| CSIRO Australia Telescope National Facility ATCA Users Guide |
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Simultaneous 20 and 13cm observations will not be available for some time (possibly 2011)
The steps to observe with the compact array at cm are:
In many cases one observing program will immediately follow a previous one, and there should be no need to restart any software. The Duty Astronomer should be on hand to help inexperienced observers set up their observations.
The main observing program that the observer interacts with is CAOBS. CAOBS usually runs in a black screen with yellow writing. Other programs that observers need to use include CACOR, VIS, MONICA
For details on using these programs see the %{REFERENCE_NAME}, %{REFERENCE_NAME}, %{REFERENCE_NAME}, %{REFERENCE_NAME} and @ref{monica} sections.
To start observing:
CAOBS > set file sched_filename
CAOBS > track # where # is the scan number in the schedule
The process for setting up observations includes
Before any other calibration can be done, the rms power levels into the CABB samplers has to be around 20V. If working at mm, the power levels into the F14 modules should also be below 20V.
To do this, CABB has to be working at the frequency of interest (with the appropriate feed on axis). The easiest way to do this is usually to be tracking on a source.
If you are working at mm frequencies (12, 7 or 3) the power levels into the F14 modules need to be less than 20V.
MONICA is used to check the levels. Navigate to Frequency 1 Conversion Summary page by:
Navigator → conversion → frequency1_summary
Check the levels in the c26 mm Splitter RF Levels (A and B). The aim is to have these less than 20V (though it isn’t always possible)
This is done with the CAOBS command:
CAOBS > set mm ca01 #_1 #
_2
where #_1 and # _2 are the mm attenuator settings. (These are also displayed on the monica conversion page.)
This step needs to be repeated for all antennas. For most antennas at most frequencies, 15 is the appropriate setting. (The levels will be adjusted at some point in the future and more care will be required to get the attenuator settings correct.)
At all frequencies, the voltages into the CABB samplers should be close to 20V (Range: Approximately 10 - 30V)
To check this, click on samplers tab in CACOR. See #REF for information
about the CACOR interface.
Check if the rms voltages on all IFs are close to 20V (Range: 10 - 30V)
If not, enter the following command in the CACOR interface
CACOR > atts[ervo] on
which will set the attenuators to adjust the levels into the samplers
Wait for the levels to settle to around 20V, then turn the automatic adjustment off again with
CAOBS > corr atts[ervo] off
(This is analogous to the old CACAL (and may well be replaced with a new CACAL in the future.))
Before observations commence, it is necessary to correct for delay offsets. It is also useful to set the phases to zero, and the gains to a preset value. Failure to correct for delay offsets will result in the data being decorrelated. Phase and amplitude corrections are mostly cosmetic, though they help monitoring the observations. Data reduction in (AIPS or MIRIAD) will redetermine the phases and gains.
To calibrate the array, track on a strong unresolved source (e.g. PKS1934-638 or,
for mm PKS1253-055, PKS1921-293, PKS2223-052 or
PKS0537-441). PKS1934-638 is not strong enough to calibrate at mm frequencies.
Correlation should be apparent before you proceed. (Phases in SPD should be stable, though they may be wrapping rapidly) and/or delays should be flat in VIS.) Check that you have correlation in both frequencies and both polarisations. If you don’t, check the Monitoring and Trouble Shooting section at the end of this chapter. If you still don’t have correlation, get help.
If working at 3mm or with significant interference, it may be necessary to average over several channels (to reduce the noise) with:
CAOBS > corr delavg #.
8 is a good value for #.
If there is still no apparent correlation, there is probably some other problem and this should be sorted out before proceeding.
Delavg should be set back to 1 after calibrating the array.
Assuming that the phases in SPD are stable, enter the command:
CAOBS > corr dcal a
This command should correct the delays so that the phases across the band on the SPD plot are flat (or fairly close to flat) and the delay offsets as seen on VIS are close to 0 nsec. If the offsets were large (greater than 100nsec) it may be necessary to do this again.
(CAOBS > corr dcal will determine the corrections and display them in the correlator window,
but will not actually apply them.)
Note: Given the scale on the delays (tens of pico seconds) it is now possible to see the effect of atmospheric variation in delays.
If observing at mm, it is useful to check the pointing before amplitude and phase calibration. For information on offset pointing, see #REF
Once the delays have been set, it is possible to set the phases and gains.
To set the amplitudes
CAOBS > corr acal #_1 #
_2 a
This command should set the amplitudes to be #_1 in the first frequency and #_2 in the second frequency.
(As with delay calibration, it is possible to check the calculation without actually applying it by:
CAOBS > corr acal #_1 #
_2
and checking the output in the CACOR display.)
To set the phases to zero
CAOBS > corr pcal a
(Again, it is possible to check the calculation without actually applying it by:
CAOBS > corr pcal
and checking the output in the CACOR display.)
Ensure that the delavg value is set to 1:
CAOBS > corr delavg 1.
(Should this section be in an Observing Strategy Section #REF)
It is important that sufficient calibration data is taken to characterise the data. This means taking Flux and Bandpass Calibration data for all observations, and Leakage calibration data for polarisation observations.
This does not have to be done at the start of the observation, but should be done at some point during the observing session. If appropriate calibration sources are not up during the observing session, contact Phil Edwards? to discuss options.
For cm observations (including 12mm observations) PKS1934-638 is the
appropriate flux calibrator and can usually be used as a bandpass calibrator.
For mm observations, a planet is needed for flux calibration and a strong
source (e.g. PKS1921-293 or PKS1253-055
#REF to information on