4.4. KEYWORDS

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ORIGIN Keyword  The value field shall contain a character string identifying the or-

ganization or institution responsible for creating the FITS file.

EXTEND Keyword  The value field shall contain a logical value indicating whether the

FITS file is allowed to contain conforming extensions following the primary HDU. This

keyword may only appear in the primary header and must not appear in an extension

header. If the value field is T then there may be conforming extensions in the FITS file

following the primary HDU. This keyword is only advisory, so its presence with a value

T does not require that the FITS file contains extensions, nor does the absence of this

keyword necessarily imply that the file does not contain extensions. Earlier versions of

this standard stated that the EXTEND keyword must be present in the primary header if

the file contained extensions, but this is no longer required.

BLOCKED Keyword  This keyword is deprecated and should not be used in new FITS

files. It is reserved primarily to prevent its use with other meanings. As previously

defined, this keyword, if used, was required to appear only within the first 36 keywords

in the primary header. Its presence with the required logical value of T advised that the

physical block size of the FITS file on which it appears may be an integral multiple of

the FITS block length and not necessarily equal to it.

4.4.2.2

Keywords Describing Observations

DATE-OBS Keyword  The format of the value field for DATE-OBS keywords shall follow

the prescriptions for the DATE keyword (§4.4.2.1). Either the 4-digit year format or the

2-digit year format may be used for observation dates from 1900 through 1999 although

the 4-digit format is recommended.

When the format with a four-digit year is used, the default interpretations for time

should be UTC for dates beginning 1972-01-01 and UT before. Other date and time

scales are permissible. The value of the DATE-OBS keyword shall be expressed in the

principal time system or time scale of the HDU to which it belongs; if there is any chance

of ambiguity, the choice should be clarified in comments. The value of DATE-OBS shall be

assumed to refer to the start of an observation, unless another interpretation is clearly

explained in the comment field. Explicit specification of the time scale is recommended.

By default, times for TAI and times that run simultaneously with TAI, e.,g., UTC and

TT, will be assumed to be as measured at the detector (or, in practical cases, at the

observatory). For coordinate times such as TCG, TCB and TDB, the default shall be to

include light-time corrections to the associated spatial origin, namely the geocenter for

TCG and the solar-system barycenter for the other two. Conventions may be developed

that use other time systems. Appendix B of this document contains the appendix to

the agreement on a four digit year, which discusses time systems in some detail.

FITS Standard