Space Telescope Science Institute
WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 6: WFC3-IR Error Sources > 6.8 Pixel Defects and Bad Imaging Regions

Various on-orbit calibration programs have been used to identify bad pixels and regions on the IR detector. All pixels found anomalous enough to potentially impact data analysis results are flagged and listed in the current bad pixel table reference file. While these pixels are listed in the bad pixel table, they will still have calwf3 calculated signals and signal rates in observer's data.
It is the responsibility of the observer to determine which flavors of bad pixels are acceptable and which are to be avoided during data analysis.
During calwf3 processing, the bad pixel table is imprinted onto the data quality (DQ) array associated with each ramp. Using the pixel values in the DQ array, observers can tailor the types of bad pixels used in their analysis. Figure 6.10 shows in white all pixels which are flagged in the bad pixel mask.
Figure 6.10: IR Bad Pixel Mask
As noted in Section 5.7.6 of the Instrument Handbook, there are several coherent features on the IR detector composed of poorly performing pixels. The “death star”, along the bottom edge of the detector, on the left side, is a collection of unresponsive pixels. Similarly, the unbonded pixels in the upper left and right corners and along the top edge of the detector are also unresponsive. These pixels have all been flagged as dead in the bad pixel mask and should be avoided during analysis. In the lower right corner of the detector is the feature known as the “wagon wheel”. This is a collection of pixels with quantum efficiencies 25% to 50% below normal. This does not mean that these pixels cannot be used during data analysis, but sources in this region will have a lower signal-to-noise ratio than they would elsewhere on the detector. This fact will be captured in the error arrays of calwf3 calibrated data. A more detailed description of these detector regions is given in WFC3 ISR 2008-28.
These are pixels with a very low quantum efficiency which measure little or no signal when illuminated. In addition to the dead pixels found through the analysis of on-orbit data, we also manually marked the pixels comprising the “death star” as dead. In total, 3,910 pixels are flagged as dead (0.4% of the detector’s light-sensitive pixels), and are marked with a 4 in the bad pixel table (see WFC3 ISR 2010-13 for details). Other than those pixels within the death star, dead pixels are scattered randomly across the detector. It is recommended that observers ignore any pixel marked as dead.
These are pixels which exhibit anomalous signals in the zeroth read of a data ramp, usually due to being shorted or unbonded (see WFC3 ISR 2003-06). This implies that many of the bad zeroth read pixels are also flagged as dead. By flagging bad zeroth read pixels in the bad pixel table, we are taking a conservative approach to bad pixel behavior. Historically, pixels with a non-nominal signal in the zeroth read displayed other non-nominal behaviors. Based on this experience, we felt it safer to flag these pixels. As with all flavors of bad pixels, observers should determine whether or not using these pixels will have a significant impact on their analysis.
In total, there are 4,990 pixels (~0.5% of the science pixels) flagged as bad in the zeroth read. These pixels are largely concentrated in the areas of the death star, the upper corners, and along the quadrant boundaries of the detector.
These pixels display an inconsistent measurement of signal in a set of nominally identical ramps. Unstable pixels are characterized more thoroughly in WFC3 ISR 2010-13. Unstable pixels observed on the WFC3-IR detector display a wide range of behaviors. Given a data set composed of many nominally-identical ramps, some unstable pixels appear stable and repeatable in almost all ramps, but will measure appreciably different signal values in only one or two ramps. Other unstable pixels display signal values that vary wildly from ramp to ramp in all observations of a data set. Pixels flagged by these searches were all flagged with a value of 32 in the final bad pixel mask. We find a total of 10,885 unstable pixels (1.06% of all science pixels) on the IR detector. Due to the unpredictable behavior of these pixels, we recommend against including them in data analysis.
Recently, a new feature was identified in ground testing data for the WFC3-IR channel. These sources have been dubbed “snowballs”, due to their extended, fuzzy appearance in the data. Snowballs are transient, extended sources with unknown origins that appear in on orbit IR channel data at rates of roughly 2 - 2.5 snowballs per hour of exposure time. Similar to the manner in which cosmic rays appear, the entire flux of a snowball is deposited into the detector's pixels instantaneously. A snowball affects between 11 and 34 pixels, and contains between 200,000 and 900,000 e. Figure 6.11 shows a 7x7 mosaic of snowballs gathered from ground testing and on orbit data. With their behavior mimicking that of cosmic ray impacts, calwf3 should be able to remove snowballs from WFC3-IR data during standard pipeline processing. That, combined with snowballs’ low rate of occurrence, implies that snowballs should have a minimal impact on science observations. Further details on snowballs can be found in WFC3 ISR 2009-43.
Figure 6.11: A mosaic of snowballs generated using ground and on-orbit data.

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