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

Constraint 2: The exhaust tube must be very small (< 1/16th of an inch) to prevent outside heat from reachingthe housing chamber and the gyroscope.Constraint 3: If more gas flows into the chamber than flows out, the pressure inside the chamber will rise. Asthe pressure rises, the helium will ionize and possibly arc. The electrical spark could knock the electricallysuspended gyroscope out of position and into the side of the housing.Solution: A gas management system was designed specifically for spinning up the GP-B gyroscopes. It usesmore than two dozen valves and numerous levels of redundancy to send ultra-pure helium gas into the chamberat 725 sccm (standard cubic centimeters per minute).Result: The gyroscopes spin up to full speed, greater than 3,500 rpm, in less than three hours.3.2.3.8 The IDA and DPA Inside the Telescope(Image Divider Assembly & Detector Package Assembly)Figure 3-17. The telescope Image Divider Assembly (IDA) and Detector Package Assemblies (DPA)Challenge: GP-B’s telescope must locate the center of the guide star to within 0.1 milliarcseconds (3x10 -8degrees). No existing telescope is accurate enough to accomplish this task.Solution: Split the incoming light with a roof-prism (inside the IDA) and direct each half toward two sensors(inside the DPA). When each sensor registers an identical amount of electrical flux, the telescope is centered.Result: Each sensor can detect the amount of light hitting it to within a few photons. The telescope can beadjusted to remain focused on the exact center of guide star, thereby precisely maintaining its originalorientation.86 March 2007 Chapter 3 — Accomplishments & Technology Innovations