ATCA

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ATCA Calibration at mm wavelengths

• Rick Forster University of California, Berkeley
• Hat Creek Radio Observatory
• Berkeley-Illinois-Maryland Association
• Topics Basics of interferometry
• The atmosphere
• Amplitude phase calibration
• Atmospheric instrumental effects
• Antenna effects

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A few definitions
Brightness B 2k TB/l2 W/Hz m2 W
Effective area Aeff hpD2 m2
Received power VA2 Aeff S Dn Watts
Janskys/K S/TA 3510/hD2 Jy/K
V2 or the total output power is TSYS TR TSKY
TA K (100200.04 K)
V (volts)
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Tsys Paddle or Chopper Wheel method
Tsys sets the flux scale and the noise level!
n0.2 Jy
a sin-1(0.2) 12o
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Basic Interferometry
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Tsys 200 K
r 5 10-4
Jy/K 150
S r Tsys Jy/K 15 Jy
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Atmosphere
absorption
delay
TA Tsou ( e-t )
Tsky Tatm (1-e-t )
Tsys TRTskyTA
Tsys TRTatm(1-e-t)Tsou(e-t)
Tsys TRet Tatm(et -1)Tsou
Tsys Tsyset
for t 1, et 2.7
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Butler Desai, 1999
Structure Function (DF vs D)
a 0
Lay, 1997
Frozen Turbulence Model (DF power vs t)
a 5/6, 1km layer, 5km/s wind, 500m D
Longer baselines shifts peak right Stronger winds
shifts peak left
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M87 data calibrated with 3C273
80m
45m
15m
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Baseline Solution C-array, March 2003
Phases AFTER Correction
Phases BEFORE Correction
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Absolute flux calibration
Test source
1310323 (QSO)
0909013 (QSO)
cal
test
source
MWC 349 (HII)
Use a test source to help quantify the effects of
atmospheric decorrelation in the image plane.
Use a primary flux calibrator (planet or HII
region) to check the value of Jy/K. Use your
phase calibrator to monitor temporal gain changes.
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Passband 32 visibility channels over 100
MHz Channel BW 3.125 MHz (11 km/s)
n 2.6 Jy (each 11 km/s channel)
s/n 12/2.6 4.6
phase noise 11 deg
(ave 100 MHz 6 times better)
At 0.2 km/s channel n 8x2.6 21 Jy
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A few final suggestions

• Calibrate Tsys often
• Check Jy/K on a flux cal
• Mosaic sources larger than ½ fwhm
• Use offset pointing for extended sources
• In a pinch total power correction might help
• Choose nearby calibrator if baseline is
  uncertain
• Choose integration time so S/N gt 5 on calibrator
• Use a guest calibrator to gauge atmospheric
  effects
• Dont push your observations beyond the seeing
  limit
• Avoid unnecessary PB calibration, check
  sensitivity
• Monitor the atmosphere abort 3mm in severe
  conditions
• Fast-switching might be worth a try in
  reasonable conditions

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