Optical mirrors (reflectors), as follows: 1. ‘Deformable mirrors’ having an active optical aperture greater than 10 mm and having any of the following, and specially designed components therefor: a. Having all the following: 1. A mechanical resonant frequency of 750 Hz or more; and 2. More than 200 actuators; or b. A Laser Induced Damage Threshold (LIDT) being any of the following: 1. Greater than 1 kW/cm 2 using a ‘CW laser’; or 2. Greater than 2 J/cm 2 using 20 ns ‘laser’ pulses at 20 Hz repetition rate; 2. Lightweight monolithic mirrors having an average ‘equivalent density’ of less than 30 kg/m 2 and a total mass exceeding 10 kg; 3. Lightweight ‘composite’ or foam mirror structures having an average ‘equivalent density’ of less than 30 kg/m 2 and a total mass exceeding 2 kg; Note : Items 2 and 3 above do not apply to mirrors specially designed to direct solar radiation for terrestrial heliostat installations.