The following list of keywords is required for metadata conributions that are provided. Note that in the case of Spitzer, FILTER or some other combination of keywords (e.g. INSTRUME + CHNLNUM) can be used.

Header Keyword	Definition
BUNIT	a character string describing the physical units in which the quantities in the array are expressed
INSTRUME	a character string identifying the instrument used to acquire the data associated with the header
TELESCOP	a character string identifying the telescope used to acquire the data, chosen from a list of facility keywords provided at this link for reference
FITS standard WCS keywords

While the above keywords are needed, the following ones are optional:

Other Keyword	Definition
FILTER	name of filter used, if applicable
OBJECT	a character string giving a name for the object observed
PSFSIZE	the typical spatial extent of point-spread function, in arcsec (optional)
TIMESYS	a character-string code for the time scale of the time-related keywords. Recommended values with well-defined meanings are avilable, but other values may be used. If this keyword is absent, 'UTC' must be assumed.
TIMEUNIT	a character string that specifies the time unit; the value should be one of 's' (seconds, default), 'd' (day), 'a' (year), 'cy' (century)

There are additional optional keywords to be used along with either TIMESYS or TIMEUNIT in the table below, though one also relates to a HDU (Header and Data Unit):

Related Keyword	Other Keyword	Definition
TIMESYS	DATE-BEG OR MJD-BEG	a character string in ISO-8601 format that defines the start time of data acquisition in the specified time system
		the value of the MJD start time of data acquisition in the specified time system
TIMESYS	DATE-END OR MJD-END	a character string in ISO-8601 format that defines the stop time of data acquisition in the specified time system
		the value of the MJD stop time of data acquisition in the specified time system
TIMEUNIT	XPOSURE OR TELAPSE OR EXPTIME	the value for the effective exposure duration for the data in specified units, corrected for dead time and lost time. If the HDU contains multiple time slices, it's the total accumulated exposure time over all slices.
		the value for the amount of time, in specified units, elapsed between the start and end of the observation or data stream
		the value for the exposure time of the observation in specified units. The exact definition is mission dependent and may include corrections for shutter open and close duration, detector dead time, vignetting, or other effects.