1. Current GBT Performance

2. Systematic Focus Errors
Axial Focus Offset: air temperature versus elevation

3. Gravitational & Temperature Focus Effects
Temperature Effect
Gravitational Effect
Measure focus over short time period
NCP source

4. Systematic Pointing Errors

5. Plan of Attack
Rationalize data acquisition
Rationalize data analysis
Implement temperature sensor system
Concentrate efforts on pointing and focus
Characterize performance with X-band (8 -10 GHz)

6. Data Quality Gaussian Fits (Az, El, Focus) Polarization (LCP – RCP)
Direction (Forward – Backward)
Jack Scan

7. Data Quality: Pointing (LCP – RCP)

8. Data Quality: Pointing (LCP – RCP)
Flux (9 GHz) > 0.08 Jy
RMS < 2.5 arcsec < 0.03 HPBW

9. Data Quality: Focus (LCP – RCP)
Flux (9 GHz) > 0.08 Jy
RMS < 3.8 mm << focus beamwidth

10. Data Quality: Pointing (F – B)

11. Data Quality: Wind

12. Observational Strategy
NCP source to probe temperature effects
Up-down sources at night to probe gravity effects
All-sky for a combination

13. All-sky (Az, El) pointing
Initial Results
All-sky (Az, El) pointing
Removed systematic pointing residuals
Simultaneous fits to gravity and temperature
Separate two effects
Refine focus and pointing models

14. Current Performance Model: Datasets: Metrics
Simultaneous solution to temperature and gravity effects
Datasets:
All-sky pointing run (2003 Nov 20)
Half-power tracking (2003 Nov 20)
Metrics
Blind pointing
Offset pointing
Continuous Tracking

15. Blind Pointing: Azimuth

16. Blind Pointing: Elevation

17. Blind Focus

18. Offset Pointing: Azimuth and Elevation

19. Offset Focus

20. Tracking: Ku-band (14 GHz) El half-power track

21. Current Performance: (NO temperature corrections)
Benign Conditions: (1) Exclude 10:00  18:00
(2) Wind < 2.5 m/s
Blind Pointing:
(1 point/focus)
Offset Pointing:
(90 min)
Continuous Tracking:
(30 min)

22. Supplemental Material

23. Blind Focus

24. Blind Pointing: residuals versus (A, Z)

25. Blind Pointing: residuals versus time

26. Blind Pointing: 2-D RMS Raw Residuals One PEAK at the start of
the evening

27. Blind Pointing: Azimuth

28. Blind Pointing: Elevation

29. Blind Focus

30. Tracking: Ku-band (14 GHz) El half-power track

31. Tracking: Ku-band (14 GHz) El half-power track