An atlas of spectrophotometric Landolt standard stars

We present CCD observations of 102 Landolt standard stars obtained with the Ritchey-Chretien 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 angstrom to 1.1 mu m. 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.