[fitswcs] WCS documents

[Bob Hanisch]

----- Original Message -----
From: "Stephen Walton" <swalton at sunspot.csun.edu>
To: <fitswcs at NRAO.EDU>
Sent: Tuesday, September 18, 2001 12:15 AM
Subject: Re: [fitswcs] WCS documents


> I cannot pretend to speak for everyone, but my quietude has been at least
> in part to incomplete understanding of all the issues involved.  I am
> curious about the current discussion:  I _think_ it is in response to the
> IRAF group's suggestion (quoting from their Web site): "we propose that
> the WCS dimension be given by the highest value of the axis in any CTYPE
> keyword, and that this value be permitted to be greater than the value of
> NAXIS (a WCSDIM keyword could also be added to make this distinction more
> apparent, but this is not necessary)."  As I understand it, this is to
> allow subimages to carry along WCS information correctly.
>
> I am afraid I don't get this.  If I have a 15 by 17 by 12 spatial by
> spectral by spatial image, let's say, and I copy the portion
> [1:15,11,1:12] to another image, most software I know of would create a 15
> by 12 image as output, not a 15 by 1 by 12 image.  If said software also
> simply copied the WCS with no checking, then there would be a mismatch;
> further processing would associate the second WCS component of the
> original image with the second dimension of the new image, rather than
> with the third.  I can't see how the proposal quoted above, with or
> without a WCSDIM/WCSAXES keyword, would help.
Indeed -- this is the very problem.  In this case, you expect -- and many
applications programs have been so coded -- to have NAXIS = 2 for the output
image of the above operation.  One can argue that, more properly, NAXIS = 3
and NAXIS1 = 15, NAXIS2 = 1, and NAXIS3 = 12.  But most software has not
been written to recognize this as a degenerate data cube and thus see the
"image" that resides in the x-z plane.

If NAXIS = 2, however, we still want to retain information about the third
coordinate axis.  The WCS still has dimension 3, and were one to take this
data plane and reinsert it into a new data cube, you would want to be able
to reconstruct the full 3-dimensional WCS.  This is why an explicit keyword
for the WCS dimensionality is needed, to avoid using one keyword to convey
two different types of information.

> Bob, would you care to elaborate on some of the practical problems viz a
> viz 'radio-optical interoperability' you ran into recently?
>
> Steve
>
> --
> Stephen Walton, Professor of Physics and Astronomy,
> California State University, Northridge
> stephen.walton at csun.edu

Sure.  A colleague was trying to compare VLA and HST (WFPC) data.  The VLA
data had been processed with AIPS, HST data with IRAF and IDL.  The HST data
had been rotated and resampled by another colleague, in IDL I think, and it
was not clear if the WCS had been updated properly.  In addition, the VLA
data uses the CDELT/CROTA WCS convention, and HST data uses the CD matrix.
AIPS apparently does not recognize the CD matrix, and the user had to
hand-compute and update the HST data headers in order to use AIPS' plotting
and display routines to compare the images.  Most astronomers are not
familiar with the details of these coordinate system representations, and if
we cannot reach better agreement we just create frustration, and worse,
wrong results, for users.

Bob