Chapter 4:
Anomalies and Error Sources
4.1 NICMOS Dark Current and Bias
4.1.1 Dark Current
4.1.2 Bias, Shading, and "Pedestal"
4.1.3 Dark Reference Files
4.1.4 What is Removed by Standard Pipeline Processing
4.1.5 Cures: How To Get Rid of What's Left
4.2 Bars
4.3 Detector Nonlinearity Issues
4.3.1 New Nonlinearity Calibrations
4.3.2 Non-Zero zeroth Read Correction for Bright Sources
4.3.3 Uncorrected Saturation
4.4 Flatfielding
4.4.1 Characteristics of NICMOS Flatfields
4.4.2 Temperature-dependent Flatfields
4.4.3 Color Dependence of Flatfields
4.5 Pixel Defects and Bad Imaging Regions
4.5.1 Hot Pixels, Cold Pixels, and Grot
4.5.2 Erratic Middle Column/Row.
4.5.3 Coronagraphic Hole
4.5.4 Vignetting
4.6 Effects of Overexposure
4.6.1 Photon Persistence
4.6.2 Cosmic Ray Persistence
4.6.3 Amplifier Ringing (The "Mr. Staypuft" Anomaly)
4.6.4 Optical Ghost Images
4.7 Cosmic Rays of Unusual Size
4.8 Scattered Earthlight
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 yields 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 good examples. 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 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 to look for new developments and software tools.
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.
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It is expected that some aspects of NICMOS performance and anomalies may be somewhat different in Cycle 11 and beyond with the NICMOS Cooling System. The discussions of data anomalies and appropriate solutions in this edition of the NICMOS Data Handbook are based on the properties and behavior of the instrument during Cycles 7 and 7N. NICMOS users should carefully monitor developments and updates posted on the STScI NICMOS web pages when analyzing data taken in Cycle 11 and beyond.
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NICMOS Problems to Watch Out For: A Checklist
- Bias and dark subtraction problems, including:
- Bars (section 4.2)
- Nonlinearity correction uncertainties
- Flatfield issues, including color dependent flat fields (section 4.4)
- Pixel defects and bad imaging regions, including:
- Effects from bright targets:
- Cosmic rays of unusual size (section 4.7)
- Scattered earthlight (section 4.8)
