The current version of the DES FITS header data model can be found in DES-doc#3427.
The PreCam FITS headers must comply to the DES FITS header data model.
Absolutely essential FITS keywords that must be in the headers for on-sky observations are the following:
TELRA, TELDEC, TELEQUIN
TIMESYS, DATE-OBS, TIME-OBS
EXPTIME