How do I plot galactic coordinates using matplotlib and astropy in python?

  • I want to make a projection of millisecond pulsars on the galactic plane, much like this one from Sala et al. 2004:
    From Josep Sala, "Feasibility Study for a Spacecraft Navigation System..."

    I have tried several methods, and have not gotten anywhere. This is my current code, along with what it is producing:

    import math as m
    import numpy as np
    import csv
    import matplotlib.pyplot as plt
    import pandas as pd
    import astropy.coordinates as coord
    import astropy.units as u
    from import ascii
    from astropy.coordinates import SkyCoord

    data = pd.read_csv('galacticwperiod.csv')

    xarr = np.array(data.iloc[:,0])
    yarr = np.array(data.iloc[:,1])
    eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
    gal = eq.galactic

    #xarr = np.array(df.iloc[:,0])
    #yarr = np.array(df.iloc[:,1])
    #zarr = np.array(df.iloc[:,2])

    #ra = coord.Angle(xarr[:], unit=u.hour)
    #ra = ra.wrap_at(180*
    #dec = coord.Angle(yarr[:], unit=u.deg)

    fig = plt.figure()
    ax = fig.add_subplot(111, projection="aitoff")
    plt.plot(gal.l.wrap_at(180*u.deg), gal.b.wrap_at(180*u.deg), linestyle='None')
    ax.scatter(gal.l, gal.b, linestyle='None')

    #ax.set_facecolor('xkcd:battleship grey')
    #ax.tick_params(axis='both', which='major', labelsize=10)
    #ax.grid(color='b', linestyle='solid')
    #plt.savefig('millisecondcoloraitoff.png', dpi=600)

    Here are a few lines of the input file 'galacticwperiod.csv':


    This is the image it produces:
    My plot

    I am almost certain this is wrong because they are not distributed along the galactic plane, which they should be. The data I am using is from the ATNF Pulsar Catalog.

    These are the sites I have already looked at, for reference:

    Any help with this would be greatly appreciated.

    Can you provide a few sample lines of `galacticwperiod.csv`? What are its fields?

    111.383,-54.849 305.898,-44.888 305.913,-44.877 They are from the ATNF Pulsar Catalog, using a search criteria of P0<0.03. This comment doesn't show it but they are in columns, with Gl (111,305,etc) in the first column and Gb (-54,-44,etc) in the second.

  • Mike G

    Mike G Correct answer

    3 years ago

    This code reads coordinates as equatorial (ra, dec) and transforms them to galactic (l, b):

    eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
    gal = eq.galactic

    The contents of 'galacticwperiod.csv' are already in galactic coordinates and should not be transformed.
    Something like this may give better results:

    gal = SkyCoord(xarr[:], yarr[:], frame='galactic', unit=u.deg)

    The other issue is that pyplot's geographic projections seem to expect angles in radians.
    This code:

    plt.subplot(111, projection='aitoff')
    plt.scatter(gal.l.wrap_at('180d').radian, gal.b.radian)

    produces this plot:

    Successful plot in Aitoff projection

    I agree that I should not be transforming my coordinates. However, this did not fix the issue.

    @Maria Updated.

    This worked. Thanks so much!

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Content dated before 7/24/2021 11:53 AM