1. Observing Strategies for the Compact Array
Making good decisions
Jessica Chapman
Synthesis Workshop September 2006

2. A: ATCA cm radio continuum observations
Frequencies
Angular resolution (/B)
Array configurations
Calibration strategies
Integration times
Interference and confusion
B: ATCA 3-mm radio continuum observations
System notes
3-mm calibrations
observing tools

3. A: cm continuum observations
(GHz):     1.78
Prim.beam
(arcmin):
Syn. beam
(arcsec) :
Bandwidth = 128 MHz split into 32 spectral channels
Switching between bands is straightforward
Allows for simultaneous observations at 3+6 cm
and cm

4. Choosing the observing band(s)
image resolution
do you want spectral indices?
is emission thermal or non-thermal?
system performance
confusion and interference
phase stability
Choice of ATCA band(s) is usually SCIENCE DRIVEN

5. Angular resolution and array choice
For point source:
Flux = S (mJy)
Brightness = S (mJy/beam area)
(same for any beam area)
rms noise = I (mJy/beam area)
signal/noise = S/ I
Sensitivity to a point source is the same for all baselines

6. Sensitivity to an extended source
beam
Flux = S mJy/arcsec2
Beam area = B (For a Gaussian beam, B = 1.13 x  y)
Signal/noise = S B/ I ( beam area)
If beam area < source size then the sensitivity to extended emission is reduced.
Note the trade off between angular resolution and brightness sensitivity.

7. WR 147
MERLIN: - total flux density = 20 mJy
VLA: - total flux density = 36 mJy

8. Choosing best configurations
Smallest angular structure longest baseline
Largest angular structure shortest baseline
Determine full size of full region for image
Select best matched array configurations

9. Compact Array Configurations
Large number available - baselines from 30 m to 6 km
39 ‘stations’ on the 3-km east-west track (Bmax = 3 km)
1 station at 6-km
5 stations on the north-south arm (Bmax = 214 m).
For complex sources – it is often advisable to use two or more configurations.
For available configurations see the
See: “Guide to Observations with the Compact Array”

10. Compact Array configurations 2006 - 2008
6A and EW352 are scheduled every term.
Other configurations are cycled over 18 months.
Three hybrid arrays offered in winter season (for mm)
Wildcard (non-standard) configurations can be requested

11. I: Primary amplitude calibration
Observations of a strong, non-variable and compact source with a known flux density are used to determine the absolute flux scale.
is used as the primary calibrator for all ATCA cm observations

12. II Bandpass calibration
Observations of or another compact source are used to correct for instrumental variations across the bandpass. A single bandpass calibration observation of about 10 minutes is usually sufficient (or use secondary calibrator).
Amplitude (Jy)
Channel number

13. III Secondary Calibration
Secondary calibration sources are observed to correct for time-dependent visibility variations caused by atmospheric and instrumental effects.
Secondary calibrator sources should be:
strong (> 1 Jy)
close to the source (< degrees)
unresolved on all baselines
have accurate positions
Calibration sources:

14. Set up array – set delays, phases and gains on a
An observing pattern for cm observations
Set up array – set delays, phases and gains on a
strong point source (CACAL). This takes ~ 10 minutes
Observe (CAOBS)
A secondary calibrator for a few minutes
The astronomical source(s) for ~ 15 – 45 minutes
Repeat cycle many times and monitor the visibilities
Observe primary calibrator at least once during the
observations.

15. Integration times Thermal noise at image centre :
Ith  Tsys . F / (nbas . BW . T. npol)0.5
Tsys (cm) ~ 340 – 450 Jy
F ~ 1.0 for natural weighting, ~ 1.5 for uniform weighting
Examples: BW = 128 MHz. Npol = 2, = 6 cm
T = 12 hours, Ith ~ 0.03 mJy
T = 10 mins, Ith ~ 0.21 mJy
Sensitivity calculator:

16. Short-cut detection experiments
In practise, to reach the thermal noise, need to have a well-sampled u-v plane.
Short-cut detection experiments
split the total time into a large number of short cuts
distribute cuts over the HA range of source
This should
reduce the sidelobes from other sources in the field
may reduce the level on interference

17. Detection of stellar winds from WR stars
WR cm
Detection of stellar winds
from WR stars
Band rms time
(mJy) (mins)
3 0.1 
6 0.1   

18. Radio continuum spectra for WR stars

19. Confusion any other astronomical source that contributes to emission
may be within the primary beam or in sidelobes
degrades final images - higher noise in images
may give spurious “detections”
Confusion from Galactic plane can be very extended

20. Confusion.. Number of extragalactic sources per square arcmin:
N (Sobs > S) = S at 6 cm
= S at 20 cm
Examples: At 20 cm, primary beam ~ 1000 arcmin2
N >20 mJy ~ 7
N > 160 mJy ~ 1
At 3 cm expect ~ one source > 0.4 mJy in primary beam

21. Stellar
detection
3 cm

22. 6 cm

23. 13 cm

24. 20 cm

25. Some Strategies for Confusion
Make a low resolution image of a large region
Identify and CLEAN sources within field-of-view
Move pointing centre away from strong confusing source
-- to minimize the primary beam response
For short cut experiments - use multiple cuts
-- improves the dirty beam characteristics
Be careful with marginal detections - are they just sidelobes?

26. Interference
20 cm band
Flux Density (Jy)
Frequency (GHz)

27. Interference can be removed using pre- and post-correlation techniques
OH maser
Satellite interference

28. characteristics strategies Interference... time variable
short bursts -- large angular scale map errors
worst on short baselines
strategies
avoid the sun (> 40 degrees)
choose ‘clean’ part of band – see guide and staff
use long exposures or multiple cuts
use longer baselines
edit data

29. 3-mm continuum observing
Antennas 1 – 5 only, so maximum baseline = 3 km
Frequency range 85 – 105 GHz
Primary beam ~ 33 arcsec
Scheduled: May – October
Phase stability best in winter and at night
Some flexible scheduling with ‘swap time’
No polarimetry at 3 mm
Observers must be present at Narrabri

30. 3-mm sensitivity Recommended frequencies are:
GHz and GHz
94 GHz
Example:
BW = 128 MHz
Freq = 94 GHz
T = 12 hours
Weather = average
rms ~ 0.6 mJy

31. 3-mm calibrations Primary calibration:
For absolute flux calibration, use the planets Mars or Uranus. However, Mars may be resolved. Uranus may not be visible.
The Miriad task plplt plots visibility functions for planets.
Could also use a strong QSO IF the flux density has been recently measured. Good options are B B
Observe the primary calibrator at the same elevation as one of the secondary calibrator observations.

32. Secondary calibration:
Observe a reasonably strong (> 1 Jy), nearby (<10o) and unresolved QSO.
Phases may vary rapidly and should be monitored.
Typically observe secondary for 2-3 minutes every 10 – 15 minutes.

33. Pointing observations:
3-mm observations use local pointing solutions. Do a pointing observation of a nearby strong QSO about once an hour.
Paddle observations:
A ‘hot-load’ paddle is used to determine the system temperatures.
Paddle observations are taken about every half hour.

34. An observing pattern for 3-mm observations
Set up array (allow about 40 minutes to set up)
detach antenna 6
track a strong source and set delays
adjust levels of attenuators
do a pointing observation of very strong source
do a paddle observation and check the paddle works OK
Take observations
do a local pointing observation (once per hour)
do paddle observation (every half hour)
cycle between source and secondary calibrator

35. Observers Tools, Archives and Information
An observing pattern for 3-mm observations
During observations
observe primary calibrator at least once (do paddle first)
monitor phases and cycle intervals
check attenuator levels are OK
For
Observers Tools, Archives and Information
See