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MIR Calibration Steps Explained


System temperature correction

The first calibration is the system temperature correction. This removes variations in instrumental gain with time, and for changes in the atmospheric opacity due to elevation or weather conditions. It uses hot and cold load measurements (via the chopper-wheel method) to scale the data from raw voltages to a real brightness temperature (~Jys). The data is weighted by the square of the measured system temperature.

Looking at the bandpass calibrator:
[1] Before applying Tsys. Units are arbitrary.
[2] After Tsys correction, units have been scaled to ~Jy.

Bandpass calibration

The bandpass calibration corrects for the frequency response across the bandpass. This profile is determined using the bandpass calibrator - a strong source with no spectral features. To build up sufficient signal-to-noise this is usually observed for ~1 hour.

Looking at the bandpass calibrator:
[1] Before bandpass calibration. Phase and amplitudes both show structure across the band.
[2] After phase-only bandpass calibration the phases have been flattened.
[3] After phase+amplitude bandpass calibration, the phases and amplitudes have both been flattened.

The profile derived from the bandpass calibrator is applied to all sources.

Gain calibration

The absolute flux of gain calibrators is determined by applying a scale factor derived from a source of known brightness - the flux calibrator. This is usually a planet or moon. The gain calibration corrects for temporal variations in the amplitude and phase of the source over the course of the observation. These variations are typically due to changing instrument responses and sky conditions. This step improves on the initial system temperature calibration.

Looking at the gain calibrator:
[1] Before gain calibration. Phase and amplitudes have been flattened by the bandpass calibration step.
[2] After phase-only gain calibration the phases have been zeroed for both sidebands.
[3] Applying the amplitude-only gain calibration scales the amplitudes to their absolute fluxes derived from the flux calibrator.

The profile derived from the gain calibrators is applied to the science targets.