Read a GWCS object from a FITS file

[nden]

Something like this (in a function or method):

from asdf.fits_embed import AsdfInFits

fa = AsdfInFits.open('miri_assign_wcs.fits')
wcsobj = fa.tree['meta']['wcs']
data = fa.tree['data']

@pllim

[pllim]

That would be useful, thanks! Is this a general standard or specific to JWST?

[nden]

This is JWST. We should link the WCS from the top object so you can do

wcsobj = fa.tree['wcs']

Also I thought @drdavella had a PR where it was possible to omit tree so fa.data would work but I may be misremembering as this does not work.

[drdavella]

@nden maybe you're thinking about doing fa['data'] instead of fa.tree['data']?

[nden]

@drdavella
Ah, you are right. I am confusing it with the JWST syntax where the dict can be accessed with ..

[pllim]

I wonder if it is worth the trouble to alias fa['WCS'] to fa['wcs']. WCS is an acronym, but in this case, it raises KeyError because the key is case-sensitive.

[nden]

I'm 👎 on aliasing. It's a key in a dictionary and it's defined to be lower case. An alias is too confusing. What about astropy where the WCS class has a wcs attribute - WCS.wcs.

[pllim]

So... fa['wcs'] (using ASDF 2.0.dev) gives me a TaggedDict. How do I convert this to GWCS object? Attempt to gwcs.WCS(fa['wcs']) gives me TypeError.

p.s. gwcs.WCS(fa['wcs']['steps']) gives me KeyError.

p.p.s. Using GWCS dev + ASDF dev + Astropy dev combo gives me AttributeError even at fa['wcs']). Dan said he'll look into it.

[nden]

Just for clarification, you don't need to convert to GWCS object.
This sounds to me like mismatched versions. Will investigate locally.

[pllim]

@drdavella says there is a small bug in Astropy (maybe he can link the PR when he has it).

@nden , don't I need a WCS object to do these things -- http://gwcs.readthedocs.io/en/latest/gwcs/using_wcs.html ? How am I supposed to do a "sky2pix" and "pix2sky" with TaggedDict?

[nden]

You are supposed to get a GWCS object back, not a TaggedDict. I can't reproduce this problem that's why I think it's a configuration issue.

[pllim]

For the record, here is a log of my attempt. And no, I don't have jwst installed.

https://gist.github.com/pllim/a1c0097daef479a5bec848aea9889d33

[nden]

Some of the transforms which define the JWST WCS are in the jwst package. Since you don't have it installed, asdf returns the raw data in a dict.
This will be a general problem with ginga, not limited to jwst, any package that defines its own transforms will run into this problem when using ginga.

[stscijgbot]

This ticket is now being tracked at AL-45