Putting matplotlib hexbin into an Aitoff projection

Question

I have the nice hexbin plot below, but I'm wondering if there is any way to get hexbin into an Aitoff projection? The salient code is:

import numpy as np
import math
import matplotlib.pyplot as plt
from astropy.io import ascii

filename = 'WISE_W4SNRge3_and_W4MPRO_lt_6.0_RADecl_nohdr.dat'
datafile= path+filename
data = ascii.read(datafile)  
points = np.array([data['ra'], data['dec']])

color_map = plt.cm.Spectral_r 
points = np.array([data['ra'], data['dec']]) 
xbnds = np.array([ 0.0,360.0]) 
ybnds = np.array([-90.0,90.0]) 
extent = [xbnds[0],xbnds[1],ybnds[0],ybnds[1]] 

fig = plt.figure(figsize=(6, 4)) 
ax = fig.add_subplot(111) 
x, y = points 
gsize = 45 
image = plt.hexbin(x,y,cmap=color_map, 
    gridsize=gsize,extent=extent,mincnt=1,bins='log') 

counts = image.get_array() 
ncnts = np.count_nonzero(np.power(10,counts)) 
verts = image.get_offsets() 

ax.set_xlim(xbnds) 
ax.set_ylim(ybnds) 
plt.xlabel('R.A.')  
plt.ylabel(r'Decl.') 
plt.grid(True) 
cb = plt.colorbar(image, spacing='uniform', extend='max') 
plt.show()

and I've tried:

plt.subplot(111, projection="aitoff")

before doing the plt.hexbin command, but which only gives a nice, but blank, Aitoff grid.

Cynewulf

In principle there is no problem of plotting a hexbin plot in any projection. Without a minimal reproducible example, noone can know where your problem lies. – Fidele

Is the (hopefully) minimal, complete code. The RA/Decl or x,y data points can be, as is given above, just a numpy array. – Cynewulf

There are basically no examples I can find of hexbin being used for an Aitoff projection, so I don't think it's as easy as just using "plotting a hexbin plot in any projection..." – Cynewulf

Answer 1

The problem is that the Aitoff projection uses radians, from -π to +π. Not degrees from 0 to 360. I use the Angle.wrap_at function to achieve this, as per this Astropy example (which essentially tells you how to create a proper Aitoff projection plot).

In addition, you can't change the axis limits (that'll lead to an error), and shouldn't use extent (as ImportanceOfBeingErnest's answer also states).

You can change your code as follows to get what you want:

import numpy as np
import matplotlib.pyplot as plt
from astropy.io import ascii
from astropy.coordinates import SkyCoord
from astropy import units

filename = 'WISE_W4SNRge3_and_W4MPRO_lt_6.0_RADecl_nohdr.dat'
data = ascii.read(filename)
coords = SkyCoord(ra=data['ra'], dec=data['dec'], unit='degree')
ra = coords.ra.wrap_at(180 * units.deg).radian
dec = coords.dec.radian

color_map = plt.cm.Spectral_r
fig = plt.figure(figsize=(6, 4))
fig.add_subplot(111, projection='aitoff')
image = plt.hexbin(ra, dec, cmap=color_map,
                   gridsize=45, mincnt=1, bins='log')

plt.xlabel('R.A.')
plt.ylabel('Decl.')
plt.grid(True)
plt.colorbar(image, spacing='uniform', extend='max')
plt.show()

Which gives

Gelt

Answer 2

I guess your problem lies in the use of the extent which is set to something other than the range of the spherical coordinate system.

The following works fine:

import matplotlib.pyplot as plt
import numpy as np

ra = np.linspace(-np.pi/2.,np.pi/2.,1000)
dec = np.sin(ra)*np.pi/2./2.
points = np.array([ra, dec]) 

plt.subplot(111, projection="aitoff")

color_map = plt.cm.Spectral_r 
x, y = points 
gsize = 45 
image = plt.hexbin(x,y,cmap=color_map, 
                   gridsize=45,mincnt=1,bins='log') 

plt.xlabel('R.A.')  
plt.ylabel(r'Decl.') 
plt.grid(True) 
cb = plt.colorbar(image, spacing='uniform', extend='max') 
plt.show()

Fidele

Hmmm.... So I can get your code e.g. working and reproducing the plot above. What I cant do is take my astropy.table and get anything sensible. – Cynewulf

I've modified the code above. The data are here:: dropbox.com/sh/4qsv753bhksn6ur/AAB4kthAcvmx_kQRa0zhkhQ0a?dl=0 – Cynewulf

I don't have astropy available. – Fidele

> pip3 install astropy – Cynewulf