[fitsbits] CRPIX clarification

[Rob Seaman]

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
>