Super Star Tracker

1. Super Star Tracker Introduction to the ISAL Study H. John Wood 8 February 2002 Revised 15 February 2002

2. Science Requirements • Two proposed projects: MAXIM and Stellar Imager are used to provide requirements on a generic star tracker. • The instruments will be located at the L2 Lagrange point and require a stable reference at the level of 30 micro arc-seconds. • This level of tracking must be held stable for hours to days. • Instruments consist of Objective and Detector spacecraft separated by up to hundreds of kilometers.

3. Derived Requirements • A laser beacon is attached to the objective s/c which appears as a star as seen from the detector s/c. • “Telescope Assembly” involves flying the two s/c into alignment with the target. • “Coarse Track” involves using the beacon and precision information on the positions of the two s/c in the inertial frame of reference to center the beacon in the super tracker FOV. • “Fine Lock” uses the laser beacon and the inertial information on the detector s/c in a feedback loop to guide the “Telescope ensemble” on the target at the required 30 micro are sec level.

4. Trade Studies • Six options were considered by the science team • Of the six options, four were looked at in detail in the ISAL study: • 1. Similar to the Hubble Space Telescope Fine Guidance Sensors (FGS) • 2. Similar to Gravity Probe-B (GP-B) • 3. Similar to GP-B but with accelerometers • 4. Similar to StarLight (Kilometric Optical Gyro w/ 4 km perimeter Sagnac Effect)

5. Option 2 Initial Design Concept • Both s/c have state-of-the-art star trackers (currently 1 arc-sec with 5 degree FOV) • The objective s/c has a laser beacon with milliwatt output – beam divergence is ~ 1 arc min • The detector s/c has a 30 cm aperture Schmidt-Cassegrain telescope with a 4-fold field detector for tracking of the beacon at the micro-arc sec level • The FOV of the beacon tracker is ~15 arc sec • Detector s/c has gyros with micro-arc sec capability

6. Option 2 Concept Con’t • The tracking loop operates on “angles only” measurements by the gyros and the beacon telescope • Control involves either angle or translation of the detector s/c • The study provides flow diagrams showing how the tracking measurements are made and how they feed into the s/c guiding loop

7. Additional Concepts • A ”Science Mode Control Loop” was developed using input from the science instrument in addition to the beacon and gyro angles • A “Beacon/Gyro Mode Control Loop” using only beacon input and gyro angles was developed using rolls and translations of the detector s/c only • This mode did not use information from the science instrument

8. Feasibility of the Concepts • The beacon tracker can be done with today’s technology and is discussed in the Optics Chapter by Dennis Evans • The gyros are based on Gravity Probe B and have a roadmap to production

9. Additional Engineering • The signal to noise ratio of the beacon tracer was studied by Eric Young to verify the optical design developed by Dennis Evans • The diffraction limited performance in object space of the 30cm diameter telescope/quad tracker looking at a mwatt-level laser at a distance of 100km is 45 µarcsec • This is more than adequate performance • The thermal evaluation by Wes Ousley showed that extreme structural stability is required between the attitude sensor and the instrument • Either extremely low CTE material would have to be developed or temperature control to fractions of a milli Kelvin would be necessary

10. Conclusion • The ISAL engineering team was generally pleased that the instrument concepts derived could be developed using existing and near-future technologies • No unobtainable technologies were required and those currently beyond state-of-the-art had credible roadmaps to fruition • Advancements in technologies discussed could change which option would be pursued