title: How to plot a tissot with cartopy and matplotlib?
body: <p>For plotting skymaps I just switched from Basemap to cartopy, I like it a lot more
  .</p>
  
  <p>(The main reason was segfaulting of Basemap on some computers, which I could not fix).</p>
  
  <p>The only thing I struggle with, is getting a tissot circle (used to show the view cone of our telescope.)</p>
  
  <p>This is some example code plotting random stars (I use a catalogue for the real thing):</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from cartopy import crs
  import numpy as np
  
  # create some random stars:
  
  n_stars = 100
  azimuth = np.random.uniform(0, 360, n_stars)
  altitude = np.random.uniform(75, 90, n_stars)
  brightness = np.random.normal(8, 2, n_stars)
  
  fig = plt.figure()
  ax = fig.add_subplot(1,1,1, projection=crs.NorthPolarStereo())
  ax.background_patch.set_facecolor('black')
  
  ax.set_extent([-180, 180, 75, 90], crs.PlateCarree())
  
  plot = ax.scatter(
      azimuth,
      altitude,
      c=brightness,
      s=0.5*(-brightness + brightness.max())**2,
      transform=crs.PlateCarree(),
      cmap='gray_r',
  )
  
  plt.show()
  </code></pre>
  
  <p>How would I add a tissot circle with a certain radius in degrees to that image?
  <a href="https://en.wikipedia.org/wiki/Tissot%27s_indicatrix" rel="noreferrer">https://en.wikipedia.org/wiki/Tissot%27s_indicatrix</a></p>