Jan 6 2006 URSI 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

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 of

σφ ≈ √ 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