Shadowing and Cross-Talk

Observations at low elevation in the C and D configurations will commonly be affected by shadowing. It is strongly recommended that all data from a shadowed antenna be discarded. This will automatically be done within the AIPS task FILLM when using the default inputs. Note that for versions of AIPS up through early incarnations of 31DEC01, FILLM is ignorant of antennas in other subarrays, or antennas which are out of the array, so flagging of antennas shadowed by antennas in other subarrays will not occur. For the final 31DEC01 AIPS and all subsequent versions, FILLM is smart enough to know about all antennas in all VLA subarrays.

Cross-talk is an effect in which signals from one antenna are picked up by an adjacent antenna, causing an erroneous correlation. At 20 cm, this effect is important principally in the D configuration. At 90 cm, C and even B configurations can also be affected. And at 400 cm, all configurations show strong cross-talk on many baselines. Careful editing is necessary to identify and remove this form of interference. For the 90 and 400 cm bands, use of the spectral line modes is strongly recommended to allow detection and removal of these contaminating signals.