>>> import numpy as np
>>> import astropy.coordinates as coord
>>> import astropy.units as u
>>> import gala.coordinates as gc

Proper motion transformations

Transforming between ICRS and Galactic proper motions is supported in Gala. To demonstrate, we again need to first define a coordinate for the object of interest:

>>> c = coord.SkyCoord(ra=100.68458*u.deg, dec=41.26917*u.deg)

Now we define the proper motion as a Quantity, and pass in to the relevant transformation function. Here, we will transform from ICRS to Galactic:

>>> pm = [1.53, -2.1] * u.mas/u.yr
>>> gc.pm_icrs_to_gal(c, pm)
<Quantity [ 2.52366087, 0.61809041] mas / yr>

Of course, these functions also work on arrays. The first axis of the input proper motion arrays should have length=2:

>>> ra = np.random.uniform(0,360,size=10) * u.degree
>>> dec = np.random.uniform(-90,90,size=10) * u.degree
>>> c = coord.SkyCoord(ra=ra, dec=dec)
>>> pm = np.random.uniform(-10,10,size=(2,10)) * u.mas/u.yr
>>> gc.pm_icrs_to_gal(c, pm) 
...