As radio telescopes have reached larger diameters and higher frequencies, it is typically not possible to
meet their surface accuracy specifications using passive homology-based designs. The most common solution to
this problem in the current generation of large, high-frequency radio telescopes is to employ a system of linear
actuators to correct the surface shape of the primary reflector.
The exact specifications of active surface actuators vary with the telescope. However, they have many common
features, some of which drive their design. In general, these actuators must provide precise and repeatable
positioning under significant loads during operation and they must withstand even higher loads for survival conditions.
For general safety, they typically must hold position in the event of a power failure and must incorporate
position limits, whether electrical, mechanical, or both. Because the number of actuators is generally high for
large active surfaces (hundreds or even thousands of actuators), they must also be reliable and of reasonable
individual cost. Finally, for maximum flexibility in their installation, they must be compact.
This paper presents a concept for an active surface actuator based on a form-closed eccentric cam (kinematically,
a Scotch Yoke mechanism). Such a design is limited in stroke, but offers potential advantages in
terms of manufacture, compactness, measurement, and survival loading. The paper demonstrates that some of
the expected advantages cannot be practically realized, due to dimensions that are driven by survival loading
conditions. As a result, this concept is likely to offer an advantage over conventional screw-type actuators only
for cases where actuator runaway and stall are the driving considerations.