After calibration on a bright point source, SIM could perform measurements while continuously rotating around the line of sight. The calibration has to be repeated every time a different baseline length is chosen. Repeating this process with the slit offset on the sky allows to mosaic large fields. Their signals fall on different pixel rows on the CCD, with their fringe frequencies being the same. If the resulting fringe frequencies are unacceptably high, then the slit has to rotate with the baseline, which causes complicated sky coverage.
More detailed study is required.
The main difference between the two is that for SIM, the phase calibration remains locked on the guide stars during the full instrument rotation, therefore re-calibration is only necessary when switching baselines. Example calculations to estimate the SIM sensitivity were given for two source models. Aurigae with an average SNR of 5 per pixel, and using about 1200 u - v points, the expected total observing time is several days. This contrasts strongly with the typical radio imaging case, in which the SNR depends only on receiver noise. This is, however, work in progress and quantitative analysis will take some more time.