The bandwidth effect appears in IRAC channels 3 and 4 (5.8 and 8.0 μm). It looks like a decaying trail of pixels 4, 8, and 12 columns to the right of a bright or saturated spot. Only in the most highly saturated cases is the effect visible 12 columns to the right. It is not corrected in the pipeline. A typical case for a star is shown in Figure 7.5. The principal cause is that the IRAC impurity band conduction (IBC) arrays were operated with insufficient current to raise the voltage on the output bus in the ROIC from a very bright pixel and settle on a dark pixel within 10 or even 20 microseconds. The small signal delays from driving the signal cable and from the lowpass filters in the preamps also come into play. And the slew rate is different in each of the four outputs of each array. The bandwidth effect is nonlinear. A rare case which gives rise to a bizarre image is shown in Figure 7.6. Here, an extremely bright star saturates an area in the last four columns. The bandwidth effect appears in the first 12 columns of the succeeding rows, making it appear as if the right edge of the image was cut and pasted onto the left side of the image.
Figure 7.6: The bandwidth effect when a bright object is in the last four columns. IRC+10216, strongly saturated, is just off the right side of the channel 3 array. Even the filter ghost is saturated. The bandwidth effect appears on the left side of the array. These data were taken from PID = 124, AORKEY 5033216.
7.2.4 Column Pull-Down/Pull-Up
When a bright star or cosmic ray on the array reaches a level of approximately 35000 DN, there is a change in the intensity of the column in which the signal is found. In channels 1 and 2, the intensity is often reduced throughout the column (thus the term “column pull-down”); see Figure 7.7. Column pull-up is caused in a similar manner, but the bias difference between the pulled-up column and its surrounding columns is positive rather than negative. When the effect occurs, it shifts the intensities of the pixels above and below the position of the responsible source, within the same column. This effect is limited to the brightest sources. The amplitude of the column pull-down does not scale linearly with the flux of the source or the brightest pixel. The effect appears to be constant on either side of the source and algorithms which fit separate DC offsets above and below the source should be effective. Cosmetic corrections are partially successful. One, provided by the GOODS Legacy team and available at the Spitzer Contributed Software web pages at IRSA, takes the median of each column, identifies columns that deviate from the local average by more than some threshold, and then adds back in a constant to the apparently affected columns. The code does not work in fields with extended emission. A more general algorithm which estimates the “true” sky value for the affected pixels and fits DC offsets is also available for observations of more structured emission. This algorithm is implemented in the BCD pipeline (see Section 5.2.4).
Figure 7.7: IRAC channel 1 (left) and channel 2 (right) observations of a crowded field with column pull-down apparent from the brightest sources. Note that the brighter sources affect a larger number of columns. These data were taken from PID=613, AORKEY 6801408.