[AstroPy] astropy.io.fits won't allow me to modify the NAXIS keywords

[Sébastien Bardeau]

Dear Daniel,


On 10/23/2017 03:49 PM, Daniel Evans wrote:
> Dear Sebastien,
>
> I suspect you're right that astropy is undoing your modifications as
> it thinks the header no longer matches the data. The solutions I can
> think of for your problem are:
>
> 1. Simply modify the array once read into Python to turn it into a 2D
> array, suitable for matplotlib.pyplot.imshow (or similar). This is
> relatively standard for viewing images in datacubes anyway, and
> requires very little effort in Python:
>
>     a = astropy.io.fits.open(sys.argv[1])
>     image4D = a[0].data
>     a.close()
>
>     image2D = image4D[0, 0]
>     matplotlib.pyplot.imshow(image2D)
>     matplotlib.pyplot.show()
>
> 2. If you do want to modify the fits file, you can convert the array
> from 4D to 2D, and reinsert this into the file. astropy will update
> the header to match:
>
>     a = astropy.io.fits.open(sys.argv[1], mode = "update")
>     a[0].data = a[0].data[0, 0]
>     a.close()

I was focusing on editing the header, but actually "editing" the data
works great! Thanks.

Do you know what would be the best syntax to add a trailing dimension
from a 2D image? I mean, in a clever way without doubling the memory
consumption.


>
> John ZuHone - the issue is presumably that matplotlib is being asked
> to display a 2D image, but is being passed a 4D array - I've done this
> many times with 3D datacubes!

@ John ZuHone: I am not a Matplotlib user (I am in charge of
reading/writing the FITS files), and I had just a comment about a
Matplotlib error without more details. But Daniel is probably right here.

Thanks,

Sebastien

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