Subject: More comments on World Coordinates

From: thompson at stars.gsfc.nasa.gov (William Thompson, code 682.1, x2040)
Date: Tue, 10 Nov 1992 02:12:00 GMT

In the document "World Coordinate Systems Representations Within the FITS
Format", it is stated.

{\tt CTYPE1, CTYPE2} -- the type of world coordinate axes.  The {\tt CTYPE}
value strings are eight characters in length, and are constructed from
two four-character segments.  The first four characters give the type
of world coordinate, and the second four characters give the type of
projection geometry.  The first four-character string is left-justified
and filled out to a length of four characters by appending hyphens; the
second four-character string is right-justified and filled out to a length
of four characters by prepending hyphens. The following coordinate pairs and
geometries are recognized: 

This is a new requirement for this keyword.  I would like to know how software
(e.g. AIPS) which is written to parse files written using world coordinates
will handle data which does not follow the conventions outlined in the world
coordinates document.  This applies both to data written before this convention
was devised, and to future data that does not conform to this convention.

Our plans are to use a unified keyword system for all aspects of the SOHO data,
both within FITS files and out.  The values of the CTYPE keyword are supposed
to be the names of valid SOHO keywords which represent the physical unit
represented by that dimension, e.g.

	CTYPE1  = 'INS_Y   '		/Instrument Y axis
	CTYPE2  = 'INS_Z   '		/Instrument Z axis
	CTYPE3  = 'WAVELNTH'		/Wavelength

In other instances, depending on the kind of data being gathered, these same
names could appear as FITS keywords

	INS_Y   =          3	/Instrument Y pointing (arcsec)
	INS_Z   =          5	/Instrument Z pointing (arcsec)
	WAVELNTH=     584.60	/Wavelength in Angstroms

There is no way we could live within the restrictions of the world coordinate
system as regards the CTYPE keyword.  (Having to live with "WAVELNTH" instead
of "WAVELENGTH" is bad enough!)

The goals of the world coordinate system are important and quite laudable.
However, the FITS community needs to recognize the existence of data sets that
are and will be outside of this convention.  Since our data will have no
relationship to the celestial sphere, none of the coordinate systems
(geocentric, galactic, or ecliptic) are relevant for our data set.  Will future
FITS readers such as AIPS be able to handle data outside of the new world
coordinate system convention.

It seems to me that CTYPE is now being asked to serve two functions.  It
specifies the physical meaning of the data, as has always been its function.
It also is now being asked to specify the kind of data projection used as well,
which is a totally new function.  I believe that these should be separated into
two separate sets of keywords.  In other words, instead of

	CTYPE1  = 'DEC--TAN'

you would have

	CTYPE1  = 'DEC     '
	CPROJ1  = 'TAN     '

This would provide for more flexibility and capatibility with existing systems.
Once that change is made, there's no reason why other data sets could be
brought into at least *partial* agreement with this convention.

Bill Thompson