Snapshot

If you do not want to image complex structure in sources, you may choose to make one or more observations with restricted hour angle coverage. Such observations, known as snap-shots or cuts, can provide positions and flux density measurements of many sources in one 12 hour observation.

Snapshots observations are best made of strong point sources, but you may be able to determine the extension and possibly the flux of very simple structures. For strong (S $>\sim5~\times~\mathrm{confusion}$), small diameter sources, two but preferably three cuts are adequate to determine fluxes and positions. For strong, extended sources more cuts are needed to determine structure. Use this table as a guide. For weak sources or detection experiments you must have sufficient cuts to reduce sidelobes of confusing sources below your sensitivity limits. This requires about three cuts at 3cm, at least 8 cuts at 6cm and more at 13cm and 20cm. Note that a single VLA snapshot is equivalent to more than six cuts with the ATCA.

You can expect to achieve a dynamic range of up to 200:1 with good snapshot observations. Typical rms noise (with three 15 minute cuts at 2.4GHz in a field free of confusing sources) is 0.7mJy/beam. Some observers have found that the total number of beam areas covered by the source is more important in determining the dynamic range than the largest angular size. Interpretation of cut ``images'' can be difficult due to the poor synthesised beam, and high sidelobe levels make confusing sources especially confusing. You cannot successfully clean a badly confused field with high sidelobes.