Subject: OFSP Proposal on the use of EXTNAME & EXTCLASn keywords (v 93-Jul-21)

From: GEORGE at HEAGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Mon, 26 Jul 1993 23:39:46 -0400 (EDT)

-------------------------------------------------------
	|   THE USE OF EXTCLASn KEYWORDS WITHIN FITS FILES    |
	-------------------------------------------------------
					  Version: 1993 Jul 21

At a meeting on 1993 Jul 21 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 OFSPs previous recommendation regarding the provision of 
a hierarchical classification scheme for various types of FITS extension 
to be used within the OGIP (and potentially elsewhere). 

In view of a number of concerns expressed to the panel, the OFSP overturned
its previous recommendation, and provisionally approved an alternate
suggestion, as outlined below. Since this is an issue which remains of
interest to the general FITS community, the OFSP would welcome any comments
(via FITSBITS) on this new proposal. 

For clarity, however, before going onto the current proposal, it may be of
use to some FITSBITS readers to first summarize the recent history/evolution 
of this idea:
- A review of the use of the EXTNAME keyword (both within the OGIP and 
	elsewhere within the FITS community) presented at a meeting of 
	the OFSP on 1993 Jun 16 revealed that this keyword was being used 
	to describe a variety of different sorts of information (often 
	simultaneously). As a result a proposal was provisionally passed by 
	the OFSP whereby 2 new keywords were introduced (EXTTYPE & EXTCLASS) 
	in an attempt to decouple essential information from the EXTNAME 
	keyword, and to provide some level of hierarchy within this information.
 	This proposal was posted to FITSBITS by George on 1993 Jun 17.
- All feedback received from FITSBITS was reviewed at an OFSP meeting on 
	1993 Jun 30. Given (in the view of the panel) the lack of serious 
	objections, and the immediate needs of the OGIP imposed by currently 
	flying missions, the proposal was raised to the status of a full 
	recommendation (known as R2) of the OFSP.
- Unfortunately however, by the time of the OFSP meeting of 1993 Jul 21, a 
	number of legitimate concerns regarding this decision had been 
	expressed to the panel. The OFSP therefore overturned recommendation R2,
 	and provisionally approved an alternate suggestion, as outlined below.
	(Essentially the earlier EXTCLASS & EXTTYPE keywords are replaced by 
	EXTCLAS1 & EXTCLAS2 respectively).

The pressures imposed by currently flying missions (ASCA, CGRO & ROSAT)
maintain the need for a convention for these keywords within the OGIP. The
OFSP therefore plans to meet again in a couple of weeks to make its final
FINAL recommendations on this issue. Thus FITSBITS subscribers are urged 
to respond with any comments etc on this timescale. 

THE PROPOSAL 
************

The OFSP recommends the use of the EXTNAME keyword be changed (at least
within the OGIP), and that a family of new keywords, EXTCLASn, be introduced 
(where "n" is an integer between 1 & 9). The EXTCLASn family of keywords is 
intended to form a hierarchy in which n = 1 is the highest level, and n = 9 
the lowest level. Thus: 

EXTNAME   - Can be any string & is essentially not used by OGIP s/w
EXTCLAS1  - Is a new string describing the highest level in the hierarchy 
EXTCLAS2  - Is a new string describing the next level down the hierarchy 
			... etc ...
EXTCLAS9  - Is a new string describing the lowest level in the hierarchy 

Currently, the OGIP only requires the use of 2 hierarchical levels, and
hence the EXTCLAS1 & EXTCLAS2 keywords. Specifically the OGIP is proposing
to use: 
EXTCLAS1  - to describe the sort of dataset stored
EXTCLAS2  - to then give more specific details on exactly what type of 
		data is stored
A few examples should make the distinctions between the keywords more
obvious:

1) For various "Good Time Intervals"
   EXTNAME  = 'ABCBEF'          (or whatever you like)
   EXTCLAS1 = 'GTI'             (some sort of GTI is stored)
   EXTCLAS2 = 'STANDARD'        (the GTIs from standard processing are stored)
or EXTCLAS2 = 'ALL'             (all times when the instrument is on are stored
                                [no filtering has been applied])
or EXTCLAS2 = 'UNKNOWN'         (uncertain what is stored)

2) For various "Events" lists
   EXTNAME  = 'GHIJKL'          (or whatever you like)
   EXTCLAS1 = 'EVENTS'          (some sort of EVENTS list is stored)
   EXTCLAS2 = 'GOOD'            (accepted events [only] are stored)
or EXTCLAS2 = 'BAD'             (rejected events [only] are stored)
or EXTCLAS2 = 'ALL'             (all events [no filtering] are stored)
or EXTCLAS2 = 'UNKNOWN'         (uncertain what events are stored)

----------------------------------- END ---------------------------------- 

Ian M George 					
NASA/GSFC OFSP
1993 Jul 26