The GBT Precision Telescope Control System

1. The GBT Precision Telescope Control System Richard Prestage, Kim Constantikes, Dana Balser, Jim Condon

2. How to make a 100m telescope work at 50GHz (…with plans for 115GHz) • Overview of GBT and the PTCS project • Thermal Effects and their compensation • Measurement of wind and servo effects

3. The GBT is large….

4. Telescope Structure and Optics

5. Telescope Structure and Optics

6. Telescope Structure and Optics

7. Telescope Structure and Optics

8. Scientific Requirements

9. PTCS Project • Aim of the project is to deliver 3mm operation. • Includes instrumentation, servos (existing), algorithm and control system design, implementation. • As delivered antenna => 15GHz operation (Fall 2001) • Active surface and initial pointing/focus tracking model => 26GHz operation (Spring 2003) • PTCS project initiated November 2002: • 50GHz operation: Fall 2003 (November) • 90GHz operation: Winter 2004/05 • Full 115GHz: Winter 2005/06

10. Gravity/Temperature Effects - Focus Temperature Effect Gravitational Effect Measure focus over short time period NCP source 0117+8928

11. Gravity/Temperature Effects - Pointing

12. Structural Temperatures

13. Structural Temperatures

14. Algorithms • Use existing GBT gravity pointing and focus models • Structure is linear: Thermal effects superpose • Temperature effect on focus, pointing assumed linear in temperatures • No dependence on air or bulk temps, just differences • Simultaneously estimate gravity and temperature model coefficients • Estimate coefficients using 9/11, 10/2, 11/10 data • Test models using 9/5, 11/20 data

15. Term Coefficient Min-Max Significance Parameter M1 1.086 13.1 14.3 SR-Pri M2 -0.697 6.2 -4.3 VFA-Pri M3 3.981 15.6 62.0 HFA M4 -7.326 0.9 -6.8 BUS V1 M5 -0.688 12.1 -8.3 BUS V2 M6 -2.576 12.1 -31.2 BUS F M7 -180.630 0.0 0.0 Offset M8 66.189 .7 43.1 sin term M9 196.949 0.6 110.8 cos term Focus Model

16. Focus Model Estimation

17. Focus Model Test

18. Term Coefficient Min-Max Significance Parameter M1 -4.6455 1.2 -5.3 BUS M2 1.7830 15.6 -27.8 HFA M3 4.4488 5.9 26.4 VFA M4 -8.4477 1.6 -14.0 Alidade M5 62.2218 0.0 +0.000 -IE,d(0,0) M6 -55.8624 0.7 -62.792 HZCZ,b(0,1) M7 -22.8268 0.9 -38.216 HZSZ,d(0,1) M8 2.4960 2.0 +2.169 -AW,c(1,0) M9 -1.3360 2.0 -1.750 AN,d(1,0) Elevation Model

19. Elevation Model Estimation

20. Elevation Model Test

21. Thermal Compensation Results • Significantly improved “static” gravity models. • Focus peformance ~< 3 mm (excludes midday) during ~30 mm thermal focus shift. • Elevation performance ~<3” 1s , <1”/hour (excludes midday) during ~ 30” thermal pointing shift. • Azimuth performance ~<3” 1s , <1”/hour (excludes midday). • Unanticipated dominance of horizontal feed arm influence.

22. Tracking Stability: Servo and Wind • Thermal effects important on timescales ~ 0.5 hours • Short term tracking stability dominated by: • Wind • Servo disturbances • We are starting to characterize the effects • Possibility of compensation looks promising

23. 14GHz half-power track

24. 14GHz half-power track

25. 14GHz half-power track

26. Servo effects

27. Effects of wind

28. Effects of Wind

29. Future Developments Prototyping, Commissioning Experiments and Transition to Production Capabilities Enable W-Band Performance Under Benign Conditions and Q-Band Performance Under Normal Conditions

30. Conclusions • GBT is capable of 50GHz operation under benign conditions: Blind Pointing: (1 point/focus) Offset Pointing: (90 min) Continuous Tracking: (30 min)

31. Conclusions • Largest “non-repeatable” effects are thermal and wind. • Thermal compensation works well apart from around mid-day; may be extended to all conditions. • Next development: inclinometers: • Az-track irregularities • Confirmation of alidade thermal pointing effects • Wind compensation on ~10s timescales • Servo disturbances are clearly visible - good chance that we will be able to compensate for these.

32. Acknowledgements • Joe Brandt, Ray Creager, Jeff Cromer, Paul Marganian, J.D. Nelson, Jason Ray. • PTCS Project Team.