Total power observations use reference positions (OFF positions) which are assumed to be free of spectral-line emission.

The criteria for searching OFF positions by ALMA depend on the science target and observing bands:

• For extragalactic objects in all bands: the OFF position is an offset of 10 arcminutes from the target in horizontal coordinates .

• For Galactic objects in Band 3, 4, 5 and 6: the OFF position is a fixed position from the ALMA Calibrator Source Catalogue within 5 degrees of the science target .

• For Galactic objects in Band 7 and 8: the OFF position is a fixed position from the ALMA Calibrator Source Catalogue within 3 degrees of the science target.

• For Solar system objects (SSOs) in all bands: the OFF position is an offset in horizontal coordinates based on the ephemeris of the SSO, its galactic latitude and its closeness to other SSOs.

Please note that it is necessary to select as Options: Catalogues: 'ALMA_TP_OFF', and Type: 'TP off position' while searching OFF positions in the ALMA Calibrator Source Catalogue.

PIs are encouraged to submit OFF positions but are not required as stated in the ALMA Phase 2 Quick Start Guide:

“Projects containing Total Power (TP, single dish) observations: all Total Power single dish observations require an OFF position that should correspond to a region in the sky where no emission is expected at the frequency of observation down to the sensitivity requested. ... ALMA staff will select TP OFF positions, and take into account any OFF positions possibly suggested by the PIs in Phase 1 (in the submitted proposal). PIs may be contacted and asked for suggestions, if the OFF position selected by ALMA staff is found to be contaminated”