Jan URSI1 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

Jan URSI2 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 –σ 2 /2 ), limits image quality by putting emission in the wrong place, variable decorrelation spoils resolution (phase errors increase with baseline length)

Jan URSI3 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)

Jan URSI4 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!

Jan URSI5 Effective phase compensation will be required for ALMA to meet ANY science goals Fast Switching? WVR? …or a Hybrid?

Jan URSI6 Effective phase compensation will be required for ALMA to meet ANY science goals Fast Switching: σ φ ≈ √ D(d) instead of σ φ ≈ √ D(baseline)

Jan URSI7 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 Δt term

Jan URSI8 Interpolation helps too…

Jan URSI9 Fast Switching Cycle can be optimized for sensitivity Optimal calibrator minimizes (vt+d): to quantify, we need source count info F.S. Efficiency == (e –σ 2 /2 ) ( t on / t cycle ) 0.5 Over-calibrating: sensitivity lost from time Under-calibrating: sensitivity lost from decorrelation

Jan URSI10 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

Jan URSI11 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

Jan URSI12 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.

Jan URSI13 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 ν target/ ν cal Need an additional calibration to determine instrumental phase drifts uncommon to ν target and ν cal

Jan URSI14 High Frequency Scheme: Details, such as the target sequence cycle time, can be determined through sensitivity optimization.

Jan URSI15 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.

Jan URSI16 An example calibrator field 90 GHz250 GHz

Jan URSI17 An example distribution of fast switching efficiency Switching Efficiency: (e –σ 2 /2 ) ( t on / t cycle ) 0.5

Jan URSI18 Median Efficiency for several observing frequencies as a function of Phase Conditions

Jan URSI19 Collapse the Distribution of Atmospheric Conditions by assuming dynamic scheduling will match high ν with high phase stability!

Jan URSI20 Results (per band): Median cal flux 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

Jan URSI21

Jan URSI22 Efficiency results factoring in atmospheric conditions and instrumental phase specification

Jan URSI23 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

Jan URSI24 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 Δ φ. This requires that we NOT interpolate the fast switching phase solutions, and also requires that the electronic phase be fairly stable

Jan URSI25 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

Jan URSI26 After NRAO?

Jan URSI27 Fast Switching Phase Compensation for ALMA