An Auxiliary Telescope for ALMA

1. Equip one of the ACA antennas with standard gain horns, 
one for each band.  Locate them around the perimeter of the 
reflector, pointed in the same direction.

2. Calibrate at one wavelength at a time.

3. Observe a bright planet with the ACA.  For the special 
antenna, switch between the regular feed being connected 
to the receiver and the standard gain horn being connected 
to the receiver.  The other antennas of the ACA form 
interferometer pairs with the special antenna as usual.

4. The ratio between the correlated signal with the horn 
as antenna and the correlated signal with the whole 
reflector as antenna is the ratio of the voltage gains 
of the horn and the whole reflector.

5. The power ratio, added to the known gain of the horn, 
gives the gain of the whole reflector.

5. With the now known gain for the reflector of the special 
antenna, a single antenna observation of the planet with 
an accurate radiometer calibration provides an accurate 
flux for the planet.

6.  A similar single antenna measurement of the planet 
with each of the other antennas transfers the gain 
measurement to the other antennas.

7. Then the ACA is used to calibrate the fluxes of the 
QSO's that will be used as phase calibrators by the 12m 
array on that day.  The phase calibrators can now be 
used for accurate flux calibration as well.

8.  The stable standard is the standard gain horn.  
The fluxes of the planets and the QSO's can vary, and 
the antennas can be repaired or modified without
affecting this calibration technique.