VLBA snap-shot imaging of scintillating sources

We present VLBA snap-shot images of weak scintillating sources and compare their morphologies with those of strong scintillating sources imaged by the USNO astrometry program. It is now clear that interstellar scintillation (ISS) is the principal cause of the intra-day variability (IDV) seen in some compact, flat-spectum radio sources at centimeter wavelengths. The MASIV (Micro Arcsecond Scintillation Induced Variability) survey has been looking for new scintillating radio sources. One result from this survey is that the fraction of scintillating sources increases strongly with decreasing flux density. Here we address the possibility that the above MASIV result arises because we are seeing different mas morphologies among the weaker sources.