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Anomalies and Error Sources NICMOS 4 Anomalies and Error Sources In This Chapter... NICMOS Dark Current and Bias / 4-4 Bars / 4-19 Detector Nonlinearity Issues / 4-20 Flatfielding / 4-22 Pixel Defects and Bad Imaging Regions / 4-24 Effects of Overexposure / 4-28 Cosmic Rays of Unusual Size / 4-34 Scattered Earthlight / 4-36 The previous chapter described the basic stages of NICMOS pipeline processing. As with any instrument, however, high quality data reduction does not end with the standard pipeline processing. NICMOS data are subject to a variety of anomalies, artifacts, and instabilities which complicate the task of data reduction and analysis. Most of these can be handled with careful post-facto recalibration and processing, which usually yield excellent, scientific grade data reductions. Careful NICMOS data processing usually requires a certain amount of "hands-on" interaction from the user, who must inspect for data anomalies and treat them accordingly during the reduction procedures. This chapter describes the most common problems affecting NICMOS data at the level of frame-by-frame processing. In some cases, recognizing and treating problems with NICMOS data requires a moderately in-depth understanding of the details of instrumental behavior; problems with dark and bias subtraction are a good example. Where appropriate, this chapter offers a fairly detailed discussion of the relevant workings of the instrument, but the reader should consult the NICMOS Instrument Handbook for further details. Each section of this chapter deals with a different aspect of NICMOS data processing, roughly following the order of the processing steps in the standard STSDAS pipeline. Various potential problems are described and illustrated. Each discussion then has a subsection labeled "Cures" which offers possible solutions Version 4.0 December 1999 NICMOS 4-1 NICMOS 4-2 NICMOS 4: Anomalies and Error Sources to the problems at hand. The art of NICMOS data processing is still evolving, and new techniques are being developed by users worldwide. Some of these have been encoded into software which is now distributed with STSDAS. New processing methods and routines are still being developed at STScI and will be provided in the future: the reader should consult the STScI NICMOS WWW pages at the following URL to look for new developments and software tools http://www.stsci.edu/instruments/nicmos/topnicmos.html. We begin with a checklist of potential NICMOS instrumental anomalies and potential data processing problems about which the user should be aware. Each of these is discussed in further detail in the sections which follow: the nature of the problem and its impact on NICMOS data is illustrated, and possible processing solutions are considered. The relevant sections for each anomaly are given in parentheses below. NICMOS / 4 Version 4.0 December 1999 NICMOS 4-3 NICMOS Problems to Watch Out For: A Checklist • Bias and dark subtraction problems, including: - residual shading (§ 4.1.2 , § 4.1.5 ) - variable quadrant bias or "pedestal" (§ 4.1.2 , § 4.1.5 ) - bias jumps or bands (§ 4.1.2 , § 4.1.5 ) • Bars (§ 4.2 ) • Nonlinearity correction uncertainties - new nonlinearity corrections (§ 4.3.1 ) - non-zero zeroth read correction (§ 4.3.2 ) - uncorrected saturation (§ 4.3.3 ) • Flatfield issues, including color dependent flat fields (§ 4.4 ) • Pixel defects and bad imaging regions, including: - Bad pixels (§ 4.5.1 ) - Grot (§ 4.5.1 ) - Erratic middle column/row (§ 4.5.2 ) NICMOS / 4 - Coronagraphic hole masking (§ 4.5.3 ) - Vignetting (§ 4.5.4 ) • Effects from bright targets: - Photon-induced persistence (§ 4.6.1 ) - Post-SAA cosmic ray persistence (§ 4.6.2 ) - The "Mr. Staypuft" Effect (amplifier ringing) (§ 4.6.3 ) - Optical ghosts (§ 4.6.4 ) • Cosmic rays of unusual size (§ 4.7 ) • Scattered earthlight (§ 4.8 ) Version 4.0 December 1999 NICMOS 4-4 NICMOS 4: Anomalies and Error Sources 4.1 NICMOS Dark Current and Bias Some of the major challenges for achieving high quality NICMOS data reduction arise from difficulties in removing additive components of the instrumental signature that are present in a raw NICMOS image. For the purpose of discussion here, we will divide these additive components into two categories, bias and dark, according to whether or not the signal is noiseless and purely electronic in origin (bias), or noisy and arising from thermal or luminous sources (dark). In practise, the NICMOS bias and dark signals each consist of several different components which exhibit a range of different behaviors. In the standard reference files used for processing NICMOS data, dark and bias components are combined together as a single DARK image and are handled in the same step (DARKCORR) of the calnica pipeline processing. NICMOS dark images (really dark + bias) are highly dependent on the readout history of the array since it was last reset, and, therefore, cannot be simply rescaled to the exposure time of the science data (as is done with most conventional CCD data). Each science file must be calibrated with a dark frame of equal exposure time and number of readouts. Because there is such a large variety of NICMOS MULTIACCUM readout sequences, it was considered to be impractical to obtain and regularly monitor NICMOS / 4 darks in every possible sequence. However, most components of the NICMOS bias and dark current are highly reproducible and can be reliably calibrated. On-orbit darks obtained during SMOV and throughout the lifetime of the instrument have been used to characterize the dependence of the major dark and bias components on pixel position, on time, and on temperature for each of the three NICMOS detectors. This information has been used to construct "synthetic" dark calibration reference files for all MULTIACCUM readout sequences, using as basic data the on-orbit darks obtained during the first part of the Cycle 7 calibration program. These are described in § 4.1.3 below. Unfortunately, some components of the NICMOS bias have turned out to be unstable or unpredictable, making it difficult or impossible to remove them using the standard reference files. In order to do a good job removing additive dark and bias signatures, it is important to understand their origin and behavior. Here we describe the various components of NICMOS biases and darks in some detail, highlighting their stability or lack thereof, and describing (briefly) how they are incorporated into the standard STScI synthetic dark reference images. In § 4.1.5 below we describe methods and tools for measuring and removing residual dark and bias artifacts from NICMOS images. Version 4.0 December 1999 NICMOS Dark Current and Bias NICMOS 4-5 4.1.1 Dark Current Linear Dark Current The true, thermal dark current is the detector current when no external signal is present. This component grows linearly with integration time: D(x,y,t) = dc(x,y) × t where D(x,y,t) is the observed signal in a given readout, t is time since reset, and dc(x,y) is the dark current. The mean NICMOS dark current for all three cameras is of order 0.1 e-/sec. The dark current has some two dimensional structure, and is roughly a factor of two higher in the corners than at the center. Amplifier Glow Each quadrant of a NICMOS detector has its own readout amplifier, which is situated close to an exterior corner of the detector. When a readout is made, the amplifier emits radiation which is recorded by the detector, an effect known as amplifier glow. This signal is largest close to the corners of the detector where the amplifiers are situated, and falls off rapidly towards the center of the array. The signal is only present during a readout, but is repeated for each readout (e.g., a MULTIACCUM sequence or an ACCUM with multiple initial and final reads). Typically the extra signal is about 20-30 DN at the corners of the detector and 2-3 DN at the center, for each readout. The signal is highly repeatable, and exactly linearly dependent on the number of reads. The amplifier glow also depends on NICMOS / 4 the amplifier-on time, which is slightly shorter for ACCUM mode. The amplifier glow is a real signal and is subject to photon statistics, so it is a source of noise in NICMOS exposures. In the processing pipeline and calibration reference files, it is considered to be a component of the dark signal, although its physical origin and temporal dependence is quite different than that of the thermal dark current. Thanks to the repeatability of the signal, images calibrated with the appropriate dark frames (same MULTIACCUM sequence or same exposure time for ACCUM images) will have the amplifier glow removed. For the STScI synthetic dark reference files, the amplifier glow is modeled as A(x,y) = amp(x,y) × NR where A(x,y) is the cumulative signal due to the glow in a sequence, amp(x,y) is the amplifier glow signal per readout (a function of the pixel location (x,y) and the amp-on time), and NR is the total number of readouts of the array since the last reset. In the corners of a full, 26-readout MULTIACCUM response there will be of order 500-800 DN due to amplifier glow, as well as the associated Poisson noise from this signal. This nominal Poisson noise is propagated into the ERR array of the NICMOS calibrated images by calnica. Version 4.0 December 1999 NICMOS 4-6 NICMOS 4: Anomalies and Error Sources Figure 4.1: Amplifier glow images for NICMOS cameras 1, 2 and 3. NIC1 NIC2 NIC3 4.1.2 Bias, Shading, and "Pedestal" There are three readily identifiable (but not necessarily physically distinct) components of the NICMOS bias: the detector reset level, shading, and variable quadrant bias or "pedestal." Bias Reset Level First, a net DC bias with a large, negative value (of order -25000 ADU) is NICMOS / 4 introduced when the detector is reset.
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