Subject: OFSP Proposal for standard RA & dec keywords (v 93-Jun-17)

From: GEORGE at ROSGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Thu, 17 Jun 1993 14:44:11 -0400 (EDT)

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	|   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

---

From: Lucio Chiappetti <lucio at ifctr.mi.cnr.it>
Date: Fri, 18 Jun 1993 09:43:01 +0200 (MET DST)

My general feeling is that it is a very reasonable proposal, in the
  framework of high energy missions at least.
> c) The orientation of the Spacecraft

  I agree with the above list of items, which is more or less what I
  was using in the past (non-FITS) for the description of a pointing.
  
  Misalignments etc. should in principle be accounted for as "calibration"
  data. However I wonder if there are any relationships or interferences
  between things like a misalignment matrix and the WCS stuff. I do not
  have time to look into the matter, but I would appreciate knowing if
  anybody has done / will do a more detailed analysis of this.   

  I still expect that each mission will need to do its own bits of
  trigonometry etc. to do things like computing collimator transmissions
  or vignetting or whatever. But it is much better to describe those
  mission-specific calculation using a common nomenclature, which is
  what is proposed here, therefore I support it.

> in decimal DEGREES. Obviously only those keyword pairs considered necessary 
   That's fine for me.
   However I remember there was some discussion in the past about preferring
   radians to degrees. How did that go ?
   
> 	DEC_OBJ		

      You mean the target here, don't you ?
      I.e. the object named in the OBJECT keyword. If yes OK for me.
      (if there are many interesting objects in the FOV one might think
      to RA_OBJ1, RA_OBJ2 etc. but I am personally not enthusiastic with
      number-indexed keywords unless REALLY necessary)

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