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

[Ludwig Schwardt]

Hi Chris,

The following discussion might be useful:

https://stackoverflow.com/questions/52900678/coordinates-transformation-in-astropy

The latest Astropy 5.0 now has an HADec frame. :-)

Cheers,

Ludwig


On Wed, 12 Jan 2022 at 04:47, Phillips, Chris (S&A, Marsfield) via
Difx-users <difx-users at listmgr.nrao.edu> wrote:

> The “celestial sphere” rotates wrt the earth. So any observations need to
> be appropriate rotated to a specific epoch (usually J2000). I want to be
> able to calculate the rotation terms myself for some tests.
>
> I’m possibly using the wrong terminology, but B1950, J2000 and “date” are
> commonly used. I’m guessing date <==> J2022 currently.
>
> Cheers
> Chris
>
> > On 12 Jan 2022, at 12:50, Leonid Petrov <Leonid.Petrov at lpetrov.net>
> wrote:
> >
> > Chris,
> >
> >  Why do you need it? Words like "equinox of date" are related to
> > efforts to simplify computations in the pre-computer epochs.
> > What do you want to achieve at the end?
> >
> > Leonid
> >
> >> On 2022-01-11 20:42, Phillips, Chris (S&A, Marsfield) via Difx-users
> wrote:
> >> 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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>
>
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