Subject: Re: DATE-OBS='31/12/99'

From: bschlesinger at nssdca.gsfc.nasa.gov (Barry M. Schlesinger)
Date: 10 Jul 1996 13:23 EDT

In article <slrn4u5kfa.nm1.davis at aluche.mit.edu>, davis at space.mit.edu writes...
>On 3 Jul 1996 09:58:08 -0400, William Pence <pence at tetra.gsfc.nasa.gov>
>wrote:
> : ... the following examples would be legal:
> : 
> :      DATE-OBS= '24/06/98' 
> :      DATE-OBS= '2010-02-13T13:53:10.3Z' 
> :      DATE-OBS= '2010-02-13'
> 
>If the statement,
> 
>   ``Reading the data values from a FITS file should not require
>   decoding any more than the first eight characters of a character
>   string value of a keyword.''
> 
>from NOST 100-1.0, section 5.3.2.1 on character string formats applies
>then there could be trouble.  Perhaps this statement should be changed
>to ... specify that ALL characters with the
>exception of trailing whitespace of a character string are meaningful.

This statement addresses values of keywords that are needed by the 
software to actually read the data of HDU in the FITS file, that is,
the actual process of parsing the data, as opposed to keywords that
are used to provide the scientific context.  The User's Guide
expresses the idea in the following way, 

`` If the reading program requires the value of the keyword to 
interpret the data properly, the information should be in the first 
eight characters.... For purely descriptive material it is not so 
critical to have the meaning clear from the first eight characters....''

As the DATE-OBS keyword is used for understanding the content, not for 
instructing the program on how to read the data, the eight-character 
rule does not apply.

The eight character rule stemmed from some limitations of hardware 
and/or software.  Every once in a while I still see notes about some 
package that has a problem with longer strings.

				Barry Schlesinger
				FITS Support Office
				GSFC/ADF