How to isolate WCS and observation information from a header

[dstansby]

When I'm reprojecting maps, I often want to change the metadata such that the WCS keywords are updated to values matching the new projection, but I want to keep the rest of the metadata (observer coordinate, observation time, observation wavelength etc etc.) the same.

When doing this, it would be helpful if there was a function that given a FITS header, could split it into two parts, one containing the projection specific WCS information, and one containing everything else.

Perhaps this should be an upstream astropy feature request?

[Cadair]

The shortest hack I can think of here is:

• Create WCS from header
• run wcs.to_header()
• Delete all the keys in the generated header from the original header.

[wtbarnes]

I tried to tackle this way back in #6502 and came to the conclusion that it was all but impossible with our current metadata model.