[AstroPy] Dependencies on large data files

[Scott Ransom]

On 07/23/2014 04:26 PM, 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?
>
> 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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Note that leap seconds and earth orientation params are automatically 
updated with astropy now.

As for barycenter calculations, we are implementing this to very-high 
precision using the JPL ephemerides for the new high-precision pulsar 
timing package PINT.  For that package we are pulling the JPL ephemeris 
files and then using SPICE for most of the ephemeris table work.  We use 
the astropy Time objects (which are sweet) for all the time calculations.

The software is in its early stages of development (and hasn't been 
updated recently because of other time commitments), but barycentering 
is already in there to few nano-sec precision, including all the 
relativistic effects in our Solar System as well.

https://github.com/nanograv/PINT

Scott


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