IRAC Instrument Handbook - IRSA - California Institute of Technology

7.3.2 Optical Banding and Internal Scattering
IRAC Instrument Handbook
The banding effect manifests itself as the rows and columns that contain a bright source having an
enhanced level of brightness. This happens only in the Si:As arrays and has been shown to be due to
internal optical scattering (inside the array). Both bright stellar sources and bright extended sources cause
banding. It is clearly different from the optical diffraction patterns and the column pull-down effect. The
SSC pipeline corrects for banding, but it does not model the flaring of banding towards the edges of the
array. Therefore, the pipeline correction is not always perfect.
Banding only appears in IRAC channels 3 and 4 (5.8 and 8 micron bands), and it is stronger in channel 3.
Banding probably occurs at all intensity levels, but only appears obvious around bright sources that are at
or near saturation levels. Banding is seen both in row and column directions, though their relative
intensities are somewhat different. In addition, there is an electronic effect. Channel 4 has a strong row
pull-up, and channel 3 has a weak column pull-up. The column pull-up is uniform across the row where
the source is bright. The optical banding intensity falls off with distance from the bright spot. Cosmic ray
hits cause electronic banding, but not optical banding.
Figure 7.15: Typical image sections showing the banding effect. These are channel 3 (left) and channel 4
(right) images of the same object (S140), adopted from a report by R. Gutermuth. These data were taken
from program pid 1046, AORKEY 6624768.
The optical banding is only an enhancement of the optical scattering in channels 3 and 4 near the row and
column where the source is. Approximately 25% of the light incident from a point source is scattered
throughout the channel 3 array. The detected scattered light falls with distance from the source. Channel 4
has the same problem to a smaller degree. Laboratory tests have confirmed the large amount of optical
scattering within the Si:As arrays. At wavelengths shorter than about 10 microns, the Si:As in the channel
3 and 4 arrays is not opaque, and most of the incident photons, especially in channel 3, reach the front
surface of the detector chip, where they are diffracted by the rectilinear grid of conductive pads. Many are
diffracted into high angles and are multiply-reflected within the detector chip, and some can travel fully
across the array before being absorbed (and detected). Other photons can pass through the detector chip
and be scattered back into the detector chip where they are detected. The interference pattern tends to
Data Features and Artifacts 125 Optical Artifacts