Differential Gain Calibration

Sometimes it is not possible to use standard calibration techniques due to a lack of SNR on the phase calibrator. This can result from the aggregate bandwidth of the spectral setup being too low or observing at high frequencies where calibrators are fainter. Observing in large configurations can also present difficulties due to the phase stability being poor. For these reasons, some observations are now carried out using Differential Gain Calibration (DGC), a technique whereby an additional spectral setup is used that gives sufficient SNR on the phase calibrator. As there is generally a phase difference between this spectral setup and that used for the science observations, additional observations are made of a strong source (“DGC calibrator”) in order to determine the phase difference.

There are two varieties of DGC observing:

B2B has allowed ALMA to accept proposals requesting the highest frequency bands (7-10) with the largest configurations (C-8-C-10), although not all of these are currently commissioned e.g. Band 10 with C-9 or C-10. The need for B2B is checked by performing a search of the calibrator catalogue to see whether a phase calibrator is present that is both bright and close enough to the science target. If yes, then in-band (normal) calibration will be assumed. If not, the OT will assume that B2B must be used, but will now check if a suitable calibrator is available at the lower frequency. If there still is no calibrator available, the OT will issue a validation error.

Bandwidth Switching is currently assumed by the OT for any “narrow-bandwidth” setup i.e. those where the aggregate non-overlapping bandwidth is less than 937.5 MHz. This is somewhat crude as DGC might not actually be required where there is a very bright calibrator near the science target. It is likely though that a similar procedure to that used in the B2B case (calibrator search) will be used in the future.

The additional calibration time required for DGC is included in the OT's time estimates.

The ALMA OT Team, 2024 May 21