We present CCD observations of 102 Landolt standard stars obtained with the Ritchey-Chre´tien
spectrograph on the Cerro Tololo Inter-American Observatory 1.5 m telescope. Using stellar atmosphere models,
we have extended the flux points to our six spectrophotometric secondary standards, in both the blue and the
red, allowing us to produce flux-calibrated spectra that span a wavelength range from 3050 A°
to 1.1 mm. Mean
differences between UBVRI spectrophotometry computed using Bessell’s standard passbands and Landolt’s
published photometry were determined to be 1% or less. Observers in both hemispheres will find these spectra
useful for flux-calibrating spectra, and through the use of accurately constructed instrumental passbands, will be
able to compute accurate corrections to bring instrumental magnitudes to any desired standard photometric system
(S-corrections). In addition, by combining empirical and modeled spectra of the Sun, Sirius, and Vega, we calculate
and compare synthetic photometry to observed photometry taken from the literature for these three stars.