  We describe measurements of the size of the Vela pulsar via scintillation,
using both fits to the distribution of intensity and measurements of the
modulation index. We briefly discuss systematic effects other than source size
that can affect the distribution, including gain variations, self-noise,
scintillation shot noise, and correlator saturation. Modulation index, a single
number, can be biased by all of these, whereas the distribution of intensity is
affected in different ways by different effects, providing means of
distinguishing among them. Self-noise and gain variations are likely more
important at long observing wavelengths, and correlator saturation and
scintillation shot noise at short wavelengths. We find a size of about 500 km
at decimeter wavelengths. Interestingly, this agrees with measurements of
modulation index by Roberts & Ables at the same wavelength. Their results (and
more recently that reported by Macquart et al.) suggest that size decreases
with increasing wavelength. Although consistent with the observations, this
conclusion is perhaps surprising from the standpoint of the traditional
radius-to-frequency mapping. However, these measurements are of size rather
than height; and of course systematic effects may play a role.
