3mm Home Page System Performance Calibration Observing Quick Start System Specs mmATCA Receivers mmATCA Results

1. Primary calibration of the flux scale

At wavelengths shorter than a few centimetres, extra-galactic sources generally prove to be too variable to be useful as flux density calibrators. So at these wavelengths, the blackbody emission from the planets are often used as flux standards. The two most commonly used planets for the ATCA at 3mm are Mars and Uranus. Note Jupiter is too large for use as an ATCA flux density calibrator. Neptune would also be a possible flux density calibrator. However Neptune is quite close to and half the strength of Uranus, and so does not offer anything over Uranus.

Uranus is the preferred flux density calibrator for the ATCA. Mars is less suitable for a number of reasons: some models of Mars' average brightness temperature suggest variations in the temperature of up to 10% as a result of Mars' diurnal rotation, its distance from the Sun and the viewing geometry of Earth-based observers. Definitive measurements and confirmation of these models are poorly studied in the literature. Depending on its distance and the array configuration, structure on Mars is also readily detected with the ATCA: Mars has hot equatorial and cold polar regions.

Although Uranus or Mars (preferably Uranus) are the best flux density calibrators, frequently they will not be accessible to an observation. To account for this, the Narrabri Observatory regularly monitors some additional sources (usually 1253-055 and 1921-293) to allow them to be used as flux density calibrators. Be warned though that these sources can vary substantially on month timescales.

Check the rise and set times of the planets. If Uranus is never above 30 degrees during your scheduled observations, then please contact Kate Brooks (Kate.Brooks @ csiro.au) well in advance of your observations. We are currently working on finding other sources for flux calibration at 3mm.

Absolute flux calibration at all frequencies with ATCA should now be applied using the miriad task MFBOOT. This has superceded both tasks PLBOOT and GPBOOT.

Use the miriad task PLPLT to plot visibility amplitudes for a given planet at a given epoch and frequency as a function of baseline.

2. Secondary calibrators

The calibrators database gives information on potential sources that may be used to measure the interferometer amplitude/phase calibration. A round-trip phase correction machinery automatically corrects for the changes in electronics path length in real time; therefore, the astronomical secondary calibration observations are for taking out the atmospheric path changes and any errors in the baseline parameter determinations. The on-line system temperature calibration takes out the temporal changes in the electronics gain but does not correct for atmospheric opacity; therefore, the secondary calibrator amplitudes are important for correcting for the elevation dependent changes in the opacity.

Ideally, you would want an unresolved calibrator with a flux density greater than 1Jy at a distance no more than 5 degrees from your program source. In practice, I would first relax the the distance to 15 degrees and then the flux density to 0.8 Jy. Check the vis function and ensure that the calibrator is not resolved in your scheduled array configuration. Take note of the defect parameter - particularly if there are measured defects at cm wavelengths.

For a 128 MHz bandwidth, a 2-min integration should be adequate to acheive a signal-to-noise of at least 10 on a 1 Jy source.

3. Bandpass calibration

Each frequency configuration requires a separate bandpass calibration (because of the different residual delays). This is acheived by observing a bright calibrator at each observed frequency setting. Observers are also advised to observe bandpass calibrators with every observing session/day and separately correct each days data for the bandpass complex gain before merging datasets.

A list of other calibrators with strong fluxes at millimetre wavelengths is available here. Be sure to avoid Cen A, 1322-427 which has strong molecular absorption lines.

4. Antenna Reference Pointing

The global pointing solutions determined after every reconfiguration can be off by as much as 20 arcsec in individual antennas. For 3mm observations reference pointing is recommended. With reference pointing, the R.M.S. pointing error is measured to be within 2 arcsec.

Reference pointing is implemented in two separate steps: (i) pointing solutions are determined from a pointing routine on an unresolved calibrator located close to the source on the sky and (ii) these solutions are used for the pointing model while observing the secondary calibrator and source.

At 3mm the reference pointing solutions should be recomputed every 1 hr or whenever the antennas move significantly in azimuth/elevation or whenever the ambient temperature or weather conditions change significantly. The pointing solutions ought to be determined on a reasonably strong calibrator: >1 Jy is ideal. The secondary calibrator may serve as a pointing calibrator also. Ensure that you update your pointing before observing your absolute flux density calibrator.

When observing compact sources in snapshot mode it is advisable to point on the source itself before observing it.

Pointing using continuum data or line data (e.g. masers) is possible.

More information is available on the web page ATCA Reference Pointing.

5. Atmosphere Related Issues

At millimetre wavelengths, the atmosphere can no longer be approximated as perfectly transparent. It degrades overall sensitivity in two ways: the atmosphere emits radiation, and so raises the system temperature, and the atmosphere attenuates the astronomical signal. For ATCA at 3-mm wavelength, the effect of the atmospheric opacity is corrected for via a measaurement of the effective system temperature - the the so-called "above atmosphere" system temperature. More details are available in the MIRIAD User Guide.

The ATCA uses the chopper wheel method (also call paddle or vane calibration) in its 3-mm system to determine an above atmosphere system temperature. This involves placing an ambient load (paddle) in front of the receiver horn.  This is needed to place the amplitudes on a brightness temperature scale.  30 seconds are spent on the absorber (P_abs) and the sky (P_sky) respectively.  The system temperature is then calculated as roughly

Tsyseff   = (300 K)

____Psky____      Pabs - Psky

6. Antenna Focus

At present, observations in all bands and frequencies are made with a single fixed focus setting: observers do not change the antenna focus.

7. Antenna Gain

TBA