[fitswcs] inherit vs. concatenate

[David Berry]

On Tue, 1 Jan 2002, Steve Allen wrote:

> I write up this exercise because I think it points out that
> concatenations of WCSs should be considered as relatively common
> operations.  Indeed, they are so common that perhaps they should be
> included as part of the WCS standard.

I've come across similar problems. I have two images of the same part of
the sky but which have different projections (one orthographic, one
tangent plane). I want to attach a WCS to the tangent plane image which
represents the corresponding pixel coordinates in the orthographic image. 
The transformation required has two part; 1) from the tangent plane pixel
coordinate system to RA/DEC, and 2) from RA/DEC to the orthographic
coordinate system. In other words, I need to concatenate the pixel->sky
mapping from the tangent plae image, with the sky->pixel mapping from
the orthographic image. Easy enough to do, but how do I represent the
resulting WCS as FITS headers in the tangent plane image? I agree with
Steve that WCS concatenation facilities would be *very* useful.

I hear groans of "more complexities!"... This mailing list always seem to
be finding good reasons for shoe-horning more and more features into
the already complex FITS WCS proposals. To what extent is this a result of
inheriting the philosophy and methodology of the previously existing
scheme - a perfectly functional scheme admittedly, but one which never
attempted to address the complexities discussed in this mailing list?

Having been working for several years with Rodney Warren-Smith's notion of 
"World Coordinate Systems as Objects" (see his ADASS 97 presentation
http://adass.org/adass/proceedings/adass97/warrensmithr.html plus
related poster papers from ADASS 99 and ADASS 2000 ), I can't
help thinking that the FITS-WCS discussions would have been a *lot*
shorter if we had started from scratch, taking a more radical
object-oriented approach which generalizes the whole problem of WCS
representation.  The AST subroutine library which implements Rodney's
ideas (http://www.starlink.rl.ac.uk/star/docs/sun211.htx/sun211.html)
has a working scheme for representing arbitrarily complex WCS objects
as FITS headers (it can also read and write WCS from other common FITS
header schemes, obviously working within the limitations of those
schemes though).

I guess the time for such comments passed several years ago though!


David