A piece of window screen held in front of the telescope or lens will produce prominent diffraction spikes on the stars (see p. 78). The spikes become much longer and more prominent as the star comes into sharp focus. This is a useful supplementary indication with any focusing method, visual or electronic. The brighter the star, the more it helps. Because the window screen does not contain glass, it does not affect the focus of the system, and it can be moved away before the actual exposure is taken.
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