The common artifacts in IRAC data are discussed in this chapter. Most of these have been mitigated by the pipeline processing (and we cross-reference the relevant section in Chapter 5 when one exists), which produces artifact-corrected images (CBCDs). Further mitigation is often possible by a judicious quality inspection of the data, and/or further processing of the BCDs. Note that many of these artifacts are quite commonly seen in IRAC images.
The most common artifacts are as follows. Persistent images usually come from a bright source observed as part of the observation. In some cases, however, persistent images from a preceding observation may be found. One way to check this is by inspecting a median of all the images in an observation (AOR). Another possible flaw in the observations would be an exceptionally high radiation dosage. The nominal rate is approximately 1.5 hits per array per second, and the radiation hits range from single pixels to connected streams (and occasionally small clouds of secondaries). High particle hit rates occurred following high intensity solar flares. There were four such events during IRAC operations. Typically, several hours of science data were rendered useless because of the large number of hits in the images. These science data were rescheduled for observation at a later time, so no science data were lost. Objects that are bright enough leave muxbleed trails and can generate pinstripe patterns over large parts of the image, and offsets along the columns and rows containing the bright source. Ghosts from internal reflections within the filters can be seen in almost every channel 1 or 2 BCD, and more ghosts in all channels are noticeable from bright objects.
We begin with a discussion of the basic characteristics of the dark frames and flat-fields that affect every image. We follow with a discussion of electronic artifacts. These effects arise from the inherent nonlinearity of the detector diodes and saturation of either the detector well, transistors in the mux (multiplexer; we use “mux” as a casual term for the ROIC, which selects four pixels at a time and puts their signals on the four outputs of the array), or the analog-to-digital converter (ADC) in the warm electronics; crosstalk within the mux or warm electronics; or from inductive coupling to currents in spacecraft cables. Most electronic effects have a short persistence, but image persistence, which is also nonlinear in photon fluence, can last seconds, minutes, hours, or even weeks. Next, we have a section on optical artifacts, which include stray light or ghosts from sources within or outside the FOV. Finally, we discuss the effects of cosmic rays and solar protons on IRAC observations. Please note that asteroids may be “contaminants” in the data as well, especially when the target is close to the ecliptic plane. Asteroids can most effectively be rejected from data sets that have been taken at least several hours apart, so that the asteroids have moved in the data and can be masked out by temporal outlier rejection routines.
