  Radiometry has been of fundamental importance in astronomy from the early
beginnings. In this review, we provide an overview of how to achieve a valid
laboratory calibration of space telescopes and discuss ways to reliably extend
this calibration to the spectroscopic telescope's performance in space.
Recently, the quest for independent calibrations traceable to laboratory
standards has become a well-supported aim and has led to plans for launching
calibration rockets for the visible and infrared spectral range. A survey of
the calibration of instruments observing from the X-ray to the infrared
spectral domains rounds off this review.
