Suppose the diameter of the starshade is D = 10 m,
       and the wavelength of light is 1 micron = 10^(-6) m.
       Then the angular radius of the first minimum of the
       pattern is theta = 1.2 x 10^(-7) radians.

              
    Q:  If the telescope has a diameter of d = 8 m
        at what distance L must we place the starshade
        so that the telescope just fits into the first minimum
        of the pattern?


           The angular diameter of the shadow is twice its 
           angular radius, 2*theta = 2.4 x 10^(-7) radians.

           We need to put the shade at a distance so that the
           linear diameter of the shadow matches the linear diameter
           of the telescope:

                 L * (2*theta)  =   d


                                   d              8 m
                 --->    L   =  --------  =   -------------  =  33,000 km
                                2*theta       2.4 x 10^(-7)