In optics, jitter is used to refer to motion that has high temporal frequency relative to the integration/exposure time. This may result from vibration in an assembly or from the unstable hand of a photographer. Jitter is typically differentiated from smear, which has a lower frequency relative to the integration time. Whereas smear refers to a relatively constant rate during the integration/exposure time, jitter refers to a relatively sinusoidal motion during the integration/exposure time.
The equation for the optical Modulation transfer function associated with jitter is
formula_1
where k is the spatial frequency and formula_2 is the amplitude of the jitter. Note that this frequency is in radians of phase per cycle. The equivalent expression in Hz is
formula_3
where u is the spatial frequency and formula_2 is again the amplitude of the jitter (note that as the jitter approaches infinity, the value of the function tends towards zero).
For spacecraft, operation in a vacuum often means low mechanical damping. Meanwhile, spacecraft are compact and rigid, to withstand high launch loads. Jitter, then, is transmitted easily and often a limiting factor for high-resolution optics. 
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