The following formula can be used in conjunction with the
typical zenith 
s for VLBA antennas given in
Table 2 to calculate the RMS thermal noise (
) expected in a single-polarization image, assuming
natural weighting (Wrobel 1995):
where 
is discussed in Section 12; N is
the number of VLBA antennas available; 
is the bandwidth
[Hz]; and 
is the total integration time on source [s].
Equation 3 also assumes 1-bit (2-level) quantization. If
simultaneous dual polarization data are available with the above
per polarization, then for an image of Stokes I,
Q, U, or V,
For a polarized intensity image of 
,
It is sometimes useful to express in terms of
an RMS brightness temperature in Kelvins (
) measured
within the synthesized beam. An approximate formula for a
single-polarization image is
where 
is as in Equation 1.