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

11.1 Telescope Readout Electronics Report SummaryHere we provide a brief overview of the telescope pointing readout system. The science telescope and the 4probe windows constitute the sensor for pointing of the science metrology reference, and the telescope readoutelectronics (TRE) provide the readout for the telescope.The light from the guide star passes through the 4 probe windows, which to first order does not disturb thewavefront of this light. The telescope brings the light to a focus on two roof prisms, one for each rotation axis (Xand Y). When the telescope is directly pointed at the guide star the intensities of light that are reflected off ofeach side of the roof prisms are equal (after compensation by weighting factors). When the intensities are notequal, their normalized difference is a measure of the pointing of the telescope with respect to the guide star.The telescope readout electronics measures the light reflected from each side of the roof prisms. The light fromthe two sides of a roof prism is transported to two pairs of photo detectors, a primary pair (A-side electronics)and a redundant pair (B-side electronics). Each photodetector pair is located on a sapphire platform with theirJFET pre-amplifiers. The platforms are mounted on a thermal standoff on the front plate of the telescope (atabout 3 K) so the platforms can be operated at 72 K, where the JFET pre-amplifiers work well. The platforms aretemperature controlled. There are a total of 4 detector pairs (8 detectors) located on 4 platforms.The TRE measures the current of each photo detector with a charge-locked loop using a feedback capacitor tothe photo detector. The voltage across the feedback capacitor is read out after appropriate scaling by an analogto-digitalconverter with a range from +10 V to -10 V at a rate of 2200 samples/second, and it is quickly resetevery 0.1 s. This provides 220 A to D samples for each charge ramp. Only the even samples between 22 and 220of the ramp are used to determine the science slope (photo detector current after appropriate scaling). The firstfew samples are ignored to avoid the transient associated with the reset, and only the even samples are used toreduce the on-orbit computation time for the slope. Little information is lost in using only the even samplessince adjacent samples have a moderate degree of correlation.The diagram shown in Figure 11-1 indicates the relationship of the TRE and portions of the SQUID ReadoutElectronics (SRE) which provides power and control information to the TRE and digital-to-analog conversion,data handling, and signal processing for the TRE output signals. The four channels of the TRE are veryindependent of each other, with four identical sets of circuit boards that are housed in two boxes in intimatecontact with a corresponding SRE forward unit. There are two identical aft SRE units, but one is always poweredoff while the other is active, so the unused unit is not shown in the diagram.Gravity Probe B — Post Flight Analysis • Final Report March 2007 309