The Luminosity Oscillations Imager (LOI) is a part of the VIRGO instrument aboard the Solar and Heliospheric Observatory (SOHO) launched on 2 December 1995.
The main scientific objectives of the instrument were to detect solar g and p modes in intensity.
The detector resolves the solar disk into 12 spatial elements allowing the detection of solar modes of degree lower than 7. The detector consists of a photodiode array custom built to specification. It has 12 scientific pixels and 4 guiding pixels. The shape of the scientific pixels is optimised to detect low degree modes of l < 7. The guiding pixels are 4 quadrants of an annulus with an equivalent inner and outer radius of 0.95 and 1.05 solar radii, respectively.
The error signals are fed back to two piezoelectric actuators that move the secondary mirror around its vertex. The actuators provide both coarse and fine pointing. The total range is ± 7.5 arc min. The -3dB bandwidth of the servo control loop is 10 Hz. The guiding keeps the Sun centred on the detector with an accuracy better than 0.1".
The photocurrent of each pixel diode is processed by I/V converters in zero-bias configuration. It is then digitized by separate Voltage-To-Frequency converters for each channel. The sampling interval and integration time is 60s, the resolution is 20 bit.
The guiding pixels are sequentially read out by the common VIRGO data acquisition system, with 16 bit precision.
Primary mirror diameter: 60 mm
Mass: 3 kg
Power: 3 W
Telemetry data rate: 10 bits per second
The LOI was designed and manufactured by the SCI-SO division. Key technical personnel were T. Appourchaux (CoI), U. Telljohann (Technical manager), J. Fleur (mechanical design)