Photometric Calibrations for 21st Century Science
  The answers to fundamental science questions in astrophysics, ranging from
the history of the expansion of the universe to the sizes of nearby stars,
hinge on our ability to make precise measurements of diverse astronomical
objects. As our knowledge of the underlying physics of objects improves along
with advances in detectors and instrumentation, the limits on our capability to
extract science from measurements is set, not by our lack of understanding of
the nature of these objects, but rather by the most mundane of all issues: the
precision with which we can calibrate observations in physical units. We stress
the need for a program to improve upon and expand the current networks of
spectrophotometrically calibrated stars to provide precise calibration with an
accuracy of equal to and better than 1% in the ultraviolet, visible and
near-infrared portions of the spectrum, with excellent sky coverage and large
dynamic range.
