IRAC Instrument Handbook - IRSA - California Institute of Technology

7 Data Features and Artifacts
IRAC Instrument Handbook
The common artifacts in IRAC data are discussed in this chapter. Most of these have been mitigated by
the pipeline processing, 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. Stray light from point sources should be masked by hand.
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 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 one solar flare during the In-Orbit Checkout, and
one in Nominal Operations. In the latter event, several hours of science data were rendered useless
because of the large number of hits in the images. 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 flatfields 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, 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 datasets 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.
7.1 Darks, Flats and Bad Pixels
The true median dark currents, due to nonzero leakage resistance or recombination in reverse-biased
detector diodes, are very small compared to the current from the background at the darkest part of the
celestial sphere. Labdarks, which were measured with the cold IRAC shutter closed, with zero photon
flux, are not zero, and have significant pixel-dependent offsets, usually positive, that depend on the frame
time and the Fowler number, as well as the history of readouts and array idling over the previous several
hours. Channel 3 is by far the most extreme case, in which, for example, a 100 second (Fowler-16) frame
can be offset as much as 370 DN (median), or the equivalent of 1400 electrons at the integrating node,
Data Features and Artifacts 103 Darks, Flats and Bad Pixels