functions in turbulence.i -

            NT CreatePhaseScreens(dimx,dimy,prefix=)  
 
     Create phase screens and save them in fits files.  
     The saved phase screens have a dimension dimx*dimy.  
     Number of phase screens = dimx/dimy.  
     The phase screens are normalized so that 1 pixel = 1 r0,  
     i.e. the variance of the squared difference of the screen  
     with itself at one pixel interval is 6.88 (rd^2).  
     dimx = long dimension of result screens  
     dimy = short dimension of result screens  
     prefix = Prefix to filename. if prefix is not set, the  
       screens are returned by not saved.  
     Example:  
     CreatePhaseScreens,2048,256,prefix="screen256"  
       
     F.Rigaut, 2001/11/10.  
     modify 2003 Feb 24 to add dimy (before dimy=256) and prefix  

            NT generate_phase(size)  
 
     Generate by Fourier an un-normalized 2D phase screen from the   
     -11/3 amplitude and a randomn phase component. Only returns the   
     real part. Beware that these screens have a effective outer scale   
     of about half the length of the screen.   
     F.Rigaut, 2001/11/10.  

            NT generate_phase(size,l0)  
 
     Generate by Fourier an un-normalized 2D phase screen from the   
     -11/3 amplitude and a randomn phase component. Returns the real and   
     complex parts.  
     Uses fftVE and cosf/sinf to keep floats for RAM use consideration  
     (the previous version of this routine was using the yorick fft,  
     thus double complex, which limits things on my machine to 4096 screens).  
     dim: desired dimension of the result phase screens  
     l0: outer scale IN PIXELS  
       
     F.Rigaut, 2001/11/10.  

             func generateVKspectrum(sdim,bdim,k0)  
 
   generate correct VoKarman spectrum including aliasing.  
       

             func generateVKspectrum(sdim,bdim,k0)  
 
   generate correct VoKarman spectrum including aliasing.  
       

            NT PhaseStructFunc(phase,npt,step,plot=)  
 
     Compute (and plot) structure function along the first dimension  
     of array. Transpose if you need to plot structure function along  
     another dimension. The structure function is defined as:  
     SF(j) = avg( ( phase - shift_along_dim1_by_j (phase) )^2. )  
     F.Rigaut, 2001/11/10.