Subject: Proposed EXTTYPE keyword

From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 2 Jul 1993 15:55 EDT

This reply is coming somewhat late because I was away at a 
meeting for a week after the original message came out, and then had 3 
weeks' messages to catch up with when I returned.

>	-------------------------------------------------------
>	|   THE USE OF THE EXTNAME KEYWORD AND THE PROPOSED   |
>	|	   INTRODUCTION OF TWO NEW KEYWORDS	      |
>	-------------------------------------------------------

> At a meeting on 1993 Jun 16 of the OGIP FITS Standards Panel (OFSP) within
> the Office of Guest Investigators (OGIP) at NASA/GSFC, there was a review
> of the use of the EXTNAME keyword within the OGIP. 

> The OFSP found that it was common practice within the OGIP to use the value
> of the EXTNAME keyword to simultaneously describe both the generic *kind*
> of dataset being stored, and additional *details* of the dataset. Thus
> values of the EXTNAME keyword were being formed by the concatenation of 2
> or more sub-strings. This gives rise to long, and often user-unfriendly,
> EXTNAMEs essentially containing a variety of different pieces of
> information. More importantly these strings are apparently incompatible
> with a number of IRAF tasks. 

> Whilst such use of EXTNAME keyword is legal FITS, the OFSP proposes &
> recommends the adoption/use of the two keywords listed below. Since this
> is an issue likely to be of interest to the general FITS community, the
> OFSP would welcome any comments (via FITSBITS) on this proposal. 

> THE PROPOSAL 
> ************

> The OFSP recommends the use of the EXTNAME keyword be changed (at least 
> within the OGIP), and that two new keywords, EXTCLASS & EXTTYPE be 
> introduced:

> EXTNAME   - Can be any string & is essentially not used by OGIP s/w
>             (NOTE: IRAF can only cope with 6 character strings)
> EXTCLASS  - Is a new string describing the sort of dataset stored (see below)
> EXTTYPE   - Is a new string available to give more specific details on
>             exactly what type of data is stored (see below)

	The precise usage of the EXTNAME keyword has not been clearly 
spelled out in the FITS papers, and so conventions of this kind are 
useful.  However, as I have mentioned in private e-mail in previous
contexts, I do not think EXTTYPE is a good name for a keyword, because
it can possibly be a source of confusion for new users. 

	A novice user might think of the value of XTENSION, such as
TABLE or BINTABLE, as the name of the extension.  However, the EXTNAME 
keyword has a different meaning.  In order to clarify the distinction, 
the value of XTENSION has been referred to in FITS documentation as 
the extension _type_.  For example, the original generalized 
extensions paper states, "the first line of the header (normally 
SIMPLE=T) is replaced by the new keyword XTENSION='type' in order to 
identify the type of the extension."  The NOST codification has 
followed this usage, "The value field [of the XTENSION keyword] shall 
contain a character string giving the name of the extension type. "  
Thus, use of the name EXTTYPE could lead to confusion between the 
established usage of extension type, which refers to its structure -- 
e. g., ASCII table, binary table, image -- and the new definition.  I 
recommend a different name.
				Barry Schlesinger
				NOST FITS Support Office