Beam Steering Unit

Our 17" clear-aperture beam-steering unit (BSU) was designed by Mr. Steve Rauenbuehler of NCAR's DFS.

BSUs are commonly used by lidars to make spatial scans. For examples, see any of NOAA-ETL’s Doppler lidar systems and the University of Wisconsin’s Volume Imaging Lidar. However, BSUs can be challenging to build because the mirrors should not introduce abberations that might cause the receiver's field-of-view to overfill the detector. In other words, the two flat mirrors in a BSU must be as carefully chosen and mechanically supported as all the other optics in the receiver. For REAL, Dr. Scott Spuler used ray-tracing to determine the maximum amount of deviation from flatness the BSU mirrors can have in order to maintain a focus on the 200-micron diameter InGaAs photodiode. His ray-trace included all the receiver optics including telescope and final focusing lenses.

In practice, keeping the mirrors sufficiently flat can be challenging since the area of the mirrors must be larger than the telescope diameter. Although new mirror construction techniques make large and lightweight mirrors possible, they are typically very expensive. Furthermore, large mirrors are typically very thick and heavy to prevent sag and image distortion. Therefore, we first attempted to build our own lightweight composite mirrors following a technique reported by Kent and Hansen (Applied Optics, Vol 38., No. 30, P. 6383-6387). Their technique affixed a relatively thin glass mirror to a thick piece of hexcore aluminum with RTV. Unfortunately, we were not able to build such a composite that maintained sufficient flatness for our system.

Months later, we decided to purchase two 1”-thick flat mirrors made from Zerodur substrates (does not expand and contract with temperature change) and coated with protected gold (for durability and high reflectivity at 1.5 microns). We chose Zerodur mainly because the supplier had blanks in stock and could deliver finished mirrors quickly for our first project.