When processing data from the community pipeline instcal files, the ccdnum that the butler uses, and which is used to create a detector object, is not the same as the ccd number specified in the original image headers. For example,
But if I check the headers, I get
where S3-164_107419-16-2 is the ID for the detector S23 aka ccdnum=11 (table of detector IDs).
The original source of this confusion seems to be in the decam policy entry for instcal, which specifies:
compared to the raw format, which is
It is likely necessary for instcal ingest to duplicate the behavior of the raw file ingest of MEF files, to prevent this conflation of HDU numbers with CCD Ids.