Room-temperature and cryogenic tests yield complementary data on surface-figure errors.
An established method of room-temperature interferometric null testing of mirrors having simple shapes (e.g., flat, spherical, or spheroidal) has been augmented to enable measurement of errors in the surface figures of off-axis, non-axisymmetric, aspherical mirrors when the mirrors are located inside cryogenic vacuum chambers. The established method involves the use of a computer-generated hologram (CGH), functionally equivalent to a traditional null lens, to modify the laser beam of an imaging interferometer to obtain a reference wavefront that matches the ideal surface figure of a mirror under test. The CGH is inserted at the appropriate position and orientation in the optical path of the imaging interferometer, which, in turn, is appropriately positioned and oriented with respect to the mirror under test. Deviations of the surface figure of the mirror from the ideal surface figure manifest themselves as interference fringes. Interferograms are recorded and analyzed to deduce figure errors.
The need for the present augmented method arises because testing an off-axis, non-axisymmetric, aspherical mirror in a cryogenic environment entails the following complications that are not present in room-temperature testing of simpler mirrors:
- There are commercial off-the-shelf CGHs for the simpler mirror shapes, but not for the more-complex aspherical, off-axis shapes.
- The wall of a typical cryogenic vacuum chamber blocks access to optomechanical alignment fiducial objects that are incorporated into or attached to the mirror.
- Thermal contraction from room temperature to the cryogenic test temperature changes gives rise to a change in the mirror surface, relative to the reference wavefront, that can be confused with a change in surface-figure error.
- The interferometer is located outside the cryogenic vacuum chamber and gains optical access to the mirror in the chamber via a window in the wall of the chamber (see figure). It is necessary to take account of the optical effects of the window, including any changes in these effects caused by imposition of the ambient-to-cryogenic temperature gradient across the window.
The augmented method includes elements of laboratory implementation and data reduction that go beyond those of the established room-temperature-only method. The most straightforward aspect of the method is the use of an off-the-shelf interferometer and, to match the complex shape of the mirror under test, a custom CGH. Other aspects of the method, too complex to describe in detail, can be summarized as follows: The method calls for a complex combination of room-temperature and cryogenic test procedures and associated data-reduction procedures formulated to minimize systematic test errors and reveal subtle thermomechanical and optical effects, and thereby to characterize surface-figure errors at ambient and cryogenic temperatures. One notable feature of the method is the use of interferometric techniques to quickly align the mirror under test when it is in the cryogenic chamber. Once the mirror has been aligned and thermal equilibrium has been established, measurements are performed on both mirror and window surfaces to obtain the data needed to computationally eliminate the optical effects of the window.
This work was done by Victor John Chambers, Raymond G. Ohl, and Ronald G. Mink of Goddard Space Flight Center and Steven Arnold of Diffraction International Ltd.