1.5 Choosing an Observing Frequency

1.5.1 Available Frequency Range

Figure 1.3: Average Compact Array system temperatures and system equivalent flux densities (SEFDs) for each observing band at high elevation under reasonable observing conditions. These are based on hot-cold load measurements, and include the atmosphere at the time of observation, thus representing the total system temperature. These measurements were made by J. Stevens in July/August 2010 with the CABB system, except for the 3mm measurements which were made by T. Wong in September 2004.

The 6cm and 3cm receivers share a common feed-horn and it is possible to observe simultaneously at any two wavelengths within these bands. You can also switch automatically to other wavelengths within tens of seconds (with the exception of the 7mm band as described previously). Similarly the 16cm receiver has a single feedhorn and observations with this package will cover the entire 1.1 - 3.1 GHz range simultaneously. Observations at two simultaneous frequencies are possible in the 15mm band or the 7mm, or the 3mm bands. With CABB frequencies currently need to lie within 6 GHz of each other (and tighter constraints possibly resulting from the exact frequencies chosen). Typical observing frequencies for continuum observations using the 128 MHz bandwidth available with the original ATCA correlator were 1384, 2368, 4800, 8640, 18496/19520, 34496/34524, 44096/44224, and 93504/95552 MHz. These frequency designations follow the ATCA custom of stating the central frequency of the chosen observing frequency range. For CABB, the nominal standard frequencies are 2100, 5500, 9000, 17000/19000, 33000/35000, 43000/45000, and 93000/95000 MHz.

Switching between the 15/3mm bands, the 6/3cm bands, and the 16cm band involves a change in feed horns by means of a turret rotation. This is done automatically under computer control and takes about 20 seconds. Turret rotation should be limited to once every 15 minutes unless a compelling scientific case is made for more frequent rotations. The additional overhead in changing to or from 7mm has been described earlier in this document.

Observing frequencies may be set to the nearest half-MHz only and no on-line Doppler tracking is done.

Observations of weak H90\alpha recombination lines may be affected by a trapped mode in the 6/3cm horn at 8857 \pm 18 MHz. There are also notches reported in the passband due to trapped modes in the receiver waveguides at 1326 at 4550 \pm 10, 5328 \pm 10 and 8780 \pm 10 MHz which may need to be flagged during data reduction.

1.5.2 Interference

In the lower frequency bands radio frequency interference (RFI) may be a problem. The interference is worse at lower frequencies with the main offenders being microwave links, microwave TV, microwave ovens, navigation satellites and self-generated interference. There has also been significant interference at 1381 MHz from the GPS L3 beacon on occasions. These channels may have to be removed from the data. It is wise to watch for interference by continually displaying the spectrum of the signal received on the shortest baseline. You can then note any channels with narrowband interference for subsequent elimination. The frequency range of 2300 to 2400 MHz, formerly occupied by microwave TV services, is presently clear again.

Recently, a very strong new source of RFI has been noted, with primary peaks of 1265 and 1310 MHz. When this RFI is present (usually during daylight hours and on weekdays, leading it to be called “midweek RFI”), large portions of the 16cm band become unusable due to harmonics of these two frequencies.

To avoid solar interference, a rule-of-thumb is to observe at a time of year when your source is further than about 40^\circ from the Sun, where possible. It is recommended to specify in your proposal dates that are not suitable for observations if the sun angle is too small. Low-frequency observations, particularly in spectral-line mode, should be made at greater distances. You are advised to observe at night in cases where good quality 21cm HI data is essential on the shortest (30m) baseline. For spectral-line observations, software exists in MIRIAD to model and subtract out solar interference. Some information about solar activity can be obtained from the Ionospheric Prediction Service.

There appears to be no significant interference within the mm bands.