7mm Home Page System Performance Calibration Observing System Specs mmATCA Receivers mmATCA Results

1. Array Configuration

Observations with compact, and preferably hybrid, array configurations are optimum for detection or sensitivity-limited experiments at 7mm (as well as 3mm). This is because:

• Atmospheric phase fluctuations lead to decorrelation of visibilities which worsens with increasing observing frequency and baseline length.




• Low spatial resolution will optimise the potential for the emission to be unresolved and thus maximise the signal-to-noise ratio.




• Higher air mass at low elevations leads to higher system temperatures. There is a dramatic increase in the system temperature at elevations below 30 degrees. Full synthesis is obtained in 6 hours for a hybrid array. This reduces the need to observe at low elevations in many cases.

2. Available Spectral Lines

The Compact Array does not carry out online Doppler corrections. If you are observing a spectral line, use the frequency calculator to determine the appropriate frequency to tune the ATCA correlator. Available spectral lines in the 7mm band are given in the table below. For a full list of commonly observed lines, see this listing.

 

Species		Rest frequency (GHz)
Silicon Monoxide	SiO	42.879916
Silicon Monoxide	SiO	43.122080
Silicon Monoxide	SiO	43.423858
Carbon Monosulfide	CS	48.990964

3. SCHED related issues

Disable the averaging option, i.e., set AVERAGING=1. This is because the changes in the differential atmospheric phase between the antennas might very well cause a decorrelation in visibility amplitudes over long integration times.

4. If changing frequencies during an observing session

Changes in the observing frequency usually cause a change in the frequency of the first local oscillator. The synthesizer is located in the central site and the phase of this reference frequency in a round trip to each antenna and back is used to estimate the instrumental phase which is used on-line as a round-trip phase correction. The consequence of this system for observers is that the recorded phase could have arbitrary jumps whenever the observing frequency changes. Therefore, observers who wish to switch between frequencies during their observing sessions are advised to commence and end observations at each frequency with a record of the visibilities on the secondary calibrator.

5. Coarse attenuators

The total power changes significantly with elevation, with weather conditions and across the 30-50 MHz band. Usually, the observing software automatically switches in/out phaseless (fine) attenuators and maintains optimal power levels at the samplers. However, operator intervention becomes necessary if a coarse attenuation step should be required (ASSISTANCE would alert the observer to the condition that the power levels are outside the range in which the sampler levels can operate). Coarse attenuation steps may be invoked using the CAOBS command "SET COARSE CA0n XXXX", where n is the antenna number and XXXX is the attenuation required. The coarse attenuations take values 0-3 and the four values correspond to the four IF chains A1, B1, A2 & B2 in that order. However, the coarse attenuators are not phase-less! Therefore, observers are advised to record visibilities on the phase (secondary) calibrator before and immediately after any change in coarse attenuators, and break (GPBREAK) gain solutions at the times when coarse attenuators were altered.

6. Atmosphere related issues

The atmospheric contribution is dependent on frequency, elevation angle and the water vapour column, which might be estimated given the ambient temperature, pressure and humidity (see the /weather page in CAMON for these parameters). There is a MIRIAD task OPPLT which estimates the sky brightness temperature (3K CMB sky as seen through the atmosphere plus the atmospheric contribution) and opacity for clear skies.

The ATCA seeing monitor is a two-element interferometer with an east-west baseline of 230m, tracking the 30.48GHz beacon on the geostationary satellite OPTUS-B3, at an elevation of 60 degrees. The seeing monitor works by taking the difference between to successive phase measurements, computing the standard deviation of this difference, and converted into a path length in microns. Results from the seeing monitor are updated in real time and displayed on MONICA in the control room. More details about the seeing monitor can be found here.

7. Notes on attenuators

You will need to check the level of attenuation in the mm system at the begining of your observations. This is tricky and you should request the help of the Duty Astronomer, even if you are an experienced observer, so that your get the most up-to-date advice. Refer also to the information available on the

Attenuation Settings web page.

Currently the attenuator levels are set with the caobs command "chklev n", where n is the scan number of the calibrator scan. For 3mm observations run this command with paddle in ("set paddle in").

8. Antenna Focus

When using the 7-mm system the focus for CA01 needs to be readjusted. For more information, contact the Duty Astronomer.