Subject: OFSP Proposal on the use of EXTNAME,EXTCLASS & EXTTYPE keywords (v 93-Jun-17)

From: GEORGE at HEAGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Thu, 17 Jun 1993 17:37:49 -0400 (EDT)

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	|   THE USE OF THE EXTNAME KEYWORD AND THE PROPOSED   |
	|	   INTRODUCTION OF TWO NEW KEYWORDS	      |
	-------------------------------------------------------
					  Version: 1993 June 17

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 concatination 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. An example is:
   EXTNAME  = 'GTI_STANDARD'
meaning the dataset is a list of all 'standard' "Good Time Intervals". An 
(admittedly extreme) example is: 
   EXTNAME  = 'SGL_SPEC_STAT_+UNC' 
meaning the dataset contains the positive statistical uncertainties 
on a single spectrum.

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. 

Furthermore due to the pressures imposed by currently flying missions
(ASCA, CGRO & ROSAT) there is an extremely urgent need for a convention for
these keywords within the OGIP (& High-Energy community in general). The
OFSP therefore plans to meet again in a couple of weeks to make its final
recommendations. Thus BITFITS subscribers are urged to response 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 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)

A few examples should make the distinctions between the keywords more
obvious:

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

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

The two new keywords follow a logical hierachy, solve all the current
problems, and seem general enough to give us (the OGIP, and general FITS 
community) flexibility in the future. 

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

Ian M George 					
NASA/GSFC OFSP
1993 Jun 17

---

From: Lucio Chiappetti <lucio at ifctr.mi.cnr.it>
Date: Fri, 18 Jun 1993 10:12:10 +0200 (MET DST)

I am really perplexed by this proposal, and I fail to see a clear
  distinction between what is used for inter-package data exchange (i.e.
  to make the file readable, or at least understandable to a "foreign"
  package) and what is used exclusively within a GIVEN package.

  My approach would be to use EXTNAME to classify in a broad sense the
  kind of file. In particular in the high energy astrophysics context
  the "kinds of files" I am using are :

     'SPECTRUM'               count/s (or photon/s) spectra
     'RATE'                   time profile (anything vs time)
     'PHOTON LIST'            list of photons or events

      in addition to these I consider images and response matrices
      (but in my view they are images, not bintables, therefore do
      not use EXTNAME)

  If each one of those supports variations, these are either in the
  form (e.g. number and names of columns; and therefore are self-docu-
  mented by other standard keywords like TTYPEn TFORMn etc.), or are
  in the semantics (which is relevant within a particular package
  only, and can be described by whatever optional keyword).

  Being my primary interest in the possibility of EXCHANGING data
  (i.e. importing/exporting them from/to my own package to/from other
  packages) I would as far as possible avoid the use of "specialized"
  convention, which may force to have a dedicated conversion utility
  for each package, instead of a generic reader. 

  As a conclusion, I do not see why you cannot use EXTNAME (instead
  of EXTCLASS) for the top-level classification (to be kept to a FEW
  classes, essentially the ones I listed above ... the exact names 
  can be agreed if you are concerned with imbedded blanks, or more 
  than 6 or 8 characters), and whatever YOU like for the bottom level
  classification. 
  Still, I perceive that the top-level classification (EXTNAME) is
  of relevance within the X-ray (HEAFITS) community (but not necessarily
  within a larger - fitsbits - community), while the lower level
  classification is only "private" to your software.

---

From: pence at tetra.gsfc.nasa.gov (William Pence)
Date: Mon, 21 Jun 93 17:59:22 EDT

>     'PHOTON LIST'            list of photons or events

I think many of us would have liked to use EXTNAME for this purpose, but
the EXTNAME keyword is already being used by different groups in 
significantly different ways.  Some use it as a unique file name,
others use it as a very long discriptive string.  So it appears too
late to try to propose a standard convention for this keyword because
it would be expensive for some of these groups to revise their software.
Plus, there are already many FITS files in data archives that would not
conform to this new restricted usage of the EXTNAME keyword.
So, it seems more practical to invent a new keyword (EXTCLASS) for this
purpose. 

-Bill Pence