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Review of WVRs in Astronomy. (Wiedner). Alan Roy MPIfR. The Troposphere as Seen from Orbit. Method: Synthetic Aperture Radar (Earth Resources Satellite) Frequency: 9 GHz Region: Groningen Interferograms by differencing images from different days. 100 mm. 0 mm. 5 km. -100 mm.

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slide1

Review of WVRs in Astronomy

(Wiedner)

Alan Roy MPIfR

slide2

The Troposphere as Seen from Orbit

Method: Synthetic Aperture Radar (Earth Resources Satellite)

Frequency: 9 GHz

Region: Groningen

Interferograms by differencing images from different days

100 mm

0 mm

5 km

-100 mm

A frontal zone

Convective cells

5 km

Internal waves in a homo-

genously cloudy troposphere

Hanssen (1997)

slide3

Coherence Loss due to Troposphere

VLBI phase time series

Coherence Function

360°

7 min

Pico Veleta – Onsala baseline

Source: BL Lac

Frequency: 86 GHz

slide5

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 (?)

slide6

WVR Performance Requirements

Opacity Measurement

Aim: correct visibility amplitude to 1 % (1 )

Need: thermal noise  2.7 K

Need: absolute calibration  14 % (1 )

slide7

Phase Correction Methods

  • Use 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)
slide8

WVR Phase Correction Performance Comparison

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 mK

Plateau 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 0.24

OVRO WLM 22 GHz uncooled 0.16 10 mK

BIMA TP 90 GHz cooled 0.17

BIMA WLM 22 GHz uncooled 0.1 5x10-3

CSO-JCMT WLM 183 GHz uncooled 0.06

SMA TP 230 GHz cooled 0.09 2x10-4

SMA WLM 183 GHz uncooled

ATCA WLM 22 GHz cooled 0.3 12 mK

Effelsberg WLM 22 GHz uncooled 0.24 5x10-4(100 s) 12 mK

VLBA TP 86 GHz cooled 0.6

Chatnantor 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

= represented at this meeting

= lowest rms phase demonstrated

slide9

Total Power Phase Correction

Plateau de Bure

Total power at 230 GHz

Correction applied to simultaneous 90.6 GHz

Phase correction

3 mm

Observed phase: rms = 0.623 mm

Corrected phase: rms = 0.167 mm

30 min

Bremer 1995, 2000

slide10

Total Power Phase Correction: VLBI demo

Pico Veleta - Onsala

Total power at 230 GHz

Correction applied to simultaneous 86 GHz VLBI

Observed phase: rms = 0.71 mm

4.7 mm

Phase correction

Corrected phase: rms = 0.45 mm

6 min

Bremer et al. 2000

slide11

Owens Valley Radio Observatory (Caltech)

(Array before moving to Cedar Flat)

Frequencies: 86 - 115 GHz

210 – 270 GHz

Antenna diam: 10.4 m

Altitude: 1220 m

slide12

Owens Valley Radio Observatory

Woody, Carpenter, Scoville 2000, ASP Conf Ser 217, 317

Downconvert to

4 GHz to 12 GHz

(cheaper components,

better characterized)

Uncooled LNA

(Tsys = 200 K)

Cold load (optional)

363 K load

Ambient load

Analog difference of line

and continuum channels

Analog sum of wing

channels for continuum

Triplexer separates 2 GHz

Bands on line and off-line

18.2 to 20.2, 21.2 to 23.2, 24.2 to 26.2 GHz

to 16-bit A/D

Alternate L and C every 1.7 ms

slide13

Owens Valley Radio Observatory

  • Two levels of Dicke switching reduce effects of gain and offset drifts:
  • 1) PIN-diode attenuators adjust the Line-Continuum output to be zero
  • for blackbody loads; output measures deviation from a flat spectrum.
  • Transfer switch reverses assignment of Line and Continuum to the
    • detectors every 1.7 ms; demodulation is performed in software
    • -> removes DC offsets and most of the gain drifts in detectors and following
    • electronics
  • Results:
  • 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
  • C1 & C2 channels derived from -10 dB coupler have 10x more noise
  • -> standard radiometer noise is not the dominant noise
  • White noise to 1 s in L or C channels separately
  • White noise to 10 s in L-C channel

Woody et al. (2000)

slide14

Owens Valley Radio Observatory

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

Woody et al. (2000)

slide15

Owens Valley Radio Observatory

interferometer path at 100 GHz

WVR predicted path

3 mm

RMS before correction = 0.53 mm

RMS after correction = 0.16 mm

26 min

Woody et al. (2000)

slide16

Owens Valley Radio Observatory

Path Length Retrieval

Observe a strong calibrator -> conversion factor

Typically use a fixed 12 mm/K

cf calculated conversion factor of 8 mm/K

Difference is “within the uncertainties of the triplexer

bandpass shapes and atmospheric model assumptions”

Woody et al. (2000)

slide18

Owens Valley Radio Observatory

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

L-C from each

WVR / K

Average L-C from

all WVRs / K

Woody et al. (2000)

slide19

Owens Valley Radio Observatory

0309+411 at 100 GHz for 5 h

Cycle: 6 min source, 6 min calibrator (0.7 degrees away)

WVR phase is transferred from calibrator to source

Before WVR

correction

(good weather)

(weather degraded)

28 Jy

36 Jy

13 Jy

After WVR

correction

40 Jy

42 Jy

34 Jy

Woody et al. (2000)

slide20

Owens Valley Radio Observatory

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 conditions

Or 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 prototype

Cooled 22 GHz WVR

Double sideband heterodyne

0.5 GHz to 4 GHz IF

16 channel analogue lag correlator (APHID)

(see Alberto Bolatto’s talk)

Woody et al. (2000)

slide21

JCMT – CSO Interferometer

James Clark Maxwell

Telescope (JCMT)

Caltech Submillimeter

Observatory (CSO)

Frequencies: 210 – 270 GHz

318 – 360 GHz Higher than OVRO

460 – 500 GHz

Antenna diam: 10.4 m & 15 m

Altitude: 4092 m Higher than OVRO

Location: Hawaii

slide22

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

Line pivot points: least sensitive to altitude of water vapour

slide23

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

The three double-sideband frequency channels of the WLM

slide24

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

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

slide25

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

slide26

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

Calibration

- Loads at 30 C and 100 C

- Load stability: 10 mK

- Flip mirror cycles every 1 s between sky and loads

Load temperature vs time

10 mK

Sectioned drawing of load

5 min

slide27

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

Mirror 2

Mirror 1

Warm load

Hot load

Corrugated horn (facing away)

slide28

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

Uncooled mixer

(Tsys = 2500 K)

183.31 GHz

+/- 8 GHz

Coupler

Power

splitter

Mixer Filter Detector V/F

Used coupler + power

splitter since no suitable

triplexer exists

1.2 GHz 4.2 GHz 7.8 GHz

Oscillators

Double-sideband mixing makes

measurement insensitive to filter shape

Gunn oscillator

91.655 GHz

slide29

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

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.

slide30

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

DSB mixing to baseband folds water line at oscillator frequency

Result is flat water line spectrum

Water line spectrum is then same as the calibration load spectrum

Calibration factor is then independent of the filter shape

slide31

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

WVR at CSO (outside, so

less stable environment)

10x10-4

2x10-4

WVR at JCMT

9 min

Gain fluctuations of WVR measured against loads each second

slide32

JCMT – CSO: 183 GHz WVRs

Wiedner 1998 PhD thesis

Wiedner, Hills, Carlstrom, Lay 2001, ApJ, 553, 1036

Maser source MWC 349

at 356 GHz

WVR correction

Before correction

RMS = 127d = 0.30 mm

1.4 mm

After correction

RMS = 48d = 0.11 mm

12 min

Atmospheric model: transition strengths from Waters (1976),

Ben-Reuven line profile, exponential atmosphere, radiative transfer calculation

slide33

Sub-Millimeter Array

CSO

JCMT

SMA

183 GHz WVRs being installed.

(Talks by Ross Williamson & Richard Hills)

slide34

Very Large Array

Dave Finley

Image courtesy NRAO/AUI

22 GHz WVRs being prototyped.

(Talk by Walter Brisken)

slide35

Plateau de Bure

22 GHz WVRs in routine operation.

(Talks by Michael Bremer & Aris Karastergiou)

slide36

Effelsberg

22 GHz sweeping WVR operating.

(Talk by Alan Roy)

slide37

Berkeley-Illinois-Maryland Array

22 GHz sweeping WVRs prototyped.

Array relocated to Cedar Flat with OVRO antennas

Now called CARMA.

(Talk by Alberto Bolatto)

slide38

VLBI Phase Correction Demo

Demonstration by Tahmoush & Rogers (2000)

3C 273

Hat Creek – Kitt Peak

86 GHz VLBI

path

4 mm

VLBI phase

WVR phase

400 s

● RMS phase noise reduced from 0.88 mm to 0.34 mm after correction.

● Coherent SNR rose by 68 %.

slide39

CARMA

Jim Stimson Photography

(Talk by Alberto Bolatto)

slide40

Chajnantor Site Testing

Two 183 GHz WVRs 300 m apart

Duplicates of JCMT-CSO WVRs (Hills/Wiedner)

Co-located with two 11.2 GHz seeing interferometers

observing a geostationary satellite

Delgado et al. 2001, ALMA Memo 361

slide41

Chajnantor Site Testing

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 (?)

Delgado et al. 2001, ALMA Memo 361

slide42

Chajnantor Site Testing

Delgado et al. 2001, ALMA Memo 361

slide43

Chajnantor Site Testing

Delgado et al. 2001, ALMA Memo 361

slide44

Australia Telescope Compact Array

Frequencies: 1.2 - 106 GHz

Antenna diam: 22 m

Altitude: 300 m

slide48

NASA Deep Space Network

22 GHz to 32 GHz WVR (Tanner et al.)

For Cassini gravity wave experiment

Naudet et al. (2000)

slide49

NASA Deep Space Network

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)

slide50

NASA Deep Space Network

2 mm

4 h

RMS before correction = 0.43 mm

RMS after correction = 0.1 mm

Naudet et al. (2000)

slide51

Conclusion

  • Reviewed 5 of 16 WVRs for astronomy

(7 radiometers tomorrow)

  • Many clever techniques are available for use
  • Lowest residual path 0.031 mm