By default, the ALMA Observatory will identify and conduct all calibrations that are necessary. Detailed information on calibration and the default ALMA approach can be found in Chapter "Calibration and Calibration Strategies" of the ALMA Technical Handbook.
In exceptional circumstances, users can request specific calibrators and/or additional calibration strategies via the OT. To do this, go to the "Calibration Setup" page of the relevant Science Goal and select "User Defined Calibration". But these should not be changed without good reason, and any changes need to be explained in the Technical Justification. For more information, please check the section "Calibration Setup" in the OT Manual. For more information on how to specify your own calibrators, click on the "?" icon in the pane that appears.