  This paper introduces a new scheme of ion beam figuring (IBF), or rather
variable-spot IBF, which is conducted at a constant scanning velocity with
variable-spot ion beam collimated by a variable diaphragm. It aims at improving
the reachability and adaptation of the figuring process within the limits of
machine dynamics by varying the ion beam spot size instead of the scanning
velocity. In contrast to the dwell time algorithm in the conventional IBF, the
variable-spot IBF adopts a new algorithm, which consists of the scan path
programming and the trajectory optimization using pattern search. In this
algorithm, instead of the dwell time, a new concept, integral etching time, is
proposed to interpret the process of variable-spot IBF. We conducted
simulations to verify its feasibility and practicality. The simulation results
indicate the variable-spot IBF is a promising alternative to the conventional
approach.
