Null corrector
A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as
However, the mirrors used in modern telescopes are not spherical – they are rotations of
Since the mirror will be ground to what the null corrector reports as the right prescription, it is critical that the null corrector be itself correct. An error in building the null corrector led to the mirror in the Hubble Space Telescope being ground to the wrong shape.[2] Less famously, this has happened in other cases as well, such as the New Technology Telescope.[3] Originally, there was no easy way to test a null corrector, so mirror fabricators needed to take extra care that the lenses were correct and spaced correctly (this second part, spacing, was the source of the Hubble null corrector failure).[2] With the advent of computer-generated holograms, it is now possible to create a hologram with the phase response of an arbitrary mirror. Such a hologram can be made to analytically duplicate the phase response of the desired mirror, then be tested with the null corrector just as the real mirror would be tested. If the combination looks like a spherical mirror to the interferometer, then both the null corrector and the hologram are correct with high probability, since the null corrector and the hologram are constructed independently by different procedures.[4] This procedure was used to test (and find an error in) the null corrector used for the MMT Observatory single-mirror retrofit.[5][6]
References
- ^ Burge, James Howard (1993). Advanced Techniques for Measuring Primary Mirrors for Astronomical Telescopes (Ph.D. thesis). University of Arizona.
- ^ a b Lew Allen (Chairman) (1990). "The Hubble Space Telescope Optical Systems Failure Report" (PDF). NASA Technical Report NASA-TM-103443. The definitive report on the error in the Hubble mirror, traced to an error in the construction of the reflective null corrector.
- ^ William J. Broad (1990-08-10). "Panel Finds Error by Manufacturer of Space Telescope". New York Times.. Mentions error also happened during the manufacture of the New Technology Telescope.
- ^ Burge, James H. (1993). Null test for null correctors: error analysis (PDF). SPIE's 1993 International Symposium on Optics, Imaging, and Instrumentation. International Society for Optics and Photonics. pp. 86–97. "The hologram is designed and fabricated independently from the null corrector, so agreement between the two indicates a high probability that both are correct."
- ^ Wilson, R.N. (1999). Reflecting telescope optics II: manufacture, testing, alignment, modern techniques. Vol. 2. Springer Verlag. p. 85.
- S2CID 1942534.