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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.