Correlation Tracker

For achieving precise measurements of the polarization with diffraction-limited images, a highly stabilized image (<0.09" in 3 sigma) is required at the focal plane of the CCDs. SOT has an image stabilization system, in which the displacement estimated from correlation tracking of solar granules is fed back to the control of a piezo-driven tip-tilt mirror in the OTA (Figure 1). The system minimizes image jitter in the frequency lower than 14 Hz (nominal gain currently defined), and the structural design damps down micro-vibration excited by mechanisms moving in the satellite in the higher frequency range. The correlation tracker produces displacement in the granules seen in the 11"×11" field of view, and therefore the SOT field of view tracks the group motion of granules in this small area. It is noted that the SOT pointing is directed to an observing target region with the satellit attitude control and the satellite attitude is controlled to track an observing region continuously in the speed of the solar differential rotation.

Figure 1. SOT image stabilization overview

Correlation tracker
CCD 50"×50" pixel (0.22 arcsec / pixel)
Frame rate 580 Hz
Spectral range 629 - 634nm
Displacement Range ±5 pixel
Error signal accuracy 0.01"
Tip-tilt mirror and its control
Signal used for closed loop control Redidual signal from correlation tracker
Actuator 3 Piezo actuators
Tilt range 10.5" in radios on the sky
Control crossover frequency 14 Hz (nominal gain)
Stability < 0.02" (in laboratory environment)

page top