Optic System Design of a 2.4m Class Telescope and Several Methods for Testing the Secondary Mirror

Optic system of a 2.4m class telescope working at Cassegrain focus and Nasmyth platforms is a R-C system using the focal ratio of primary mirror corresponding to f/1.5 and f/3. A number of methods for testing the quality of aspheric convex secondary mirror with primary focal ratio f/3 have been presented. The 2.4m class telescope requires secondary mirror larger,more aspheric and more accurately figured. In this paper we list a comparison with data of measuring this giant optics using the Hindle spheric mirror,reflective compensator,transmission compensator and computer-generated holograms (CGH). Hindle arrangement for testing convex hyperboloid can be implemented by retroreflection from a sphere whose center is at the inaccessible focus of the hyperboloid. But this test requires the additional mirror more than twice larger than the aspheric mirror. A reflective compensator must contain an element that is at least one and a half larger than the tested morror.So convex secondary mirrors are notoriously difficult to fabricate because of the tremendous cost and difficulty of making accurate measurements of the optical surfaces. The new measurement technique using full-aperture test plates with a CGH fabricated onto the spherical reference surface has been considered with higher accuracy. When supported a few millimeters from the secondary and properly illuminated with laser light,an interference pattern is formed that shows the secondary surface errors.The hologram consists of annular rings of metal drawn onto the curved test plate surface using a custom-built writing machine. There are several important advantages to measuring convex aspheres. The high accuracy of the test is a result of the quality of the hologram and the spherical reference surface.The additional optics to perform the test do not need to be of interferometric quality and can be made quickly and economically. The CGH test is easily performed and has very little noise from the vibration or air motion because the interference cavity is only a few millimeters. And unlike classical testing methods which require the mirrors to be conic sections of revolution,the holographic test can be used for measuring both bare glass surfaces and mirrors with highly reflective coatings. Finally we evaluate the costs for four testing methods.