RSTO Solar Spectrometers

CALLISTO Spectrometer Set-up at RSTO

RSTO operates three CALLISTO receivers fed by a broadband log-periodic antenna and a biconical antenna (Figure 1). Nominally, the RSTO set-up operates at 600 channels with a sampling time of 250 ms seconds per sweep. CALLISTO 1 observes at 10–100 MHz, CALLISTO 2 at 100–200 MHz, and CALLISTO 3 at 200–400 MHz. The system has been optimised to measure the dynamic spectraof Type II radio bursts produced by coronal shock waves, and Type III radio bursts produced by near-relativistic electrons streaming along open magnetic field lines. It can also record other radio bursts, such as Type IV bursts and Type I noise storms.

Figure 1 RSTO Antennas 

The log-periodic antenna has a frequency band of 50 to 1300 MHz with a 50 degree half-power beam-width (HPBW). The antenna is fixed to an alt-azimuth drive which tracks the Sun to optimize its response. The biconical antenna is 4 m long and has a nominal frequency sensitivity from 10 to 300 MHz. It is also mounted on a motorized rotator to track the Sun. CALLISTO 2 and 3 operates with a pre-amplifier that has a frequency range of 5–1500 MHz, and a typical noise figure of 1.2 dB, while a similar pre-amplifier is separately connected to CALLISTO 1. The system set-up is optimized to reach the ionospheric cut-off frequency at ∼10 MHz. In order to do this, the receiver with a nominal operational band between 45 to 870 MHz has to operate  with a frequency up-converter, shifting the range between 10–100 MHz to 220–310 MHz. The observed frequencies are then down-converted in software.

Figure 3 The set-up of the array of three CALLISTO spectrographs at RSTO.