Make that "astrometry.net", of course.

On May 30, 2008, at 12:48 AM, Rob Seaman wrote:

Steve Allen wrote:

We also do not have a set of altered words for Bill Pence to
propose which make it clear that traverse along array indices isn't
really pixels and does not really have units until and unless the
WCS says they do, and that in the case where the coordinate along
the array axis can reasonably be interpreted as a real-valued
entity the data value is intended to correspond to the measured
quantity at the integer values which run from 1 to NAXISj.

This discussion reminds me of the innumerable "which way is up?"
questions in the IRAF mail over the years. The answer goes
something like "the question is meaningless until you display the
image".

A couple of points for semantic musing. What does binning of pixels
do to those integer values? (Or decimation, subsampling or block
averaging.) Coordinates (center of the pixel or otherwise) may
start as integer values 0, 1, 2, 3. Bin by two and this turns into
0.5, 2.5, 4.5, ... Also, "measured quantity" isn't really the
extent of it in a world of theory and simulations. It's more like
the distinction between independent and dependent variables.

And many times the WCS is implicit, as with an image display causing
the up/down/left/right wave function to collapse. Call this the
astronomy.net effect. Even in an image completely devoid of
metadata, celestial fiducial marks express an implicit WCS.

Rob