Non-standard keywords

Besides the standard FITS keywords defined in the FITS documents, it is possible to use other, non-standard keywords. Such non-standard keywords have no global definition and may be interpreted differently by different FITS readers.

A number of non-standard keywords has been used by ESO e.g. for definition of frames acquired at the La Silla observatory. They are given in Table 7.1.3 and should not be used with other definitions. An extended set of FITS keywords are defined for describing data from the VLT (see [7]).

 

Table 7.4: Name translation between special non-standard FITS keywords and MIDAS descriptors.

FITS keyword	Data type	MIDAS descriptor	Remarks
MJD-OBJ	Real	O_TIME(4)	Modified Julian Date of start of exposure
TM-START	Real	O_TIME(5)	UT of start of exposure, in sec. after midnight
TM-END	Real	-	UT of end of exposure
EXPTIME	Real	O_TIME(7)	Exposure time in sec.
RA	Real	O_POS(1)	Right Ascension in degrees
DEC	Real	O_POS(2)	Declination in degrees
POSTN-RA	Real	O_POS(1)	Right Ascension in degrees
POSTN-DE	Real	O_POS(2)	Declination in degrees
AIRMASS	Real	O_AIRM	Mean airmass of exposure
FILENAME	String	FILENAME	Original file name
ESO-LOG	String	ESO-LOG	Observing log

 

Due to the freedom to use non-standard keywords it is important to know their exact definitions when transferring data between different systems.