The number of extragalactic sources N per square arc minute of sky with flux densities greater than S mJy at 6 cm can be written approximately as
over the flux density range that is relevant for confusion calculations at the VLA (e.g., Ledden et al.\ 1980). The corresponding expression at 20 cm is
The analogs of these expressions for 2 cm and 1.3 cm are not known
directly from measured source counts. They could be estimated from
the 6 cm count in Equation 9 by scaling flux densities to 6 cm
with an effective mean spectrum of 
.
Images made at 20 cm will contain, on average, one extragalactic source of flux density 110 mJy closer to the field center than the 15' HWHM of the primary beam. The 6 cm primary beam (4.5' HWHM) will contain, on average, one extragalactic source of flux density 2 mJy, the 2 cm beam (1.85' HWHM) a source of < 0.1 mJy and the 1.3 cm beam (1' HWHM) a source of < 0.01 mJy. At 92 cm, there is always at least one several-Jy source in the primary beam and there may be many. Individual pathological cases aside, confusion is unlikely to be a problem at wavelengths shorter than 6 cm. At 20 cm and 6 cm it may be a problem in the VLA's more compact configurations. At 92 cm it is a problem in all configurations.
Confusion is particularly troublesome for snapshooters using the compact configurations below 2 GHz, because the sidelobes resulting from the ``snowflake'' u-v coverage of snapshots are extensive. Images at low galactic latitudes are also more confused than those at high galactic latitudes. Snapshooters should plan to reduce their data using the ungridded subtraction algorithm (MX or IMAGR in AIPS) both because it permits imaging of multiple subfields and because it eliminates the effects of sidelobe aliasing.
