Add option to use gWCS

[pllim]

@mcara suggested that Ginga should add the option to support gWCS (https://github.com/spacetelescope/gwcs) for future JWST support. I am just opening an issue here so I don't forget that this request happened.

c/c @larrybradley, @nden, @stscieisenhamer

[ejeschke]

I think that should be straightforward, given that we already have the pluggable WCS support. I note from the link that asdf is required. I think Mike D. did a quick hack to see if he could make a version of MultiDim that browsed the hierarchical structure of an asdf file and load the images. This was at the PiA2015 session, I believe. And I think he had something working within an hour or so.

[pllim]

Thanks, that sounds promising! Unfortunately, Mike D. is no longer at STScI. Do you happen to have a copy of his hack?

[pllim]

According to @nden , for uncalibrated JWST data, Astropy WCS will not be able to read the distortion information but GWCS can.

[pllim]

Update: After some internal discussions, it looks like I might have to move this one to stginga instead. If that is indeed the case, I'll close the issue here. FYI.

[ejeschke]

Given the community interest in gWCS, it seems like it would be useful to have that as a WCS option in Ginga. But perhaps it can be added later copying the approach in stginga.

[pllim]

My understanding so far is that it is only useful if you have data model schemas to go with it (e.g. schemas embedded in JWST pipeline). Discussion is still ongoing. Hopefully I'll know more next week. Thanks for your patience.

[nden]

@pllim It's more like, if you want to display JWST raw data you will need the jwst package installed.
Asdf works with schemas in third party packages through extensions defined through entry points. The only hard dependence for ginga would be asdf and gwcs. Depending on where the data comes from and whether they are final products, users may need to have some other packages installed. I don't find this unreasonable especially if working with raw data.

[pllim]

Asdf works with schemas in third party packages through extensions defined through entry points

Exactly. It only works with schemas. And currently only JWST uses it (and perhaps SunPy but in a different way). I can't envision how to support such a thing in a generic way over here. It is not like FITS WCS where all you need is a header (for better or worse). Might be better to contain this in stginga for now until it can be more generic. That is my current thought but we can discuss next week.

[pllim]

Update: @hcferguson has decided that it is best to hold this off until astropy/astropy#7326 and its counterpart for GWCS (spacetelescope/gwcs#146) is merged. FYI. Thank you for your continued patience.

[ejeschke]

Sounds reasonable.

[pllim]

APE 14 is done -- http://docs.astropy.org/en/latest/wcs/wcsapi.html