The banding effect manifests itself as the rows and columns that contain a bright source having an enhanced level of flux. This happens only in the Si:As arrays (channels 3 and 4) 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 pulldown effect.
Again, the “truth image” of the background is used to compute a robust weighted DC offset. The banding artifact is extra flux above the background and it will be subtracted out and saved into the CBCD image. TheIRAC pipeline does not model the flaring of banding towards the edges of the array. Therefore, the pipeline correction is not always perfect. More information about banding can be found in Section 7.3.2.
5.2.6 Muxstripe Correction (Channels 1 and 2)
For a very bright source, muxbleed is accompanied by a pinstripe pattern (“muxstripe”; every 4th column from the bright source is affected) that may extend over part of the image preceding or following the bright pixel (for example, see Figure 7.2 and Figure 7.3). Stars, hot pixels, and particle hits can generate muxbleed and muxstripe. In the artifact correction pipeline, a procedure was developed to mitigate the muxstripe in the image.
Figure 5.10. An image showing all four readout channel images side by side. These have been obtained by rearranging the columns in the original image. Muxbleed is apparent in the bottom right of the 4th readout channel image.
The algorithm involves converting the BCD image into 256X64 pixel arrays (each of the four readout channels into a separate image; every fourth column is read out by the same channel; see Figure 5.10). The muxstripe for one source contaminates only one readout channel, and therefore only one of these separate arrays. The median of the four arrays is created and subtracted from each array, which allows deviation from the normal background to stand out.
Figure 5.11. Subtraction of the median background from the readout channel images. This makes the muxstripe much more apparent in the 4th readout channel image (on the right).
Figure 5.12. Profiles showing the column median versus row values for identifying muxstripe. The muxstripe is now identifiable between rows 125 and 200 (significantly lower values than the median background).
From each of these median-subtracted arrays, a one-dimensional array of the values along each row is then created, simply by combining the pixel values along the X-dimension. A profile that represents the median versus row is created. For each profile, statistics arre calculated to identify any muxstripe. It will be identified as a deviation larger than 3% of the median value for several rows. This will miss the weakest muxstripe or a case where all readout channels have muxstripe in the same position (cluster of very bright stars), but this will be rare. The subset of pixels that are affected by the muxstripe is identified and this column is corrected using the difference of the median of the ‘clean’ pixels and the median of the affected pixels. The readout channel arrays are then read back out to recreate the original image, and the corrected image is written to the CBCD file. More information about muxstriping can be found in Section 7.2.2.