Amplitude Correction and System Temperature

A major concern at submillimter wavelengths is the contribution to the noise by the atmosphere modulation in amplitude. The system temperature (T$_{\rm sys}$) measurements determined by comparing the receiver output power from an ambient temperature load with that of the sky are essential to tracking noise variations of the receivers and therefore to calibration for the amplitude gains.

The current SMA data from the on-line system has not been corrected for system temperature. However the frequently measured T$_{\rm sys}$ values for each antenna are stored in the header of MIR data for each pair of antennas. The antenna-based T$_{\rm sys}$ are computed and stored in the header of uvdata in Miriad while the data are loaded using smalod. The measured T$_{\rm sys}$ can be used to calibrate the atmosphere. The effects of varying opacity, both in time and in elevation, are then removed from the measured fringe amplitude. Amplitude calibration is critical to the achievable dynamic ranges in imaging.