[AstroPy] Dependencies on large data files

Tim Jenness tim.jenness at gmail.com
Wed Jul 23 16:34:25 EDT 2014


On Wed, Jul 23, 2014 at 1:26 PM, Peter Williams <peter at newton.cx> 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.
>
>
Suggesting there should be a command people type to dynamically pull the
ephemeris from the server and compile it?

How accurate do you need to be for usual BJD calculations? Do you need to
know the planets to within 0.1 arcsec? There are routines around that can
do a reasonable job of estimating the positions of the planets without
requiring the full JPL ephemeris. We could even have a scheme where astropy
defaulted to something like SLALIB precision if the JPL version wasn't
around (Pat Wallace was also telling me about an intermediate level
calculation between SLALIB and JPL but I haven't got the link handy).


> 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?
>

ERFA is in astropy already so there are some astrometry routines that
exist without having python visibility at the moment.

Which routines do you have?

-- 
Tim Jenness
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