Spectral Line and Continuum

Since narrow bandwidth filters are only available in the 1st LO chain, spectral line observing uses small bandwidth correlator configurations, usually 4 or 8MHz. However, it is common to use the second frequency chain for continuum observations with a larger bandwidth either at the same or another frequency. Ideally continuum observations would be taken over only one channel 128MHz wide, however the effects caused by bandwidth smearing and narrow band interference can be reduced by splitting the band into several channels. The maximum number of channels is dictated by the correlator design. The inherent need for high velocity resolution for spectral line work results in the use of a large number of channels over a smaller bandwidth.

Note: that the ATCA operates only at the frequencies that are requested in the ATCASCHED file and no Doppler shifting is carried out.

Strong interference from the Sun can cause havoc in 20cm spectral line observations on the shortest baselines. Observations of sources that will be within approximately 50° of the Sun are not recommended. However, software exists in MIRIAD to model and subtract this interference. Interference problems are most severe for narrow-band observations, but rarely affect a whole 12hr observation. It is recommended that during your observations you display the spectrum from the shortest baseline on the SPD display. Other interference problems to watch for in spectral line mode are

• any harmonics of the 128MHz sampler clock (eg 1408MHz). For further information see this web page.
• intermittent interference from the GPS L3 beacon at 1381MHz
• a trapped mode in the 6/3cm horn at 8857$\pm$18MHz
• a trapped mode at 1326MHz

Schedule files made for spectral line observations use particular ATCASCHED fields that are not displayed by default. Enable these using the #line command or by entering a value for the fields:

#Start

The lower limit of the channels to be recorded.

#Stop

The upper limit.

#Hanning

Apply a Hanning smoothing function to the data.

Note that the prefix (#) can be either 1 or 2 and indicates which frequency. Enabling these line options means that you wish them to be applied on-line. This is not recommended to novice users as you are exposed to a number of calibration problems which are best avoided and all the corrections can be applied off-line.