  Celestial standards play a major role in observational astrophysics. They are
needed to characterise the performance of instruments and are paramount for
photometric calibration. During the Herschel Calibration Asteroid Preparatory
Programme approximately 50 asteroids have been established as far-IR/sub-mm/mm
calibrators for Herschel. The selected asteroids fill the flux gap between the
sub-mm/mm calibrators Mars, Uranus and Neptune, and the mid-IR bright
calibration stars. All three Herschel instruments observed asteroids for
various calibration purposes, including pointing tests, absolute flux
calibration, relative spectral response function, observing mode validation,
and cross-calibration aspects. Here we present newly established models for the
four large and well characterized main-belt asteroids (1) Ceres, (2) Pallas,
(4) Vesta, and (21) Lutetia which can be considered as new prime flux
calibrators. The relevant object-specific properties (size, shape,
spin-properties, albedo, thermal properties) are well established. The seasonal
(distance to Sun, distance to observer, phase angle, aspect angle) and daily
variations (rotation) are included in a new thermophysical model setup for
these targets. The thermophysical model predictions agree within 5% with the
available (and independently calibrated) Herschel measurements. The four
objects cover the flux regime from just below 1,000 Jy (Ceres at mid-IR
N-/Q-band) down to fluxes below 0.1 Jy (Lutetia at the longest wavelengths).
Based on the comparison with PACS, SPIRE and HIFI measurements and pre-Herschel
experience, the validity of these new prime calibrators ranges from
mid-infrared to about 700 micron, connecting nicely the absolute stellar
reference system in the mid-IR with the planet-based calibration at sub-mm/mm
wavelengths.
