Subject: OFSP Proposal for a HDUCLASn hierarchy of keywords (v 93-Aug-08)

From: GEORGE at HEAGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Sat, 7 Aug 1993 9:55:09 -0400 (EDT)

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	|   THE USE OF HDUCLASn KEYWORDS WITHIN FITS FILES    |
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					  Version: 1993 Aug 06

(Formerly, "THE USE OF EXTCLASn KEYWORDS WITHIN FITS FILES")

At a meeting on 1993 Aug 04 of the OGIP FITS Standards Panel (OFSP) within
the Office of Guest Investigators (OGIP) at NASA/GSFC, there was a review
of the OFSPs previous proposal for the provision of a hierarchical
classification scheme for various types of FITS extension to be used within
the OGIP (and potentially elsewhere). 

As a result of a comment received from Maureen Conroy (CfA), the previous
proposal (as posted to FITSBITS by George 93-jul-26) has been revised such
that the proposed hierarchy is now achieved using HDUCLASn keywords (rather
than the previous suggestion of EXTCLASn). The current proposal is included
below in full. As ever, the OFSP would welcome any comments (via FITSBITS)
on this new proposal. 

====================== Current Proposal in Full ============================

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	|   THE USE OF HDUCLASn KEYWORDS WITHIN FITS FILES    |
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					  Version: 1993 Aug 06

Introduction
------------
We consider there to be a common need for a set of keywords which 
precisely define the scientific 'type', 'sub-type', 'sub-sub-type'..etc 
of the data contained within a given FITS extension (including the 
Primary Array). It should be stressed that by scientific 'type' we 
are referring to whether a given extension contains (say) a 'light curve',
a 'PHA spectrum', a 'background map', and possibly sub-classes of these.
We are NOT explicitly referring to how that data is actually stored internally 
within the FITS extension.

As far as we are aware, no existing FITS keywords are commonly used for
SOLELY for this purpose, or (can easily) provide the necessary & desirable
number of hierarchical levels (at least for our intended usage). Indeed, we
are aware that a variety of existing keywords have been employed for
similar purposes within the community (specifically EXTNAME), but often
using mutually incompatible conventions. Here we therefore propose a family
of new keywords which forms such a hierarchy of descriptive strings
EXPLICITLY for this usage. We believe these new keywords compliment &
clarify the use of existing FITS keywords, and are likely to be of general
use throughout the FITS community. 

The Proposal
------------
We propose that the scientific 'type', 'sub-type', 'sub-sub-type'..etc 
of the data contained within a given FITS extension be specified using
a set of keywords of the form HDUCLASn (standing for Header-Data Unit Class), 
where "n" is an integer between 1 & 9. These keywords should be considered 
hierarchical with n = 1 as the highest level, and n = 9 the lowest level. 

Usage
-----
The need & use of such keywords can be illustrated as follows. The input to
a specific application task is a FITS file consisting of a large number of
extensions. The application requires access to a particular type of 
(scientific) data -- lets say a "heavy particle background map" for a given 
observation using a given detector. Under the proposal, an extension 
containing such data would contain (say):
   HDUCLAS1 = 'BKGDMAP'
   HDUCLAS2 = 'PARTICLE'            
   HDUCLAS3 = 'HEAVY'            
and the application would thus first search through the file looking 
for HDUCLASn keywords with these values. If found, the application can then 
check for appropriate values of OBS-DATE, TELESCOP, INSTRUME etc. 
[Of course for generality/safety, the application might also want to continue
checking the remainder of the input file for additional extensions
fulfilling the above criteria. If additional extensions are indeed found,
then the application may, for example, prompt the user for which they wish
to use supplying information from other keywords etc] 
Once the required extension has been identified, the data can obviously be 
accessed. 

Advantages
----------
We consider the advantages of these new keywords over the use existing 
keywords (in particular the EXTNAME keyword) for this purpose to be 
as follows:
1) The new keywords are explicitly designed for this purpose, and thus 
	will not impact the ways in which other keywords are currently used.
2) The new keywords are equally appropriate for use with the Primary Array
	or an extension. 
3) The hierarchical nature of the information is explicit, and allows 
	smaller, user-friendly strings to be employed

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Ian M George 					
NASA/GSFC OFSP
1993 Aug 08