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Referring back to Fig. 3.1, the mirror M opposing the transmission flat reference mirror T is now replaced by a 90-deg combination of two flat mirrors (Fig. 5.1). The two mirrors, mounted on a fixture as shown, can be adjusted to 90 deg while observing defoci in an interferometer. The 90-deg fixture stands on a tilt table, allowing alignment of the orthogonality of the a edge relative to the interferometer axis (not shown), angle b. The two mirrors M1 and M2 forming the 90-deg angle are flat all the way to the edge.
Obtaining an interferogram is made convenient by first observing the three focal-plane images produced by the interferometer’s autocollimator, as shown in Fig. 5.1.Onthe axis is the focus T, representing the reference. Next to the focus T are the two foci, W1 and W2, of the two mirrors. The distance W1 – W2 is changed by controlling angle a; W1 ¼W2 only for a ¼ 90 deg ±0. Nutating the stand (changing the angle b) allows T,W1, andW2 to be on a line, represented in the bottom of Fig. 5.1. Rotating the 90-deg fixture in the b direction will neither change distance W1 – W2 nor move these two foci closer together along the a axis. Before W1 and W2 fully coincide, one can already observe two partial interferograms with potentially confusing variations [Figs. 5.2(a) and (b)] that behave as though they are part of only one wavefront having a kink angle.
Varying the alignment of T by minute amounts and letting the 90-deg mirror assembly be close to 90 deg, one obtains three characteristically different but equivalent interferograms. These alignments can be selected for convenience of evaluation [see schematic diagrams in Figs. 5.2(a) and 5.5].
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