[fitsbits] CRPIX clarification

[Rob Seaman]

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