Add milli- prefix to Angstrom unit #11745

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1. github-actions commented on May 17, 2021

   

   github-actionsBot

   on May 17, 2021

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2. pllim

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   units

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   on May 17, 2021
3. embray commented on May 17, 2021

   

   embray

   on May 17, 2021

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   I do believe this is pretty non-standard, though I also see it is used in some papers, especially related to x-ray spectroscopy.

   I'd lean against allowing arbitrary SI units with Angstrom, but I'll let the experts bikeshed over that. However, in the meantime, you can define your own like:

   mAA = def_unit(['milliAngstrom', 'mAA', 'milliangstrom'], 1e-3 * u.AA,
                  format={'latex': r'm\mathring{A}', 'unicode': 'mÅ'})

   Now you can use mAA in any calculations with units, and it will work as expected. However, if you want to do reductions to units of mÅ it might take a bit more finessing.
4. dhomeier commented on May 25, 2021

   

   dhomeier

   on May 25, 2021

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   I'd lean against allowing arbitrary SI units with Angstrom, but I'll let the experts bikeshed over that.

   I don't think this is arbitrary at all, in my experience it is (or historically has been) fairly standard use in any contexts with corresponding spectral resolution. Might perhaps need some justification why we allow one specific SI prefix, but no others (but certainly would not want to open this to abuse like DÅ or cÅ ;-).
5. mhvk commented on May 25, 2021

   

   mhvk

   on May 25, 2021

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   I've seen it too, which I think makes enough of us to enable just the milli prefix. @byrdie - would you be willing to make a PR? Just change the angstrom definition in astropy/units/si.py to have the same prefix definition as bar in astropy/units/misc.py.

   EDIT: I guess bar is pretty good precedent too - I've never seen any other prefix than milli.
6. mhvk

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   Effort-low

   💤 Package-novice

   on May 25, 2021
7. roytsmart

   added a commit that references this issue on May 26, 2021

   Update si.py

   ...

   134bd30
8. mhvk

   added a commit that references this issue on May 27, 2021

   Added milli-prefix to Angstrom unit.

   ...

   30bd04d
9. mhvk

   closed this as completedin #11788on May 27, 2021