We present very detailed simulations of fast switching phase correction which treats, as accurately as we can, fast switching at all frequency bands available to ALMA, with the calibration being performed at either the target frequency or at 90 GHz.
This work leads us to the following results and potential problems:
Fast switching will typically have efficiencies (considering both residual decorrelation and time spent calibrating) ranging from 0.70 at the highest frequencies to 0.90 at the lowest.
Residual phase errors will range from 35 deg rms at the highest frequencies to 15 deg rms at the lowest frequencies.
For target frequencies above 345 GHz, calibrating at 90 GHz (where the sources are brighter and the sensitivity is better, but requires an extra calibration observation to determine the difference in the instrumental phases at the two frequencies) will
result in about a 5% increase in efficiency over calibrating at the target frequency.
It appears that below 345 GHz, there is little gain in performing the calibration at 90 GHz, and we may very well calibrate at the target frequency.
Performing fast switching at 90 GHz, rather than at the target frequency, only improves the efficiency by about 5% in the sub-millimeter, and may be deemed too much trouble for too little gain.
Problems with modeling dispersive delay in the sub-millimeter regime, or dry pathlength fluctuations together with even well-modeled dispersive delay, could push us to perform calibrator observations at the target frequency above 345 GHz.
Calibrator sources (ie, flat spectrum quasars) might not look the same at 90 GHz and the target frequencies because of dust emission at the higher frequency, or because the position of the core changes with frequency. These effects could make it difficult to calibrate at a frequency other than the target frequency.
The fast switching pointing specification (being able to move 1.5 deg in 1.5 sec with a 3 arcsec residual pointing error) is not acceptable for mosaicing or sub-millimeter 1 observations, and we need to study how quickly the antenna returns to an acceptable
The specification on changing frequencies is 1.5 s, which was intended to match the fast switching antenna motion specification. If this could be shaved to 1 s, we could probably make good use of that improved frequency change time.
At the best 5% conditions in the joint opacity/phase stability distributions, we determine that the residual phase jitter over short times (ie, 20 s or less) is 111 fs rms delay.
Also at the best 5% atmospheric conditions, simulations indicate that longer timescale (ie, 300 s) atmosphereic delay variation will produce residual errors in visibility phase corresponding to about 32 fsec rms.
View a pdf version of MMA Memo 523.