Expose bug in selecting wavelength for moment map

[maartenbreddels]

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Related #283

I haven't been able to solve #283, but managed to get a handle on where things go wrong and expose them in a unit test.

• This issue is not related to the frontend, it happens all on the Python side
• Putting a breakpoint at https://github.com/spacetelescope/jdaviz/blob/3edb474fc0046fbddc56f4355c0304d84956483f/jdaviz/configs/cubeviz/plugins/moment_maps/moment_maps.py#L101 I see that spec_sub.center.x is correctly 5.5e-7, and width is 1e-7,

This might be an issue with specutils, or we're holding specutils wrong.

[maartenbreddels]

It seems https://github.com/astropy/specutils/pull/710 fixed this, because of a dev install, and not running pip install -e . again, I did not pick up the new specutils, and I guess @PatrickOgle had a similar situation.

[maartenbreddels]

@rosteen maybe it makes sense for you to pick this up from here, because now this triggers another issue with specutils, and you see to have fixed that original issue with specutils as well.

[pllim]

So... where was this PR going? Should we get this test in as xfail until it is fixed later?

Or if your goal was to open this to show something will fail, it did. Can we close it without merge?