Subject: ORIGIN and CREATOR keywords

From: forveill at gag.observ-gr.fr (Thierry Forveille)
Date: Wed, 27 Oct 1993 18:24:58 +0100

My impression is that the definition of ORIGIN as the organisation writing
the file dates back to days when there was to a large extent a one-to-one 
mapping between institutions/telescopes and data reduction programs. With
fewer packages spreading to more places, the meaning probably shifted to
follow the usefull information. I cannot come up with an example where
the place a file was written at is relevant, rather than the program (with
its control parameters if any). A FITS file written at ESO la Silla could as
well be a theoretical simulation as the output of any telescope there...
I would thus we keep ORIGIN to mean the writing program/package (and 
accordingly adjust the standard) unless somebody uses it with its initial
definition.

It is on the other hand unfortunately not true that the writing program is
irrelevant when decoding a FITS file. FITS is mostly a grammar, and only
to a very limited extent a vocabulary. It is still quite common to see
images with no axis description (thus implicitely CRVALi=0,CDELTi=1,CRPIXi=0)
storing the RA and Dec of the central pixel as something like RA-POINT
(or RA_PNTG, RA-OBS, RA_TELES....), plus the focal scale and orientation
of the pixel array. They are perfectly valid FITS files and the axis 
information is there, though arguably not in a very convenient shape.

The 8 character limit as worded in the Standard does mean that a FITS reader
is at present only requested to read the first 8 characters. I have fortunately
never encountered any program that enforces this limitation, but I do
agree that it should be suppressed as soon as feasible.

	Thierry Forveille

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From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 28 Oct 1993 09:27 EDT

In article <9310271725.AA04437 at fits.cv.nrao.edu>, forveill at gag.observ-gr.fr (Thierry Forveille) writes...
>follow the usefull information. 

FITS Paper I defines ORIGIN to be "Tape writing institution."  That 
meaning is the one that has been intended all along. 

>I cannot come up with an example where
>its control parameters if any). 

One usage, touched on by Lucio Chiappetti, is that when particular
institutions or groups have defined keywords to have particular
meanings, and the same keyword may be used in different ways by
different organizations, the ORIGIN keyword identifies whose
conventions are being used. 

>accordingly adjust the standard) unless somebody uses it with its initial

We would then have a keyword with an ambiguous definition.  And we
can't *change* the meaning because doing so would cause existing FITS
files to be out of conformance.  Any existing usage of ORIGIN with a
meaning other than that defined in the first FITS paper is simply not
in conformance with standard FITS.  If we are going to have a keyword 
to identify the software that produced a file, it should be a new 
keyword; we should not redefine an existing one.

>information is there, though arguably not in a very convenient shape.

I would discourage the practice of using keywords other than the
standard keywords to express concepts for which a standard keyword
exists.  In the NOST User's Guide, we say, "Different keywords should
not be used in the place of reserved keywords to express the same
concepts."  FITS Paper I describes the purpose of reserving keywords
in FITS as "to facilitate and encourage the specification of detailed
intensity, coordinate, and documentary information with each image."
Expression of concepts in ways other than that defined in the standard
complicates the task of the reader, defeating the purpose of
standardization.  As Wells, Greisen and Harten noted in the first FITS
paper, "... we will all have to be careful to avoid compromising this
tool by basic departures from the basic FITS standard. 

>agree that it should be suppressed as soon as feasible.

The 8 character limit on values of a character keyword applies only 
when the information from that keyword is needed by the software to 
actually read the data from the file.  It does not apply to 
information required only for scientific interpretation of the header 
and data.  

				Barry Schlesinger
				NSSDC/NOST FITS Support Office