review of wvrs in astronomy alan roy mpifr (wiedner)

Review of WVRs in Astronomy

Alan Roy MPIfR

(Wiedner)

The Troposphere as Seen from Orbit

Method: Synthetic Aperture Radar (Earth Resources Satellite)Frequency: 9 GHzRegion: GroningenInterferograms by differencing images from different days

5 km

5 km

Internal waves in a homo-genously cloudy troposphere

A frontal zone Convective cells

0 mm

-100 mm

100 mm

Hanssen (1997)

Coherence Loss due to Troposphere

Pico Veleta – Onsala baselineSource: BL LacFrequency: 86 GHz

Coherence Function

7 min

360°

VLBI phase time series

Phase Referencing Errors due Troposphere

WVR Performance Requirements

Phase Correction

Aim: coherence = 0.9 requires / 20 (0.18 mm rms at = 3.4 mm) after correction

Need: thermal noise 14 mK in 3 s Need: gain stability 3.9 x 10-4 in 300 s

Zenith Delay for Phase Referencing

Aim: transfer phase over 5o with 0.1 rad error at 43 GHz

Need: absolute ZWD with error < 1 mm (?)

WVR Performance Requirements

Opacity Measurement

Aim: correct visibility amplitude to 1 % (1 )

Need: thermal noise 2.7 KNeed: absolute calibration 14 % (1 )

Phase Correction MethodsUse a nearby strong calibrator

a) Interleave source and calibrator observations BUT: must cycle fast -> short integrations -> few calibrators strong enough

b) Dual beam: observe simultaneously calibrator and source (VERA) BUT: need duplicate moveable receiver

c) Dual frequency: observe target source at lower frequency scale up phase to calibrate the higher frequency

BUT: scaling up multiplies the phase noise; need very good low-frequency observation

d) Paired antennas: one observes target, one observes calibrator (Asaki 1997)

Measure the water vapour and infer the phase

a) Total power method

b) Radiometric phase correction (eg at 22 GHz, 183 GHz or 20 um)

Telescope Technique Freq Path Residual / mm dG/G dT in 1 s

VLA WLM 22 GHz cooled 0.81 0.6x10-4 (100 s) 20 mKPlateau de Bure WLM 22 GHz uncooled 0.031 7.5x10-4 (30 min)

Plateau de Bure TP 230 GHz cooled 0.041 2x10-4

Pico Veleta TP 230 GHz cooled 0.24OVRO WLM 22 GHz uncooled 0.16 10 mKBIMA TP 90 GHz cooled 0.17BIMA WLM 22 GHz uncooled 0.1 5x10-3

CSO-JCMT WLM 183 GHz uncooled 0.06SMA TP 230 GHz cooled 0.09 2x10-4

SMA WLM 183 GHz uncooledATCA WLM 22 GHz cooled 0.3 12 mKEffelsberg WLM 22 GHz uncooled 0.24 5x10-4 (100 s) 12 mKVLBA TP 86 GHz cooled 0.6Chatnantor WLM 183 GHz uncooled 0.08 2x10-3 (100s)

DSN WLM 22 GHz uncooled 0.21 25 mK (8 s)

IRMA WLM 15 THz cooled

WVR Phase Correction Performance Comparison

= represented at this meeting = lowest rms phase demonstrated

Total Power Phase CorrectionPlateau de BureTotal power at 230 GHzCorrection applied to simultaneous 90.6 GHz

Bremer 1995, 2000

3 mm

30 min

Phase correction

Observed phase: rms = 0.623 mm

Corrected phase: rms = 0.167 mm

Total Power Phase Correction: VLBI demoPico Veleta - OnsalaTotal power at 230 GHzCorrection applied to simultaneous 86 GHz VLBI

Bremer et al. 2000

4.7 mm

6 min

Observed phase: rms = 0.71 mm

Corrected phase: rms = 0.45 mm

Phase correction

Owens Valley Radio Observatory (Caltech)

(Array before moving to Cedar Flat)

Frequencies: 86 - 115 GHz 210 – 270 GHzAntenna diam: 10.4 mAltitude: 1220 m

Owens Valley Radio ObservatoryWoody, Carpenter, Scoville 2000, ASP Conf Ser 217, 317

Uncooled LNA(Tsys = 200 K)

Downconvert to 4 GHz to 12 GHz(cheaper components,better characterized)

Triplexer separates 2 GHzBands on line and off-line18.2 to 20.2, 21.2 to 23.2, 24.2 to 26.2 GHz

Analog sum of wingchannels for continuum

Analog difference of line and continuum channels

Alternate L and C every 1.7 ms

to 16-bit A/D

363 K load

Ambient load

Cold load (optional)

Owens Valley Radio Observatory

Woody et al. (2000)

Two levels of Dicke switching reduce effects of gain and offset drifts:

1) PIN-diode attenuators adjust the Line-Continuum output to be zerofor blackbody loads; output measures deviation from a flat spectrum.

2) Transfer switch reverses assignment of Line and Continuum to thedetectors every 1.7 ms; demodulation is performed in software-> removes DC offsets and most of the gain drifts in detectors and following electronics

Results:1) Allan Variance -> noise in L - C < 10 mK for 20 s to 20 min while noise in L & C > 30 mK -> analog L – C differencing and transfer switch modulation valuable

2) C1 & C2 channels derived from -10 dB coupler have 10x more noise -> standard radiometer noise is not the dominant noise

3) White noise to 1 s in L or C channels separately White noise to 10 s in L-C channel


Woody et al. (2000)

Calibration

Once per hour hot & ambient load

Solve for gain, Tsys, and drift in offset of L-C channel

Accuracy of gain determination: 1 %Noise in offset determination: 20 mK


26 min

3 m

m

interferometer path at 100 GHz WVR predicted path

RMS before correction = 0.53 mmRMS after correction = 0.16 mm

Woody et al. (2000)


Woody et al. (2000)

Path Length Retrieval

Observe a strong calibrator -> conversion factor

Typically use a fixed 12 mm/Kcf calculated conversion factor of 8 mm/K

Difference is “within the uncertainties of the triplexerbandpass shapes and atmospheric model assumptions”


Woody et al. (2000)


Woody et al. (2000)

Transferring phase between calibrator and source: hard! (due to gradient in sky brightness)must normalize gains among the WVRs using the step due to elevation change

Average L-C from all WVRs / K

L-C from eachWVR / K


Woody et al. (2000)

0309+411 at 100 GHz for 5 hCycle: 6 min source, 6 min calibrator (0.7 degrees away)WVR phase is transferred from calibrator to source

Before WVRcorrection

After WVRcorrection

(weather degraded)(good weather)

28 Jy

40 Jy

36 Jy

42 Jy

13 Jy

34 Jy


Woody et al. (2000)

Conclusion

Can correct tropospheric phase fluctuations down to < 0.2 mm.

Allows 3 mm observations in previously unusable weather.

Not sufficient for improving images during typical conditionsOr for routine use during 1 mm observations.

Developing a cooled version to decrease noise to reach 0.05 mm.

Staguhn et al. 2001, ASP Conf: First light on prototypeCooled 22 GHz WVRDouble sideband heterodyne0.5 GHz to 4 GHz IF16 channel analogue lag correlator (APHID)

(see Alberto Bolatto’s talk)

JCMT – CSO Interferometer

Frequencies: 210 – 270 GHz 318 – 360 GHz Higher than OVRO 460 – 500 GHzAntenna diam: 10.4 m & 15 mAltitude: 4092 m Higher than OVROLocation: Hawaii

James Clark Maxwell Telescope (JCMT)

Caltech SubmillimeterObservatory (CSO)

JCMT – CSO: 183 GHz WVRs

Line pivot points: least sensitive to altitude of water vapour

Wiedner 1998 PhD thesisWiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

JCMT – CSO: 183 GHz WVRsWiedner 1998 PhD thesisWiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

The three double-sideband frequency channels of the WLM


Advantages of 183 GHz over 22 GHz:

- line is 10 x stronger than 22 GHz. -> can build uncooled systems - optics are small -> easier to install in existing telescopes

Disadvantages of 183 GHz:

- line saturates easily -> suitable only for dry sites - retrieval coefficient depends on amount of water vapour and conditions


Calibration- Loads at 30 C and 100 C- Load stability: 10 mK- Flip mirror cycles every 1 s between sky and loads

10 mK

5 min

Load temperature vs time

Sectioned drawing of load


Hot load Warm load

Mirror 2

Mirror 1

Corrugated horn (facing away)


Uncooled mixer(Tsys = 2500 K)

Coupler Mixer Filter Detector V/F Powersplitter

1.2 GHz 4.2 GHz 7.8 GHz Oscillators

Gunn oscillator91.655 GHz

183.31 GHz+/- 8 GHz

Double-sideband mixing makesmeasurement insensitive to filter shape

Used coupler + power splitter since no suitable triplexer exists


A small shift in the centre frequency of a filter makes a big change in the measured brightness temperature since the line is steep.Thus, need filter shape within 5 MHz of spec. No triplexer matched this.


DSB mixing to baseband folds water line at oscillator frequencyResult is flat water line spectrumWater line spectrum is then same as the calibration load spectrumCalibration factor is then independent of the filter shape


Gain fluctuations of WVR measured against loads each second9 min

2x10-4 10x10-4

WVR at JCMT

WVR at CSO (outside, so less stable environment)


12 min

1.4 mm

Maser source MWC 349at 356 GHz

After correctionRMS = 48d = 0.11 mm

Before correctionRMS = 127d = 0.30 mm

WVR correction

Atmospheric model: transition strengths from Waters (1976), Ben-Reuven line profile, exponential atmosphere, radiative transfer calculation

Sub-Millimeter Array

183 GHz WVRs being installed.

(Talks by Ross Williamson & Richard Hills)

CSO JCMT SMA

Very Large Array

22 GHz WVRs being prototyped.

(Talk by Walter Brisken)

Image courtesy NRAO/AUI

Dave Finley

Plateau de Bure

22 GHz WVRs in routine operation.

(Talks by Michael Bremer & Aris Karastergiou)

Effelsberg

22 GHz sweeping WVR operating.

(Talk by Alan Roy)

Berkeley-Illinois-Maryland Array

22 GHz sweeping WVRs prototyped.

Array relocated to Cedar Flat with OVRO antennas Now called CARMA.

(Talk by Alberto Bolatto)

VLBI Phase Correction Demo

Demonstration by Tahmoush & Rogers (2000) 3C 273Hat Creek – Kitt Peak86 GHz VLBI

400 s

4 mm

path

● RMS phase noise reduced from 0.88 mm to 0.34 mm after correction.● Coherent SNR rose by 68 %.

VLBI phase

WVR phase

CARMA

(Talk by Alberto Bolatto)

Jim Stimson Photography

Chajnantor Site Testing

Delgado et al. 2001, ALMA Memo 361

Two 183 GHz WVRs 300 m apartDuplicates of JCMT-CSO WVRs (Hills/Wiedner)Co-located with two 11.2 GHz seeing interferometers observing a geostationary satellite



Correlation coefficient between WVR and interferometers varied.

Cause: when turbulence is lower than 300 m it lies in near-field of interferometer antennas causing large beam differences between the instruments (?)

Australia Telescope Compact Array

Frequencies: 1.2 - 106 GHz Antenna diam: 22 mAltitude: 300 m

ATCA 22 GHz WVR

ATCA WVR Frequencies

ATCA Phase Correction Demo

NASA Deep Space Network

22 GHz to 32 GHz WVR (Tanner et al.)For Cassini gravity wave experiment

Naudet et al. (2000)


Need: 10 mK radiometric stability from 100 s to 10000 s

Focus: improve precision and stability of noise diode and Dicke switch

Methods: 1) Regulate temperature in radiometer box to 1 mK. 2) bought commercial noise diodes. 3) follow instructions to bias with regulated 28 V.

-> poor stability: 20 x 10-4 in 10 s - 100 s 4) try current-regulating bias circuit -> immediate improvement to 1 x 10-4 in 100 s, 5 x 10-4 in 1 day

5) replace magic T power combiner with directional couplers due to extreme sensitivity to mismatch (-40 dB reflection caused 4 % change of noise diode power) -> 1 x 10-4 in 1 day 6) regulate the relative humidity -> 0.3 x 10-4 in 1 day

7) Dicke switch using absorber inserted in slotted waveguide by loudspeaker voicecoil

Tanner et al. (1998)


RMS before correction = 0.43 mmRMS after correction = 0.1 mm

Naudet et al. (2000)

4 h

2 mm

Conclusion

Reviewed 5 of 16 WVRs for astronomy (7 radiometers tomorrow)

Many clever techniques are available for use

Lowest residual path 0.031 mm

review of wvrs in astronomy alan roy mpifr (wiedner)

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