Metrology for the adaptive optics system

Precise registration of telescope pupil, wavefront sensor camera, and science camera is critical to optimum performance of an adaptive optics system. For this reason great care is taken in design and construction to ensure optical rigidity and stability. However, some residual flexure is unavoidable, particularly for a system like the one being built at the Jet Propulsion Lab for the Cassegrain focus of the Palomar 200 ", which will experience changing gravity loads as the telescope points and guides. Such flexure has direct impact when it occurs in the non-common-path segments of the wavefront-sensor and science beams. The diffraction-limited image size in K-band at the 5 m telescope is 30 microns. To maintain non-common-path flexure errors at a small fraction of this scale would require costly, gravity-compensated optomechanics. A more practical approach is to use precise measurements of the flexure in a periodic recalibration of the wavefront sensor offsets. As an alternative to mapping flexure over a grid of sky pointing positions, we have investigated a low-bandwidth metrology scheme in which we sense tilts and translations of optical components in the various light paths using visible laser signals. We find that such a system is relatively easy to implement, and can be expected to provide dramatic improvement in adaptive optics performance when the "real-world" effects of telescope orientation are considered.