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With increased surface radiance of the IR source, antireflection coating of all optical surfaces, high reflectivity of all mirror surfaces, and the special preamplifier design, the noise level of the fourth Purkinje image servosystem has been reduced to a peak-to-peak value of about 1 minute of arc or 20 seconds of arc rms, as compared to about 1 minute of arc rms in the Generation-IV instrument. f. Frequency Response The frequency response of the servosystems determines the delay through the system. Delay is a critical feature when using the eyetracker in an image stabilization mode (especially in the presence of saccadic eye movements). For ideal retinal image stabilization, the eyetracker output should change simultaneously with eye rotation. In the latest design, frequency response has been significantly improved through redesign of the servomechanisms. In particular, the servomotors are much smaller in physical size; they also drive smaller mirrors. The limiting frequency response is usually determined by secondary mechanical resonances in the servo structure. Phase shifts that occur in the servo position signals as a result of these resonances limit the total feedback gain and thus the bandwidth of the servos. These resonances are inherently higher in frequency in the new design because of the reduction in size.