7.9 Pixel Defects and Bad Imaging Regions

7.9.1 Bad Pixels

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 and propagated into the data quality (DQ) arrays of the observer's ima and flt files produced by calwf3, the flagged science pixels will still have calwf3-calculated signals and signal rates in the science (SCI) arrays. It is therefore important for observers to pay attention to the DQ arrays in their data, and use them to identify which pixels to ignore in any post-calwf3 data reduction and analysis (such as AstroDrizzle).

It is the responsibility of the observer to determine which types of bad pixels are acceptable and which are to be avoided during data analysis. Use the DQ arrays for this purpose.

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. The current default for IR channel data is for Astrodrizzle to ignore (treat as bad) all pixels with any flag except 512 and 64. The 512 flag is used to identify pixels affected by blobs, while the 64 flag is currently not attached to any type of bad pixel and not used. Observers can change the driz_sep_bits parameter within Astrodrizzle to modify this default and adjust which types of bad pixels to use or ignore.

Figure 7.9: Example of an IR Bad Pixel Mask. White pixels are flagged in the bad pixel mask.

7.9.2 Off-nominal Detector Regions

As noted in Section 5.7.7 of the WFC3 Instrument Handbook, there are several coherent features on the IR detector composed of poorly performing pixels. The “Death Star”, the large circular feature at the left bottom edge of the detector, is a collection of pixels which have extremely low QE and exhibit unstable behavior. The unbonded pixels in the upper left and right corners and along the top edge of the detector are unresponsive to illumination. 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.

7.9.3 Dead Pixels

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, about 3300 pixels are flagged as dead (0.3% of the detector’s light-sensitive pixels), 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. We recommend that observers ignore any pixel marked as dead.

7.9.4 Bad Zeroth Read Pixels

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 off-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 about 5000 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.

7.9.5 Unstable Pixels

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 and WFC3 ISR 2012-11. Unstable pixels observed on the WFC3/IR detector display a wide range of behaviors: 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 are all flagged with a value of 32 in the final bad pixel mask. There are a total of 16,500 unstable pixels (1.6% of all science pixels) on the IR detector. Due to the unpredictable behavior of these pixels, we recommend against including them in data analysis.

7.9.6 Snowballs

Curious but relatively innocuous anomalies which occasionally appear in IR observations are "snowballs". These sources have an extended, fuzzy appearance in the data. Snowballs are transient, extended sources that appear in IR channel data at rates of roughly 1.2 snowballs per hour of exposure time. The snowballs are suspected to be caused by natural radioactivity within the detector itself. Specifically, alpha particles emitted by thorium and/or uranium at ~1 ppm concentration in the detector can explain the observed characteristics of the snowballs ( WFC3 ISR 2009-44). Similar to the manner in which cosmic rays appear, the entire flux of a snowball is deposited into the detector's pixels instantaneously. A typical snowball affects about 10 pixels, depositing between 200,000 and 500,000 electrons, and saturating 2-5 pixels. Figure 7.10 shows a 7×7 mosaic of snowballs gathered from ground testing and on orbit data. With their behavior mimicking that of cosmic ray impacts, calwf3 is able to remove snowballs from WFC3/IR data during standard pipeline processing. That, combined with 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 (initial snowball characterization) and in WFC3-2015-01 (longterm characterization). In 6200 hours of exposure time (over 5 years of operation) 7400 unique snowball events were detected on the IR array. No trend in occurrence is seen.

Figure 7.10: A mosaic of snowballs generated using ground and on-orbit data.