Yang, Ho Soon; (1999) Developments in stylus profilometry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

As astronomy advances, bigger telescopes on the ground are demanded as well as space telescopes. The mirrors of modern large telescopes are generally aspheric, which are difficult to test with conventional interferometric methods. Even null tests, which are most widely used in testing aspheric, sometimes lead to incorrect test results, for example, the primary mirror of the Hubble Space Telescope. Two complementary methods - interferometry and an independent test - are therefore imperative. Stylus profilometry can be a very useful candidate for the independent test as it does not require any additional optics and it can be applied to the convex as well as concave form. Previous studies at Optical Science Laboratory of University College London have taken some steps towards a new method. This consists of a low-contact force type of probe system and a laser reference system, to achieve the capability of testing a large aspheric mirror. In this thesis, the author indicates the weak aspects in previous developments and describes improvements he implemented. By hardware and software modifications of the probe system, the repeatability is better than 10 nm for at least 8 degrees slope surface. Also, the author develops the pulley and belt system as a motorised horizontal drive system. For the laser reference system, the hardware based closed loop system is developed, and the whole instrument is enclosed by the polythene curtain to reduce the fluctuation of the reference beam. Test results of the optical flat and spherical mirror are presented, which validates the potential usefulness of the laser reference system and opens the possibility of measuring the form of the large asphere accurately. Finally, the thesis concludes with suggestions of further works for meeting ambitious target accuracy for the instrument.

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