New UCD1+ for SIA protocol

[Doug Tody]

Hi Alberto -

On Wed, 24 Mar 2004, Alberto Micol wrote:
> Only two points (in the temporary absence of an established data model):
> 
> > VOX:Image_Scale
> > --> instr.scale
> > with the array value giving the scale in degrees per pixel of each 
> > image
> > axis.
> >
> 
> Wouldn't it be better to have it as --> pos.wcs.scale ?
> This parameter has to do with the way the data are sampled,
> and any software could resample it (eg drizzle) onto a different grid
> than the original instrumental pixel scale (and think of the case where
> data from different instruments are mosaic'd together).
> In the end the instrument (instr.) has nothing to do with the pixel 
> scale.
> 
> And, anyway the pixel scale can be computed from the pos.wcs.cdmatrix...
> why do we need it? Probably for queries ...

All we are doing here is normalizing the UCDs for SIA - for V1.1 we would
prefer to minimize serious changes to the interface.  Anyway, although
the image scale can be computed from the CD matrix, 1) scale is a major
image attribute which we prefer to have readily available to the client
without having to process the WCS, 2) while the image scale is a required
parameter, the WCS is merely "strongly recommended" and data providers are
not required to provide it.  If the WCS is missing, it can be approximated
from the required image parameters, i.e., CRPIX the image center, CRVAL
is POS in IRCS coordinates, the image is assumed to be nonrotated, and
the CD matrix is thus defined by the naxis and scale values.

> Actually, where is the resolution in this scheme?  > Probably it was
not in the SIAP, but I should read the SIAP specs again.

SIA does not currently have the full sampling/bandpass data characterization
which we have discussed elsewhere.  At some point we would like to add this.
V1.1 is conceivable if it comes along fast enough.

> > VOX:Image_AccessReference
> > --> meta.ref.url
> > specifying the URL to be used to access or retrieve the image.
> 
> This is complex ... there could be various URLs associated to
> the same image, e.g., to access the full product or only a preview;
> but I need to read the SIAP doc again before insisting on this.

These each get a different row of the query response VOTable, hence each
has a different acref URL.  We have another proposal (again from Roy)
to define an optional logical name which can be used to tag each row in
the query result.  Rows which refer to the "same image" but differ only
in image format, bandpass, or whatever, would have the same logical name
to indicate that they are associated.

	- Doug