Optimizing the frequency response of a steering mirror mount for interferometry applications

The Navy Prototype Optical Interferometer (NPOI) in Flagstaff, Arizona, makes use of separate smaller telescopes spaced along a Y-array and used simultaneously to simulate an equivalent single large telescope. The performance of the NPOI can be improved by increasing the steering response of the 8-in. diameter Narrow Angle Tracker (NAT). The mirrors of the NAT correct the image position for atmospherically induced motion. The current tracker has a slow response due to the low fundamental frequency of the mount and limits the quality of the data. A higher frequency will allow a faster servo feedback to the steering mirror, which will enhance the tracking performance on stellar objects resulting in final fringe data of higher quality. Also, additional and fainter objects could be observed with a faster response system, and the interferometer as a whole would be less sensitive to fluctuations in atmospheric quality. Improvements in the NAT performance over the current cast aluminum frame and glass mirror were achieved by the use of advanced composite materials in the design of the frame and mirror. Various design possibilities were evaluated using finite element analysis. It was found that the natural frequency of the NAT can be increased from 68 to 217 Hz, and the corresponding weight decreased by a factor of 5.6, by using a composite mount with a composite mirror.