Subject: OGIP/93-001 (Vers 1993 May 21) - The specification of unit strings

From: GEORGE at HEAGIP.GSFC.NASA.GOV (Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum))
Date: Tue, 1 Jun 1993 20:02:38 -0400 (EDT)

OGIP/93-001 (Vers 1993 May 21) - The specification of unit strings

"wlupton at keck.hawaii.edu (William Lupton)" writes (on a wide screen):
> I have been scanning the FITS literature for some definite guidance 
> on the units to use for
> angular keywords. For example, should I write RA as floating point 
> radians (as it is repres-
> ented internally), as an "hh:mm:ss.ss" character string, as floating 
> point hours, or as
> floating point degrees? In any case, the FITS remark will show the value 
> as "hh:mm:ss.ss".
> Similarly for Dec, Az, El and various offsets, where arc seconds is often 
> a possibility (or
> seconds of time of course!).

I know it doesn't completely solve his dilemma, but he and few other
subscribes may be interested to know that the HEASARC (High Energy
Astrophysics Science Archive Research Center) here at NASA/GSFC has
recently produced a new version of a memo (OGIP/93-001) which lists the
"standard" strings to specify most (all ?) physical units within FITS files
produced by the HEASARC (see below). The memo has been "round the
High-Energy circuit" via the "heafits" e-mail exploder and I think fairly
well recieved. 

The memo is heavily influenced on the IAU Style Guide (which I've just
noticed we dont actually acknowledge), though naturally probably slightly
biased towards X- & gamma-ray energies. Within the memo planar angles can
be specified in radians, degrees, arcmins or arcsec (you'll have to read
the memo to get the recommended strings :-). We purposely avoided making
many recommendations since we thought up too many "ifs" and "buts", however
it is clear that radians are the official (MKS) SI units, though most
"users" seem to prefer degrees. 

Personally, I would suggest decimal (RA) hours be avoided, and strings
never used for any keyword software may be interested in (I know the latter
are trivial to parse and convert, but why both ?) 

Anyroad, should you be interested the OGIP/93-001 memo is available via
anonymous ftp on 
	legacy.gsfc.nasa.gov (128.183.8.233) 
as 
	.caldb/docs/memos/ogip_93_001.tex	the LaTeX source
	.caldb/docs/memos/ogip_93_001.ps  	PS version of complete doc.

Constructive comments are welcomed.
						
regards
Ian M George 			(george at lheavx.gsfc.nasa.gov)
HEASARC