Originally every PSF assessor has its own offset. During the changes of ticket AST-960 it was discovered the PSFs for the different channels need to have the same PSF. This means the PSF offsets can be stored in the WorkTable.
When no direction dependent PSF is used there's no need to store any offset entries in the WorkTable.
Without this change it's quite hard to implement AST-960. The image accessors of the PSF are converted to images in Radler's Perform step. This meant the PSF offsets needed to be "attached" to the images, by storing them in the WorkTable they can be passed as a function argument.