GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

14.1.6 Telescope Dither—Correlating the Gyro & Telescope Scale FactorsWhen used in reference to technology, the term, dither, generally refers to the seemingly paradoxical concept ofintentionally adding noise to a system in order to reduce its noise. For example, a random dithering technique isused to produce more natural sounding digital audio, and another form of visual dithering enables thousands ofcolor shades, used on Internet Web sites, to be derived from a limited basic palette of 256 colors. With respect tothe GP-B spacecraft, the dither refers to an oscillating movement of the science telescope and spacecraft that wasactivated whenever the telescope was locked onto the guide star. This intentional telescope movement produceda calibration signal that enabled us to relate the signals generated by the telescope photon detectors to thesignals generated by the SQUID magnetometer readouts of the gyro spin axis positions.14.1.6.1 Science Telescope—Centering on the Guide StarThe sole purpose of the GP-B science telescope was to keep the spacecraft pointed directly at the guide starduring the “guide-star-valid” portion of each orbit—that is, the portion of each orbit when spacecraft was “infront of ” the Earth relative to the guide star and the guide star was visible to the telescope. This provided areference orientation against which the spin axis drift of the science gyros could be measured. The telescopeaccomplished this task by using lenses, mirrors, and a half-silvered mirror to focus and split the incoming lightbeam from the guide star into an X-axis beam and a Y-axis beam. Each of these beams was then divided in halfby a knife-edged “roof ” prism, and the two halves of each beam were subsequently focused onto a pair of photondetectors. When the detector values of both halves of the X-axis beam were equal, we knew that the telescopewas centered in the X direction, and likewise, when the detector values of both halves of the Y-axis beam wereequal, the telescope was centered in the Y-axis direction. (The telescope actually contains two sets of photondetectors, a primary set and a backup set for both axes.)Figure 14-14. Schematic diagram of the GP-B telescope optics414 March 2007 Chapter 14 — Data Collection, Processing & Analysis