9 July 2008 An intelligent modulator system
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In 2001, the GONG+ instruments began acquiring solar magnetic field images (magnetograms) every minute. These observations offer a useful resource for the solar physics community. However, the quality of the magnetograms was reduced by a significant zero point error in the observations that varied across the solar image and with time. This precluded using the magnetograms for meaningful extrapolations of weak photospheric fields into the corona. The error was caused by the slow, asymmetric, locally varying switching of the LCD modulator (LCM) from one retardation state to the other. This generated a false magnetic field pattern as a result of different responses to weak instrumental linear polarization ahead of the LCM. The original modulator driver used a very simple design to excite the LCM. Liquid crystals like those in the LCM take different times to switch from one polarization state to the other than to return to the first polarization state. To eliminate the difference in switching times, a driver capable of varying its output during the change in LCM state was needed. A microcontroller-based design was chosen. The final driver design resulted in a factor of 100 improvement in the zero point error.
© (2008) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
George Luis, Jack Harvey, Tim Purdy, Mike Soukup, Patricia A. Eliason, "An intelligent modulator system", Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701460 (9 July 2008); doi: 10.1117/12.787996; https://doi.org/10.1117/12.787996

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