Subject: Standard RA & Dec keywords (OFSP recommendation R3)

From: GEORGE at HEAGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Fri, 30 Jul 1993 15:08:36 -0400 (EDT)

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	|   PROPOSED STANDARD KEYWORDS FOR THE SPECIFICATION  |
	|	    OF RA & DEC WITHIN FITS FILES	      |
	-------------------------------------------------------
					  Version: 1993 Jul 27

At a meeting on 1993 Jun 30 of the OGIP FITS Standards Panel (OFSP) within
the Office of Guest Investigators (OGIP) at NASA/GSFC, there was a review
of a preliminary recommendation (see FITSBITS post by George 1993 Jun 17)
to standardize the keywords used to specify RA & DEC within (at least
OGIP-produced) FITS files. 

All comments received from the community via FITSBITS were reviewed:
- Most of the comments questioned the need for these new keywords,
suggesting instead that the standard World Coordinate System (WCS) FITS
keywords already provide this information.  However, the Panel agreed that
this was not the case, and that the proposed new RA and DEC keywords supply
additional information and that they can also be used in cases where the
WCS keywords would be inappropriate (as in some Binary table extensions). 
- The Panel accepted a suggestion by Pat Wallace (FITSBITS post 1993 Jun 22)
to clarify the wording of the EQUINOX keyword, and further, to recommend
that the EQUINOX always be either 1950.0 or 2000.0, with 2000.0 being
strongly recommended for all new data sets. 

With these minor changes, the OFSP then raised the proposal to the status
of a "full recommendation" (R3). The full text of this recommendation is
included below for convenience. 

			*** OFSP RECOMMENDATION R3 ***
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On standard RA & Dec keywords giving the position of the observed object, 
the telescope pointing, and the spacecraft axes.
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1) Reference Frame keywords
---------------------------

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 (in decimal years)
		  of the Besselian epoch or Julian epoch appropriate 
		  for the specified RADECSYS.
		  It is strongly recommended that the value of this 
		  keyword is restricted to be either 
		    EQUINOX = 1950.0 (for RADECSYS = 'FK4'), or
		    EQUINOX = 2000.0 (for RADECSYS = 'FK5').
		  with the latter strongly recommended for all new datasets.

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 position of a source is given using the keywords:

	RA_OBJ
	DEC_OBJ		

   These values will usually correspond to the target specified by the 
   OBJECT keyword.

b) The Pointing Direction of the Instrument.

   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 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 Jul 27