leave persistent afterimages, but under typical conditions they are most noticeable for sources which have collected 20,000 or more ADU during the previous exposure. It is not unreasonable to expect afterimage signals of ~1 e-/second immediately following a severe over-exposure. Occasionally, persistent images result from bright targets imaged by the previous NICMOS observer.
Figure 4.10: Persistence induced by an exposure of a bright star.
The left and right images show dark frames taken 32 and 512 seconds after the exposure of the star
Cures:
There is no easy cure for persistence in existing data; just be aware that it may be present. For dithered observations you can mask out persistent images from previous exposures before combining into a mosaic. It may be possible to subtract a rescaled version of the previous image that caused the persistent afterglow from the persistence image itself and thus remove it (see the discussion of cures for cosmic ray induced persistence below)
4.8.2
More insidiously, during regular passages of HST through the South Atlantic Anomaly (SAA), the arrays are bombarded with cosmic rays, which deposit a large signal into nearly every pixel. The persistent signal from these cosmic rays may then be present as a residual pattern during exposures taken after the SAA passage. This appears as a mottled, blotchy or streaky pattern of “noise” (really signal) across the images, something like a large number of faint, unremoved cosmic rays. These persistent features cannot be removed by the MULTIACCUM cosmic ray processing done by the standard pipeline (i.e. the CRIDCALC step of calnica) because they are not transient. However, for data taken in Cycle11 and beyond, a method for removing the persistence has been developed (saaclean) and it has been a part of the standard the pipeline since 2008. This method is further described under "cures" below and in Section 3.3.3
For Cycle 7/7N data and Cycle 11 and beyond data that has not been processed with saaclean, effects of cosmic ray persistence may affect image quality. The cosmic ray persistence adds non-Gaussian, spatially correlated noise to images, and can significantly degrade the quality of NICMOS data, especially for exposures taken less than 30 minutes after an SAA passage (see examples in Figure 4.11). Count rates from moderately bad cosmic ray persistence can be of order 0.05 ADU/second, with large pixel-to-pixel variations reflecting the spatial structure of the signal. The effective background noise level of an image can be increased by as much as a factor of three in the worst cases, although 10% to 100% is more typical. This “noise” is primarily due to the spatially mottled structure in the persistence, not the added Poisson noise of the persistence signal itself. Because HST passes through the SAA seven or eight times a day, a large fraction of NICMOS images are affected by cosmic ray persistence to one degree or another. Observations of bright objects are hardly affected, since the persistent signal is usually quite faint. Similarly, short exposures are not likely to be badly affected because the count rate from persistence is low and may not exceed the detector readout noise. But deep imaging observations of faint targets can be seriously degraded. The NICMOS ISR-98-001 (Najita, Dickinson and Holfeltz 1998) presents a detailed discussion of this phenomenon and its effects on imaging observations.
Figure 4.11: Cosmic Ray Persistence in Three Dithered NIC2 MIF1024 Images.
For each image, the histogram at bottom shows the distribution of pixel values near the sky level. The shoulder of positive values for the persistence-impacted images demonstrates the non-Gaussian nature of CR persistence noise. The “normalized RMS” is the measured value divided by the mean for many “clean” images free of CR persistence
Cures:
For Cycle 7/7N data there is no task to remove the persistence signal. However, if you have multiple exposures within a given SAA-impacted orbit, the amplitude of the persistence should decay, and you may want to give later exposures higher weight when combining them. Sometimes it may be preferable to discard the worst frames altogether. Well-dithered data lessens the impact of persistence, since objects will move relative to the persistent CR signal and it will not sum coherently when the data are registered. With well-dithered data (at least three positions), one can also take advantage of the MultiDrizzle software to combine images after masking the persistence.
Starting in Cycle 11, the operating procedure has been to automatically perform a pair of ACCUM mode darks for each NIC camera following every SAA passage, provided there is a NICMOS observation scheduled before the next SAA passage. Post-SAA darks can be identified by the target name “POST-SAA-DARK.” Calnicb generates a product that is currently a simple average of the two dark exposures and the product filename is ipppssoot_saa.fits. The post-SAA dark exposures that are associated with a given observation are automatically retrieved with the science data whenever a post NCS NICMOS science observation is retrieved from the HST Archive.
Keywords saa_exit and saa_time contain information about the last exit from the NICMOS SAA contour. saa_dark and saacrmap1 contain information about the post-SAA dark that was taken closest in time to the science exposure. The values of the saa_exit and saa_time keywords are used by the OPUS pipeline to identify the filenames of the post-SAA darks closest in time to the respective NICMOS observation. These filenames are written into the header keywords. If no post-SAA dark is appropriate for a given science observation, the value of these keywords will be set to ’n/a’. The new keywords are presented in the following example:
The NICMOS team has developed and tested a PyRAF task (based on the original algorithm developed by L. Bergeron) called saaclean (see Section 3.3.3) that utilizes post-SAA darks in order to remove cosmic ray persistence. This task is delivered with STSDAS version 3.6 and located in the stsdas.hst_calib.nicmos package of PyRAF version 1.3 or later. Starting March 31, 2008 the saaclean task is included in the OTFR, therefore all data retrieved from the archive after that date will have the SAA cosmic ray persistence removed. For more information, please see the NICMOS ISR 2007-001 by Barker, Laidler, and Koekemoer, and the task help file. The original detailed algorithm and IDL software tool developed by L. Bergeron are described in Both the algorithm and the tool are described in NICMOS ISR by Bergeron and Dickinson (2003, NICMOS ISR 2003-010).
4.8.3
It has recently been found that the NICMOS detectors may be exposed to another type of persistence, Bright Earth Persistence (BEP). This occurs in rare instances when HST is pointed towards the bright earth and one or more of the NICMOS filter wheels are not yet in the blank position. The result is an effect similar to that encountered during SAA passages, image persistence with multiple trap decay time scales, but with a different source of excitation, soft photons instead of cosmic rays. The persistence image resulting from exposure to the bright earth is similar in appearance to the persistence resulting both from intense cosmic rays and from using onboard lamps. However, the bright earth exposes the detectors more evenly than in the case of cosmic rays and more intensely than from the onboard lamps. The procedure used for removing cosmic ray persistence therefore needs to be modified to account for this. A description of the BEP effect and an outline of how it can be removed is given in NICMOS ISR 2008-001 (Riess & Bergeron 2008). For further updates please refer to the NICMOS Web page at:
http://www.stsci.edu/hst/nicmos/
The occurrence of BEP depends on several factors and it is currently not know to what extent NICMOS data is affected. It is likely to be a fairly small fraction, but large enough to require a cure (see below). In order to avoid exposing the detectors to the bright earth, each NICMOS filter wheel will normally place the blank in front of the detectors whenever the HST is pointing within 20d of the earth’s bright limb (and 6d from the earth’s dark limb). However, if NICMOS is operating in parallel with another instrument, the read-out of this instrument will take precedent and could, in rare circumstances, lead to a delay in moving the NICMOS filter wheel.
The combination of ACS as primary and NICMOS as parallel instrument is known to have resulted in BEP in NICMOS. Another instance that could result in exposure to the bright earth is if a slew is initiated immediately following a NICMOS visit and this visit ended before reaching the Earth Avoidance Zone. The telescope will then be slewing, possibly across the bright earth, while the NICMOS filter wheels are in motion.
Figure 4.12: NIC2 Persistence Images
The far left is the BEP image derived from the 10 impacted visits of the SHOES program. The middle panel is the persistence seen from the scattering of the full detector by CR hits in numerous SAA passages. The version on the right, like SHOES, is caused by persistence from photons, in this case after the use of the lamps. The best signal-to-noise ratio for modeling the persistence comes from the BEP on the left due to its more uniform and greater excitation.
Cures:
Removal of the BEP is done in a manner similar to the removal of cosmic ray induced persistence from SAA passages. A stored BEP persistence image is scaled to the measured persistence and subtracted from the science image. This procedure is repeated until less than 50% of the background pixels are affected. This rather high limit is set to avoid affecting sources in the image. Due to the large coverage of afflicted pixels, the scaling is done by minimizing the residuals relative to a flat sky (without sources). In contrast, SAA cosmic ray persistence is scaled by minimizing residuals relative to adjacent pixels because of the quantized "illumination" by the random cosmic ray hits. For more details about the routine, nic_rem_persist that removes BEP, please refer to Section 3.3.3
4.8.4
There is another sort of NICMOS ghost image, wholly separate from those induced by persistence. Bright targets which appear in a given detector quadrant can produce an electronic ringing effect in the readout amplifiers, which may induce ghost images at the same pixel locations in the other three quadrants. This is believed to be due to the pull-down of the power supply, which does not completely recover from reading a large number by the time it’s asked to read the next number (from the next quadrant). In some cases a whole row (or column, whichever direction the “fast” read clocking runs) can be anomalously high. The amplitude of the ghost images is of order a tenth of a percent of the real image count rate. Inside the STScI NICMOS group, this effect is fondly known as the “Mr. Staypuft anomaly” (Figure 4.13).
Figure 4.13: The Mr. Staypuft Anomaly.
The bright source at lower right produces electronic ghost images in the other three quadrants, plus vertical streaks. In Camera 1 images, these streaks run horizontally
Cures:
The STScI NICMOS group has developed the STSDAS "puftcorr" task which identifies and removes the Staypuft column/row effect in post-processing, based on an empirical model for its electronic behavior. For example, if your image has a bright source at pixel (58,143), then you may see ghost images around (186,143), (186,15), and (58,15). If the observation was made with Camera 3 (so that the fast clocking direction is along columns), there may also be elevated signal at all rows on and around columns 58 and 186. Ordinarily it is not possible to eliminate these ghosts from dithered data by masking, since they will move about on the array along with the astronomical targets. If, for some reason, observations were obtained at multiple roll angles, then it would be possible to mask the ghosts and eliminate them; this was done for the Hubble Deep Field South NICMOS observations.
Some NICMOS observers studying relatively blank fields have dealt with the “elevated columns/rows” phenomenon by fitting medians to the affected columns/rows (or to portions of the columns/rows, avoiding regions affected by bright or extended targets), and subtracting the median values from each column or row of the data. This can at least provide a cosmetic fix. In principle, this correction should probably be applied before flat fielding the images, since it is most likely that the “Staypuft” signal is not responding to the array QE variations, although this has not yet been formally tested. However, if the fit is done before flat fielding, then the sky background itself will not be uniform, and in fact will be improperly subtracted by the procedure. For this reason, in practice it is probably safer (if not strictly correct) to apply a median column (or row, for NIC1) correction after flat fielding instead. For updates, please refer to the NICMOS Electronic Ghosts Web page.
4.8.5
In addition to persistence and electronic ghosts, NICMOS polarimetric images of bright targets are subject to optical ghosts (Figure 4.14). These are most evident for the short wavelength polarizers in NIC1. The location of the polarizer ghosts relative to the source changes direction with respect to the principal axes of transmission. For POL0S, the brightest ghost is found roughly (-10,+35) pixels away from the target position, and a second one at (-16,+70) pixels. It is possible that for a brighter object more ghosts would appear in the same angle and direction. POL120S images appear to be ghost-free. For POL240S, ghosts appear (+10,+54) pixels from the source position and (+18,+80) pixels, with the possibility of more appearances for brighter targets.
Figure 4.14: Optical ghosts in NIC1 polarizers.
Ordinary (non-polarimetric) NICMOS images of bright sources may have very faint filter ghosts which result from back-reflections off the faceplate and the filters. The position of the ghosts changes from one filter to another. They are roughly 8 magnitudes fainter than the star which produces them. The ghosts cannot be completely subtracted from the field unless the reference PSF is at the exact same location as the star. They are typically seen as a residual in the PSF subtracted images, and look like large “donuts” because they are out-of-focus reflected images.
Cures:
No true cure is possible: be aware that such ghosts may exist; their positions are predictable. If images are available with multiple spacecraft roll angles, the ghosts may be masked out before combining the data.

1
The SAACRMAP keyword is no longer being used by the OTFR pipeline


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