We compare two models—disk and straight-edge—for diffraction effects when distant stars are occulted by the Moon and asteroids. For a large obstacle, the error that occurs when using the straight-edge model for a round obstacle is shown to be inversely proportional to the radius of the obstacle and proportional to the square roots of the wavelength and the distance to the obstacle. The error in the relative intensity incurred in employing the straight-edge model for lunar occultations for light (500?nm wavelength) is 3 × 10-6 and for infrared (10 μm wavelength) 2 × 10-5, whereas for large asteroids (about 500 km radius) it is 10-3 in infrared and for small asteroids (2?km radius) about 0.3.
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