[AstroPy] FITS image data dtype

[Miguel de Val-Borro]

On Wed, Jul 17, 2013 at 12:14:16PM -0400, Erik Bray wrote:
> FITS files store arrays in big-endian order natively.  Actually swapping the 
> bytes to native order can be costly, especially when the data is being accessed 
> by way of an mmap'd file (the default in PyFITS 3.1 and up).  And it's generally 
> unnecessary when working within Numpy--it is only necessary if passing those 
> bytes to other software that is expecting values in native byte order.

The matplotlib function imshow shows the original map differently than
after swapping the byte order to little-endian format.

imshow(hdulist[1].data)
https://docs.google.com/file/d/0B3Hv-zV2q5IcSjk2amw3ZmM3Mk0/edit
imshow(hdulist[1].data.byteswap().newbyteorder())
https://docs.google.com/file/d/0B3Hv-zV2q5IcTWZ2WXduMWttQUE/edit

If I remember correctly I also had problems with numpy array operations
on the maps with the '>f8' dtype for this kind of image data.

Miguel