MMA Memo 211 Accurate Amplitude and Flux

The 10% amplitude calibration accuracy achievable with the current standard calibration techniques may be suucient to produce images with dynamic range of 10 3 , but achieving a dynamic range of 10 4 or higher with the MMA requires better than 1 percent accuracy in amplitude calibration. A self-calibration technique may be applied to improve images, but it may not be possible in all cases. Therefore, an emphasis is given to achieving accurate initial calibration. Multi-transition spectroscopic studies and multi-array synthesis also require high accuracy in absolute sense as well. The conventional \chopper wheel" and a two temperature load method for amplitude and ux calibrations are examined. The two temperature load calibration ooers a potential to achieve the 1% accuracy in amplitude and ux calibration, but it is technically challenging. In comparison, the chopper wheel gain calibration and astronomical ux calibration cannot provide better than 5% accuracy. Whether the more complex two temperature load system is justiiable for the MMA may ultimately dependent on how well the radiometric phase correction will work. Several other relevant issues including establishment of astronomical ux standards are discussed, and engineering goals are identiied.