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:
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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:
The profile derived from the bandpass calibrator is applied to all sources.
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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:
The profile derived from the gain calibrators is applied to the science targets.
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