Therefore, if r0 is only 10cm, the performance of a 10m telescope, in terms of its long-exposure image resolution, is reduced by a factor of 100. It needs to be bright enough so that sufficient photons are collected in order to make a good estimate of the geometry of the wavefront distortions. A Laser Beacon is an artificial reference star which may be manufactured if a sufficiently bright natural star is not available close to the science object. It is made by shining a very high-power laser into a small region of the atmosphere and collecting and observing light which is scattered back to the ground. It often consists of a mirror which can be rapidly deformed to the equivalent shape of the wavefront which must be subtracted out.
Alternative technologies involve the use of liquid crystal devices.
Such a device may also be used to perform seeing measurements . Open-loop is the mode of operation of an adaptive optics system in which the phase aberrations are measured but not corrected. In this situation, the wavefront sensor observes the full aberrations introduced by the atmosphere during each cycle. The open-loop frequency is simply the inverse of the time taken to detect the phase distortions, calculate the signals required for correction and drive the phase corrector. Closed-loop is the mode of operation of an adaptive optics system in which the phase corrector is used to correct measured phase aberrations.