Subject: OFSP Proposal for standard RA & dec keywords: Note of a FITSBITS post

From: [email-masked] (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Thu, 17 Jun 1993 15:45:11 -0400

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	|   PROPOSED STANDARD KEYWORDS FOR THE SPECIFICATION  |
	|	    OF RA & DEC WITHIN FITS FILES	      |
	-------------------------------------------------------

The 1st meeting of the newly-formed of the OGIP FITS Standards Panel (OFSP)
at NASA/GSFC was held on 1993 Jun 16. During this meeting the panel
reviewed the various keywords currently used to describe the RA,dec within
FITS files. 

It was found that the current scheme to be rather confusing, in some cases
ambiguous, and insufficient to meet all the needs of the OGIP. After a
review of many possible options, the OFSP has therefore recommended the
adoption & use of a (new) set of standard keywords for this purpose. 

Since this is an issue likely to be of interest to the general FITS
community, the OFSP felt the best forum for discussion of such a proposal
is the FITSBITS exploder, and thus the proposal was posted there (today).
Since some HEAFITS recipients may still not be members of the FITSBITS
(details below), a copy of the proposal is appended below. However 
it is **STRESSED** that all discussion should take place on FITSBITS.

To subscribe to FITSBITS - the international forum for general FITS format 
and standards issues:
	send a mail message to: [email-masked]
        In the body of the message, simply ask to be added to the fitsbits
        mail exploder.
It should be noted that copies of future OFSP proposals which have been 
sent to FITBITS are extremely unlikely to also be posted on HEAFITS.

For your information, the current membership OFSP is
	Bill Pence (chair)
	Mike Corcoran (ROSAT)
	Richard Fink (ASCA)
	Ian George (HEASARC)
	Tom McGlynn (GRO)
	Arnold Rots (XTE)
	Lorella Angelini (HEASARC)

----------------------- Start of included message ---------------------------
----------------------- (as exploded on FITSBITS) ---------------------------

	-------------------------------------------------------
	|   PROPOSED STANDARD KEYWORDS FOR THE SPECIFICATION  |
	|	    OF RA & DEC WITHIN FITS FILES	      |
	-------------------------------------------------------
					  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 various keywords currently used to describe the RA,dec within FITS 
files.

The OFSP found the current scheme (at least as often used within the
High-Energy community) to be rather confusing, in some cases ambiguous, and
insufficient to meet all the needs of the OGIP. After a review of many
possible options, the OFSP therefore proposes & recommends the adoption &
use of the (new) 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. 

It should be noted that this proposal is restricted to FITS extensions
containing data from a single 'pointing' (ie EXCLUDES extensions containing
datasets from instruments capable of simultaneously observing in more than
one direction, and from scanning experiments).

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 FITBITS subscribers are urged to response with any
comments etc on this timescale. 

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

1) Reference Frame keywords
---------------------------

The OFSP recommends both the following keywords are mandatory if one or
more pairs of keywords specifying RA & dec (as defined in 2) are given. 
	
RADECSYS 	- a string denoting the stellar reference frame in use
		  e.g. values: 'FK4','FK5' etc
EQUINOX		- a real giving the date of the equinox in decimal years (AD)
		  e.g. values 2000.0, 1950.0 etc

2) The RA & dec coordinates
---------------------------

In all cases listed below, the values of keywords are reals expressed
in decimal DEGREES. Obviously only those keyword pairs considered necessary 
need be specified. However, the values of ALL such keyword pairs MUST be 
given in the same reference frame as specified by the values of the RADECSYS 
& EQUINOX keywords (see 1).

a) The Positions of astronomical objects/sources

   The OFSP recommends the position of a source is given using the keywords:

	RA_OBJ
	DEC_OBJ		

b) The Pointing Direction of the Instrument.

   The OFSP recommends the pointing direction of the instrument is given 
   using the keywords:

	RA_PNT	
	DEC_PNT

   where, 
   - in the case of imaging instruments, the values of these keywords 
        give the direction of the optical axis of the instrument (after 
	all borsighting corrections etc have been applied).
   - in the case of non-imaging instrumentation, the values of these 
	keywords give the direction of some other (instrument-specific)
	vector. It is anticipated that in most cases this will be 
	the direction of maximum instrument sensitivity.
   For instruments with <100% stable pointing accuracy, the above pair 
   of keywords should be the mean values during the observation.	

c) The orientation of the Spacecraft

   The OFSP recommends the orientation of the spacecraft (or telescope 
   platform) is given by the specification of any/all of the following 
   keyword pairs:

	RA_SCX
	DEC_SCX

	RA_SCY
	DEC_SCY

	RA_SCZ	
	DEC_SCZ

   giving the orientation of the spacecraft X-, Y- & Z-axes respectively.
   For instruments with <100% stable pointing accuracy, the above pairs 
   of keywords should be the mean values during the observation.	

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

Ian M George 					
NASA/GSFC OFSP
1993 Jun 17