[AstroPy] [astropy-dev] Coordinates subpackage - request for help

David Berry d.berry@jach.hawaii....
Thu Nov 15 02:29:28 CST 2012

On 14 November 2012 22:01, Demitri Muna <demitri.muna@gmail.com> wrote:
> Hi,
> We are in the process of testing the new astropy coordinates package. One test that I think would be useful is this (which Tom also mentioned):
> * generate 1000 ra/dec J2000 random points on the sky
> * convert them to, in steps,
>     - galactic
>     - B1950
>     - ecliptic
>     - equatorial J2000
> Can someone do this in IDL? The result should be
> * the file containing 1000 points
> * the resulting points at the end of the conversions
> * the IDL script
> The aim is to write a script using the coordinates package and see if there are any discrepancies. We will add this to the testing suite.
> I thought I'd cc the astropy list as I'm sure there are lots of people who are proficient in IDL and can probably do this pretty quickly.

1) Different conversion algorithms are always (or nearly always) going
to produce different answers. So do you have a target accuracy for the
astropy coordinates package? Do you want arcseconds accuracy or
milli-arcsecond accuracy? The more accuracy you want, the more
sophisticated the algorithms need to be.

2) Why does it need to be IDL? It's proprietary software, so many
people will not use it. There are many tools in many freely available
languages for creating tables of corresponding positions in different
coordinate systems.   For instance, I could create such a table
quickly using PyAST, which since we're talking Python seems an obvious

3) You include ecliptic in the list. Does the coordinates package
support ecliptic? I could not see it in the code.

4) Your list of coordinate systems starts at " ra/dec J2000" and ends
at "equatorial J2000" - are you making a distinction between these?
And I presume you mean FK5 J2000 (there is a common usage of "J2000
RA/Dec" which uses the mean dynamical equator and equinox of the J2000
epoch as reference, which is different to FK5)?

5) Was the omission of ICRS intentional?

6) No mention of the epoch of observation. As I mentioned in another
message you need to specify the epoch of observation, to be able to
convert accurately to and from an FK4 RA/Dec system (which I presume
is what you mean by "B1950"). Strictly, you also need the epoch of
observation when converting between FK5 and ICRS, albeit the
dependency on the epoch is much weaker.

7) You need to specify an equinox for the ecliptic coordinates.


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