The ctop function uses bicubic interpolation to find image values
at non-integer Cartesian pixel locations corresponding to sampled polar
coordinates. For radius r pixels, the number of samples
(regularly distributed in azimuth) is approximately equal to
#2pi*r, so that each circle is sampled at a resolution of about 1
px. The exact number of samples is selected so it has factors from only
the set 2, 3, and 5, for the speediest Fourier transformation. The
sampled data values are then Fourier-interpolated over the full width of
the return image, so that a particular value of the first dimension
corresponds to the same polar angle for all values of r.