[Difx-users] J2000 to date conversions {External}

[Phillips, Chris (S&A, Marsfield)]

Hi al

Apologies for using the DIFX mailing list for a decidedly non-DIFX question. I’m guessing there is a number of people on this list who can help me with this query.

I am trying to convert some J2000 coordinates to “date coordinates” (ie precess to a specific date).

The initial idea was to use python astropy (rather than perl/slalib) to get with the times. However I am getting inconsistent results, and cannot find any documentation on how to do this in astropy.

As an example I am trying to convert 1934-638 from J2000 to epoch 2020-1-1. Various different approaches are here:


1934-638 19h39m25.026s -63d42m45.63s J2000
astropy 19h41m15.463s  -63d39m55.69s
perl 19h41m13.563s -63d39m56.94s
SLA 19h41m10.545s -63d40m02.77s
Perl SLA 19h41m10.559s -63d40m02.76s
Online 19h41m15.5s     -63d39m56s http://www.robertmartinayers.org/tools/coordinates.html

“Perl” is code I wrote years ago for Astro:Coord (on CPAN) based on Fortran code at Hobart observatory
SLA is a hacked version from Caltech VLBI package “PRECESS” which uses SLALIB
PerlSLA is a perl version which makes the same calls (mostly for coding consistency check)
Online is a random webpage found via google.

Clearly astropy and the online version use the same algorithm (or online is using astropy under the hood).

My perl version is similar to astropy, but slalib is different by a lot. I remember years ago looking into this sort of stuff
 (maybe B1950-J2000 rather than epoch precession) and SLALIB used a very different implementation to astronomical
Almanac, following a paper which basically said the almanac implementation was wrong.

FYI, the relevant code is:

Astropy:

from astropy.coordinates import SkyCoord, FK5
from astropy.time import Time

c = SkyCoord('19h39m25.026s', '-63d42m45.63s', frame='icrs')
t = Time('2020-01-01 00:00:00', scale='utc')

my_date = FK5(equinox=Time(t))
c_date = c.transform_to(my_date)


I’m a little dubious this is correct, given the use of “Equinox”, while I really want to change Epoch.

The SLALIB implementation is:


      CALL SLA_MAPPA(2000D0, DATE, AMPRMS)
      CALL SLA_MAPQKZ(R2000, D2000, AMPRMS, RA, DA)


Where DATE is the MJD (checked to be consistent).

Does anyone have code which they trust to test these conversions (or comments)?

Thanks
Chris
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