New UCD1+ for SIA protocol

[Anita Richards]

Bob Hanisch wrote
> I just want to add my support for Alberto's objection to instr.scale -- I
> think this could be very confusing.  The scale here is not an attribute of
> the instrument, but of the image.  Glancing quickly through the UCD-UCD1+
> mapping, however, I do not see a UCD that fits very well.

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

To underline these points, I think thaat there may be a tendency to think
of images as products of instruments using pixel detectors, where the
pixel scale of the instrument determines the resolution of the image and
the sampling interval of the image.  But of course that ain't necessarily
so.  As has been pointed out, data processing can change the image pixel
size of even CCD images let alone other forms of data. The resolution is
determined by the PSF, but even that means different things in different
regimes.   Both pixel size and resolution are variable e.g. by weighting
radio data.

Hence for describing images you need an image pixel size.

Resolution is tricker since even in a single image that can be brightness
dependent.  In the Registry we would use a range but in describing images
I think that the choices are

1) Just use image pixel size and assume that everyone uses something close
to the convention of 3 pixels across a resolution element most of the time
(but as resolution is unspecified the user will have to think about it and
hence will not be misled).  This is probably best for images but maybe
not for catalogue data.

2) Use a 'characteristic resolution'.  However, that is problematic for
images as this may not be specified in the FITS header (e.g. in radio
images the restoring beam size is usually in there but can be buried deep
in the history).  However it is often what is given in catalogues.

cheers

a
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AVO Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).