Working on obs_comCam I'm not sure that AssembleCcdTask covers all the cases, and I'm pretty sure that the Detector attached to the resulting Exposure is not always correct.

The cases that I know of are:

• All the science data from all amplifiers is in one image and is contiguous (e.g. DECam, SDSS)
• All the science data from all amplifiers is in one image and is not contiguous (e.g. HSC)
• The science data from different amplifiers is in different files/HDUs (LSSTCam, comCam)

Additionally, there is a config parameter doTrim of doubtful usefulness.

The first of these is a special case of the second (it allows us to omit a copy), but I think that this is an optimisation that we need not yet face. Currently I think that the IsrTask is expected to do the assembly, but it is also possible to get the butler to do this via a butler composite and an assembler that calls AssembleCcdTask, which is what obs_comCam does. If we adopt this approach for all obs packages we can write a simpler IsrTask that handles basic processing for all cameras.

Whether or not we choose this rule, the Detectors coming out of AssembleCcdTask must always be correct.