Full-aperture interferometric test of convex secondary mirrors using holographic test plates

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 6.5- and 8-m-class telescopes require secondary mirrors that are larger, more aspheric, and more accurately figured than those for existing telescopes. The challenge of measuring these giant optics has spurred the development of a new measurement technique using holographic test plates. This test uses a full-aperture test plate with a computer-generated hologram (CGH) fabricated onto the spherical reference surface. 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. The accuracy of the surface measurement using this technique is expected to be 35 nm P-V and 6 nm rms for a 1.7- m secondary mirror for the MMT. Considerably higher accuracy is expected for less aspheric surfaces.