1. Oct 13-15, 2003 ADASS 2003 The GBT Precision Telescope Control System Kim Constantikes

2. ADASS 2003 2 Introduction Brief overview of the Green Bank Telescope (GBT) and scientific requirements on pointing, efficiency,etc. Experimental results Brief overview of the Precision Telescope Control System (PTCS) Overview our exploratory data processing environment, the Engineering Measurement System (EMS)  The GBT is performing surprisingly well  We are enhancing instrumentation and investigating GBT phenomenology at level of arcsecs, 100’s of microns wavefront error  The EMS has been used to perform GBT laser rangefinder surveys at millimeter levels, will be used for other GBT investigations and prototype real-time compensations

3. ADASS 2003 3 Telescope Structure and Optics Offset-Gregorian design Operation to 115 GHz, 40 GHz winter 2003-2004 Optics: 110 m x 100 m of a 208 m parent paraboloid Effective diameter: 100 m Off axis feedarm Elevation Limit: 5° Slew Rates: Azimuth - 40°/min; Elevation - 20°/min Main Reflector: 2209 actuated panels with 68  m rms. Total surface: rms 400  m FWHM Beamwidth: 740"/f(Ghz) Prime Focus: Retractable boom Gregorian Focus: 8-m elliptic subreflector with 6-degrees of freedom Rotating Turret with 8 receiver bays

4. ADASS 2003 4 Telescope Structure and Optics

5. ADASS 2003 5 Scientific Requirements for High-Frequency Observing

6. ADASS 2003 6 Pointing Accuracy, 5° C Gradient, 5° El

7. ADASS 2003 7 The Real vs. Ideal GBT: All-Sky Pointing

8. ADASS 2003 8 The Real vs. Ideal GBT: Half-Power Tracking

9. ADASS 2003 9 The Real vs. Ideal GBT: Efficiency and Beam Shape

10. ADASS 2003 10 The Real vs. Ideal GBT: Wind Effects

11. ADASS 2003 11 The Real vs. Ideal GBT: Servo Effects and Structure Vibration

12. ADASS 2003 12 The Real vs. Ideal GBT: Temperature and Focus

13. The Real vs. Ideal GBT: Temperature Gradients

14. ADASS 2003 14 PTCS: Paradigms, Models PTCS composed of four elements (April 2003 CODR): –High Frequency Observing System: The observer’s interface –Engineering Measurement System: Algorithm/analysis/control exploration tools –Precision Measurement System: Production measurements –Precision Control System: Control and models wrapped around existing Antenna Manager (M&C) components Alternative approaches for risk mitigation: –Direct measurement of figure/position/orientation, additional closed loop control –Empirical corrections to pointing, focus –Model structure, measure perturbing influences (mainly wind and thermal gradients) Direct measurements not currently sufficient for 100 Ghz, –Empirical methods in development and test –Some models in use: FEM adjustment of primary, structure linearity/superposition –Considering new models to identify/predict perturbations (thermal-structural model), plate scales and abberations (optical model) Additional closed loop control not currently needed –Structure vibration is typically small, not significantly excited by shaped trajectories or wind-pumping, lowest structure mode ~ 0.6 Hz Current emphasis is antenna characterization using multiple instruments and astronomical inferences

15. ADASS 2003 15 PTCS: Instruments (Current) 12 laser rangefinders in ring around GBT 19 precision structural temperature sensors –Thermal corrections for pointing, primary shape (?) 2 precision air temperature sensors –Characterize group refractive index variation degradation to laser rangefinder accuracy, convective heat transfer model for GBT structure Quadrant Detector –Measure feed arm position (angle-angle) from elevation axle 3 weather stations Servo monitoring for azimuth and elevation drives –Torques, rates, etc. 1 2-axis 10  G accelerometer set on FA tip

16. ADASS 2003 16 Laser Rangefinder Geometry

17. ADASS 2003 17 PTCS: Instruments (in Development) 2 2-axis clinometers (0.2 arcsec) on elevation bearings –Map azimuth track variations Additional air temperature sensors –Characterize vertical refractive index, convective heat transfer Additional 3-axis accelerometer sets –Compensation for vibration (need high-bandwidth tertiary element) –Structural health monitoring via modal analysis Hot wire (self-heating thermistor) wind speed –Measure 3-D wind with low time constants, compensate with tertiary element Wide-field star tracker –Measure differential orientation of locations on structure, e.g., subreflector actuator mount points

18. ADASS 2003 18 Engineering Measurement System Algorithm development and numerical analysis Initially for real-time laser rangefinder multilaterations Continues as exploratory environment for production measurement and control Consists of: –Top level signal flow-graph representation, data-driven process: Wit –Data transport to/from monitor and control: TCP and SOAP –Database connectivity for calibrations, configurations, etc. –Scripting/numerical/visualization environment: Matlab –API to C, C++, etc Wit provides easy probing of graph, drag and drop dataflow design Matlab has rich algorithm and visualization environment Databases organize large datasets, e.g., retro glass offsets, instrument calibrations SOAP transport provides direct access to GBT M&C data Wit and Matlab provide migration to standalones, libraries

19. ADASS 2003 19 Example: Multilateration with laser rangefinders –Initialize coordinate transforms, rangefinder calibrations, retro excess path lengths, etc. from database –Calculate and smooth path group refractive index, rangefinder zero points and “leakage” signals –Correct measured path phases –Predict range from rangefinder to target using structural FEM –Calculate measured range from corrected phases, predicted ranges –Select range data by target –Non-linear least-squares estimate of retro position from ranges and rangefinder positions –Save intermediate,final results to database; real-time probe and plot quantities of interest

20. ADASS 2003 20 Example: Multilateration with laser rangefinders Graph edges carry primitive data types, structures, generic objects Graph nodes execute when input data are available Hierarchical graphs modularize operators Graph state defined by edge data, Matlab engine retains state in global structures Can probe or break on edges, pause execution, and use full MATLAB IDE in paused state Node parameters can be changed on the fly, parameters can be promoted to inputs, etc. Database operations via ODBC and SQL

21. ADASS 2003 21 Example: Multilateration with laser rangefinders

22. ADASS 2003 22 Other Applications Prototype real-time visualization and correction of low-rate effects: –Thermal pointing errors: Structural and air temp monitoring Predictive algorithm, e.g. linear regression SOAP/TCP to/from GBT Monitor and Control System –Alidade tilts and azimuth track flatness: Elevation bearing clinometers SOAP/TCP. –Wind-induced feed-arm motion: Quadrant Detector Pointing coefficients SOAP/TCP

23. ADASS 2003 23 Additional Information on PTCS Project documentation can be found at: http://wiki.gb.nrao.edu/bin/view/PTCS/WebHome CODR Project and System Notes Various design and status information Information for Astronomers (under construction) Experiments and results See our poster The GBT Engineering Measurement System (P9.9) for more details…

24. ADASS 2003 24 Additional Slides

25. ADASS 2003 25 Telescope Structure and Optics: Active Surface

26. ADASS 2003 26 Scientific requirements for high-frequency observing

27. ADASS 2003 27 Scientific requirements for high-frequency observing

28. ADASS 2003 28 Pointing Accuracy, 6 m/s wind

29. ADASS 2003 29 The Real vs. Ideal GBT: Gravity Elevation Delta-X (mm) Delta-Y (mm) Delta-Z (mm)

30. ADASS 2003 30 The Real vs. Ideal GBT: Temperature and Azimuth

31. ADASS 2003 31 The Real vs. Ideal GBT: Temperature and Elevation

32. ADASS 2003 32 Architecture of current GBT Observing System

33. ADASS 2003 33 Architecture of HFOS

34. ADASS 2003 34 Architecture of PCS