[fitsbits] CRPIX clarification

[Maren Purves]

Rob Seaman wrote:

> 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".

unless ...

we had a rather interesting thing happening with one of our
instruments that uses sub-arrays:

Until very recently (this is for an instrument that's been
in operation for about 10 years) the CRPIX etc. coordinates
followed the array (array controller) numbering, which
starts in the top right corner rather than in the
bottom left.

This works for almost everything, the one exception
being if you use a full array flat field (or dark)
for a subarray observation, because you'll get the wrong
quadrant.

Following this I implemented the "up" that the data
reduction expects which is the opposite of what the
array controller thinks, i.e. I had to flip the
coordinate axes (rotate 180), but only for the header,
not for the readout - independent of displaying.


Maren
(UKIRT)