Using Aperture Masks

As we have seen above, the circular form of the telescopic image is due to the shape of the diffracting aperture. The effect of the secondary mirror of a reflector modifies the size of the Airy disk and the radius and intensity of the diffraction rings.

The use of an aperture mask has been applied in several ways to modify the imaging of a telescope to deal with particular problems in imaging double stars, in particular with binary stars such as Sirius where the companion star is very much fainter than the primary, 10,000 times as faint in fact. Unless Sirius B (also called The Pup) is near its widest separation (about 11'') it is impossible to see visually with a small telescope. This is because the glare from Sirius A spreads out to envelop the companion star.

One means of reducing the glare is to use a hexagonal aperture mask, a fact that seems to have been discovered by Sir John Herschel. The effect is to produce a six-pointed diffraction pattern, with most of the light being directed into these spikes, and the sky between the spikes, relatively near the brilliant primary star, being much darker than without the mask. E.E. Barnard10 used this method to measure Sirius B. By rotating the mask around the optical axis, it can be used to glimpse faint companions at any position angle to bright stars.

Another form of aperture mask is the coarse diffraction grating. Used by professional astronomers to reduce the brightness of the components of double stars whilst maintaining the resolution, the grating can also be used as a basis of a simple micrometer, the principle and operation of which can be found in Chapter 14.

Experiments have been made with other shapes of aperture masks. G.B. van Albada11 describes the use of an objective mask made from several lentil-shaped slits which were used in double-star photography on the 23.5-inch refractor at Lembang in Java. It was possible to just record the companion of Procyon (a considerably more difficult star than Sirius B) using this method.

A new application of this principle is being considered for imaging extrasolar planets close to bright stars. Whilst a sharp aperture produces a fuzzy image, it turns out that the converse is also true. By using a square aperture with a fuzzy edge, thus directing most of the light into four diffraction spikes at right angles to each other, NASA astronomers hope to find planets by direct imaging. The process of producing a fuzzy aperture is analogous to apodising where, by coating a lens with a film which is progressively thicker towards the outer edge, the effect on the Airy disk is to increase it in size but the diffraction rings are suppressed. A fuzzy-square mask should make it possible for telescopes to see Earth-like planets about five times closer to their star than with an ordinary telescope.

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