A new arrangement for testing Cassegrain mirrors is described which does not require the use of large mirrors as auxiliaries. It consists in placing a convergent lens in front of the mirror so as to form a real image of the illuminating pinhole and measuring the resulting aberration. The aberration of the lens is numerically eliminated after an independent measurement with the help of a small plane mirror placed at an appropiate distance. The longitudinal aberration of the Cassegrain alone can also be directly obtained by displacing the small plane mirror. The method permits high precision. It can be applied to any convex surface.
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