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SIMPLETON location and instrument setup

SIMPLETON is a two-element 20 MHz interferometer. First it was located at the geographic coordinates l=149.66128, b=-30.57805 (south of Narrabri, New South Wales, Australia). Later it moved to l=149.95873, b=-30.43617 (east of town). A portable GPS unit was used to determine its exact coordinates and to align the telescope precisely East-West. The dipoles are then oriented North-South, each antenna consisting of two parallel dipoles separated by 7.5 meters (half a wavelength), thus defining a 7.5x7.5 m square surface.

SIMPLETON can be set up to work with different baseline lengths (antenna separations; see status log). The characteristic unit for measuring the size of an interferometer is the (maximum) separation between antennas, in units of wavelengths (λ). For example, at a frequency of 20 MHz, which corresponds to λ = 15 meters, a 37.5 m baseline corresponds to 2.5 λ.

The resulting antenna setup does not really look very imposing, but it does its job! I have collected a few photos here. Each of the two antennas can see the whole sky at any given time. The two signals are synchronised by using a local oscillator and a fixed delay line between the two amplifiers. The signals at the radio frequency of 20.12 MHz the signals are mixed with a second wave. The upper sideband (near 40.2 MHz) is blanked out by a low-frequency filter, leaving only a signal at 3000 Hz (which is the frequency of an acoustic wave and can thus be made audible via the soundcard of a computer).

Both antennas look straight up at the sky and the signal of one antenna is delayed with respect to that of the other by means of a delay line (coax cable of given length) such that the waves of an incoming radio signal from a celestial source are received at the same time when it goes through the meridian (the two signals are then "in phase"). However, through their various so-called "sidelobes" the antennas can in fact see the entire sky above the horizon at any given time. The data from both telescopes can be combined with each other ("correlated") to form an interference pattern. Note that the two antennas used here do not track sources on the sky, while most professional telescopes do. This leads to significant differences in the kind of results one obtains. Have a look at the pages on interferometry for more details.

Results

Results from SIMPLETON are summarised here now.

Status log

Since this table grew significantly over time, it was moved into a separate file.