[AstroPy] Dependencies on large data files

[Thomas Robitaille]

Hi Peter,

Peter Williams wrote:
> Hi all,
> 
> (Apologies in advance if this topic has already been brought up -- I
> haven't checked the archives/issues very thoroughly.)
> 
> One of the features I'd like to see in Astropy would be computation of
> BJDs -- this is the kind of subtle calculation that many people could
> use, and you really don't want people hand-rolling their own solutions.
> 
> The thing is, do to BJD calculations correctly, you need a solar system
> ephemeris. The compiled version of the standard JPL ephemeris weighs in
> at 14 MB and is not something that you want to have hanging out in the
> Git repository.
> 
> Has any thought been given to having astropy depend on large data files
> such as an ephemeris table? One could imagine similar dependencies on
> leap second tables, earth orientation parameters, etc. These kinds of
> files can be large and it's often desirable to be able to update them
> out-of-band with regard to the software itself. Is there a model for how
> to deal with such resources?

To answer this original question about large data files, we have indeed
put some thought into this and the ``get_pkg_data`` and related routines
in ``astropy.utils.data`` do have the ability to fetch data from a
data.astropy.org server automatically. We haven't really used it until
now because we haven't had the need, but this sounds like a good
application.

Cheers,
Tom


> 
> Cheers,
> 
> Peter
> 
> PS. I have code to compile the JPL ephemerides and perform a
> low-precision BJD calculation, based on the NOVAS library:
> 
>    https://github.com/pkgw/precastro/
> 
> This library also uses NOVAS to provide various astrometric routines
> that I think might also make valuable additions to AstroPy. Is there
> interest in that kind of functionality?
> 
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