title: Creating a 2d mesh in python
body: <p>I am really new to programming...</p>
  
  <p>But here is my question :</p>
  
  <p>I cannot post images but the plot I wish to have is a "crown" (two concentric circle with radius a
  I mean, mathematically speaking is really easy to define but how can I do it with a python program ?</p>
  
  <p>I thought of something like this :</p>
  
  <pre><code>def Fm1(X, Y):
      r =r = sqrt(1.*X**2+1.*Y**2)
      cos = 1.*X/r
      sin = 1.*Y/r
      teta = where( sin &gt;= 0. , arccos(cos) , -arccos(cos) )
      teta = where(r == 0. , 0., teta)
      return r, teta
  
  
  def F(r,teta):                                                                  
      X = r*cos(teta)                                                             
      Y = r*sin(teta)                                                             
      return X,Y
  </code></pre>
  
  <p>Those are simply the function that let you pass from the cartesian to the polar coordinates, and then :</p>
  
  <pre><code>r=sy.linspace(a,b,N+1) # radius division
  t=sy.linspace(0,2.*pi,2**NN) #angle (theta) division
  R,T=meshgrid(r,t) #creating a mesh
  
  X,Y = F(R,T)#transform from polar to cartesian
  
  #Plotting :
  fig=plt.figure()
  ax=fig.add_subplot(111)                                 
  ax.plot(X, Y)
  plt.show()
  </code></pre>
  
  <p>But the result is : concentric polygons. I wish I had  N+1 circles at equidistance from radius a to radius b and 2**NN lines (origin center and a given angle). </p>
  
  <p>Sorry I know it's really a trivial question,</p>
  
  <p>Thanks</p>