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6. Calculate the period for the helix and the helix radius in mask L. The angle 0 can be found in a similar manner as step 2 above, using the distance between layer lines as X. The relation sin 0 = M/P can be used to find the helix period, which in turn can be used in the equation for the helix radius: r = P/2nt cotan (a/2) where a is the measured angle between the arms of the X shape. 2.4 cam X = 0 . 8 cm x a 6. 10. , 1%, spencer, B. (1995) Laser Experiments for Beginners, Univers