Jan 6 2006 URSI 1
Fast Switching Phase Compensation for ALMA
Mark HoldawayNRAO/Tucson
Other Fast Switching Contributors:Frazer Owen
Michael Rupen
Chris Carilli
Simon Radford
Larry D’Addario
Frank Bertoldi
Jan 6 2006 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)
Jan 6 2006 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)
Jan 6 2006 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!
Jan 6 2006 URSI 5
Effective phase compensation will be required for ALMA to meet ANY science
goals
Fast Switching?
WVR?
…or a Hybrid?
Jan 6 2006 URSI 6
Effective phase compensation will be required for ALMA to meet ANY science
goals Fast Switching:σφ ≈ √ D(d)
instead ofσφ ≈ √ D(baseline)
Jan 6 2006 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 Δt term
Jan 6 2006 URSI 8
Interpolation helps too…
Jan 6 2006 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
Jan 6 2006 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
Jan 6 2006 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
Jan 6 2006 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.
Jan 6 2006 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 and νcal
Jan 6 2006 URSI 14
High Frequency Scheme:
Details, such as the target sequence cycle time, can be determined through sensitivity optimization.
Jan 6 2006 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.
Jan 6 2006 URSI 16
An example calibrator field
90 GHz 250 GHz
Jan 6 2006 URSI 17
An example distribution of fast switching efficiency
Switching Efficiency:(e –σ2/2) ( ton / tcycle )0.5
Jan 6 2006 URSI 18
Median Efficiency for several observing frequencies as a function of Phase Conditions
Jan 6 2006 URSI 19
Collapse the Distribution of Atmospheric Conditions by assuming dynamic
scheduling will match high ν with high phase stability!
Jan 6 2006 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
Jan 6 2006 URSI 21
Jan 6 2006 URSI 22
Efficiency results factoring in atmospheric conditions and instrumental phase
specification
Jan 6 2006 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
Jan 6 2006 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
Jan 6 2006 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
Jan 6 2006 URSI 26
After NRAO?
Jan 6 2006 URSI 27
Fast Switching Phase Compensation for ALMA