The mosaicking procedure first includes the projection of BCDs onto a common frame. The projection is done using a linear interpolation (other current options are drizzle and grid, and there are others in development) and takes into account the distortion corrections. The initial projection uses pixel masks for each BCD produced by the pipeline. Remaining pixels affected by cosmic rays that were not detected during the pipeline processing can be flagged using a multiframe spatial-temporal outlier detection (other options include single frame and multiframe spatial). MOPEX keeps track of these outliers in a ''rejection mask'' (*brmsk.fits) and these masks are used when the images are re-projected and mosaicked. (For the specific mosaicking parameters used when running MOPEX as part of the automatic pipeline at the SSC, please see the IRSA website.) The final mosaic is oriented in the file such that the size of the image is minimized; it is generally maximized in the y coordinate, and you can really see this in long scan maps. This means that the mosaics are rotated in order to make the scan direction roughly along the y-axis.
Jailbars (see section 7.1.2 below) can be seen in some of the MIPS-24 BCD frames, particularly at the beginning of a scan leg or in frames with saturated sources. By superposition, some of these jailbars are removed in the outlier rejection step of the mosaic construction, but sometimes they persist. The frames can be corrected by applying an additive correction to the affected columns (or portions of columns) at the BCD level, or you may choose to exclude the affected frames entirely from the mosaic construction.