astlib coordinates code to astropy coordinates code

Question

I'm trying to fix this code which used astlib to convert coordinates to use the astropy coordinate modules. Here's a copy of the code:

from astLib import astCoords as coords

#Convert B1950 coordinates (as given)
clra=zeros(3)
cldec=zeros(3)
for i in range(len(clra)):
    clra[i], cldec[i] = coords.convertCoords(
                                'B1950', 'J2000', clra1950[i],
                                cldec1950[i], 1950
                            )

#Convert input coords to Galactic coords
lgal,bgal = radians(coords.convertCoords(
                        'J2000', 'GALACTIC', ra, 
                        dec,2000
                    ))

I need help with 2 things.

1. What are the proper imports for astropy if I want to change from B1950 to J2000, and the proper imports to go from J2000 to galactic coordinates?

2. in the areas that start with coords.convertCoords(), What are the functions and arguments from astropy that go in its place. In other words, what do I replace it with?

Also, I had done some research into this issue. I found this link here: http://docs.astropy.org/en/v0.2.1/coordinates/

It describes the notation for astropy's coordinate related functions. However, there is a lot there, and I'm not sure what to use and how to use it.

Answer 1

This is one way to do it:

In [1]: import numpy as np

In [2]: from astropy import units as u

In [3]: from astropy.coordinates import SkyCoord, FK4, FK5, Galactic

In [4]: clra = np.zeros(3)

In [5]: cldec = np.zeros(3)

In [6]: c1 = SkyCoord(clra * u.deg, cldec * u.deg, frame=FK4)

In [7]: c2 = c1.transform_to(FK5(equinox='J2000'))

In [8]: c3 = c2.transform_to(Galactic)

In [9]: print(c3.l.degree)
[ 97.74220094  97.74220094  97.74220094]

In [10]: print(c3.b.degree)
[-60.18102359 -60.18102359 -60.18102359]

Note that you can convert c1 to Galactic directly too.