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 8.0 µm channels), 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 7.18: 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 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 µm, 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 concentrate the scattered light along the rows and columns, causing the optical banding. The pattern is due to interference that depends on wavelength and the spatial extent of the source at each wavelength. The banding/scattering pattern does not vary much for point sources with a continuous spectrum, but a narrow-band source has a complex banding/scattering pattern.
Users should be aware of the uncertainties resulting from banding, specifically when attempting measurements of faint sources near the affected rows or columns. For bright sources with significant banding, aperture photometry may not be successful, and it would be better to measure these sources using frames of shorter exposure times. Users are encouraged to experiment with image restoration techniques of their choice. Algorithms similar to the pull-down corrector may have some effectiveness in mitigating banding. The pipeline-performed banding correction is discussed in Section 5.2.5.
