Fast Switching Phase Compensation for ALMA

1. Fast Switching Phase Compensation for ALMA Mark Holdaway NRAO/Tucson Other Fast Switching Contributors: Frazer Owen Michael Rupen Chris Carilli Simon Radford Larry D’Addario Frank Bertoldi URSI

2. At Millimeter wavelengths,The Atmosphere Messes Us Up So, we went to 5000m to escape the atmosphere! 55% of the oxygen, 5% of the water vapor Opacity: absorbs mm radiation, thermal radiation increases noise, and opacity fluctuations make calibration and imaging problematic. Phase flucuations: wrecks sensitivity – decorrelation ( e –σ2/2 ), limits image quality by putting emission in the wrong place, variable decorrelation spoils resolution (phase errors increase with baseline length) URSI

3. Site Testing at Chajnantor Since 1995, site monitoring has found that 230 GHz opacity is very low and often shows no diurnal effect (ie, many continuous hours of high frequency observing) URSI

4. Site Testing at Chajnantor Monitoring of phase errors at 11.2 GHz on a 300m baseline indicate phase errors are still a huge problem! The median phase errors on 300m baselines at 230 GHz result in 50% decorrelation loss in sensitivity if not corrected! So, lets get CORRECTING! URSI

5. Effective phase compensation will be required for ALMA to meet ANY science goals Fast Switching? WVR? …or a Hybrid? URSI

6. Effective phase compensation will be required for ALMA to meet ANY science goals Fast Switching: σφ≈√ D(d) instead of σφ≈√ D(baseline) URSI

7. Fast switching “cuts off” the structure function at some “effective baseline” That effective baseline is about: (v Δt+ d)/2 v = atm. vel. =10-15m/s Δt = cycle time We are dominated by the Δtterm URSI

8. Interpolation helps too… URSI

9. Fast Switching Cycle can be optimized for sensitivity • Optimal calibrator minimizes (vt+d): to quantify, we need source count info • F.S. Efficiency == (e –σ2/2) ( ton / tcycle )0.5 • Over-calibrating: sensitivity lost from time • Under-calibrating: sensitivity lost from decorrelation URSI

10. Source counts at 90 GHz Blind Survey at 90 GHz too slow (400 sq deg down to 10 mJy) So, we target compact, flat spectrum quasars, observe at 90 GHz, determine the spectral index distribution, and scale 5GHz flat spectrum counts URSI

11. Source counts at higher freqs By solving for a distribution of break frequencies and assuming optically thick α~0, optically thin α~0.8, we can extrapolate to higher frequencies URSI

12. Source counts at 250 GHz Flat spectrum quasars observed with MAMBO as pointing sources can be used to estimate 250 GHz source counts: slightly higher than our extrapolation by fitting a break frequency distribution. URSI

13. What will we do at high frequencies? • At high ν, source counts decline, sensitivity declines, can’t fast switch! • Plan: calibrate at 90 GHz and scale the phase solutions by νtarget/νcal • Need an additional calibration to determine instrumental phase drifts uncommon to νtarget andνcal URSI

14. High Frequency Scheme: Details, such as the target sequence cycle time, can be determined through sensitivity optimization. URSI

15. How do we quantify the switching details? Statistical approach: • Simulate ~1000 calibrator fields, with S = f(ν) • Select the optimal calibrator: min(vt+d) • Calculate efficiency for calibrating at the target ν AND for calibrating at 90 GHz. • Results: for each band, we get a distribution of residual phase errors and a distribution of efficiencies. URSI

16. An example calibrator field 90 GHz 250 GHz URSI

17. An example distribution of fast switching efficiency Switching Efficiency: (e –σ2/2) ( ton / tcycle )0.5 URSI

18. Median Efficiency for several observing frequencies as a function of Phase Conditions URSI

19. Collapse the Distribution of Atmospheric Conditions by assuming dynamic scheduling will match high ν with high phase stability! URSI

20. Results (per band): • Median cal flux 50-100 mJy • Median cal time <1 s • Median slew time <1 s • Median cycle times 20-30s • Median Eff: 0.7 – 0.9 • Note: calibrating at the target frequency will be more efficient below about 300 GHz • Inst. Cal flux: 1 Jy  0.3 Jy URSI

21. URSI

22. Efficiency results factoring in atmospheric conditions and instrumental phase specification URSI

23. What About WVR? • Fast Switching has significant decorrelation • but: • WVR cannot solve for absolute phase, just incremental phase fluctuations • WVR cannot solve for instrumental phase, just atmospheric phase fluctuations • WVR cannot solve for any dry fluctuations URSI

24. Phase Calibration Hope: Instead of doing 20-30s fast switching cycles, to perform 300s switching cycles and use WVR to determine phase increments. FS can help determine the variable conversion between ΔT and Δφ. This requires that we NOT interpolate the fast switching phase solutions, and also requires that the electronic phase be fairly stable URSI

25. Of course, MORE WORK IS NEEDED! • Prototype antennas do meet slewing spec: 1.5deg in 1.5s • Check out fast switching interferometrically on P.I. • Start collecting information on fast switching calibrators (down to about 25 mJy – about 30,000 sources) • Understand more about the cal sources at high frequency • Observe these sources on long baselines • Simulations of WVR + Fast Switching: In progress • Keep in touch with the Software Guys URSI

26. After NRAO? URSI

27. Fast Switching Phase Compensation for ALMA URSI