6.1 Introduction

The measurement of radiant power is an important aspect of astrophysical research. The material in the previous chapters concentrates on image location, size, orientation, geometric image quality, throughput or étendue, and transmittance. This chapter concentrates on radiometry, defined as the science of the measurement and characterization of electromagnetic radiation (power). The nomenclature used by engineers of optical and IR systems will be used here.

Astronomical applications of radiometry include the measurement of stellar magnitudes or the intensity of point sources and the measurement of the surface brightness of the moon, planets, nebulae, and galaxies in the visible and the IR regions. The IR spectrum is typically partitioned into regions depending on the detectors and the need for cooling telescopes and instruments. The performance of a ground-based IR telescope system is very sensitive to the thermal background radiation from the telescope, instrument, and atmosphere. Spacebased telescopes for far-IR astronomy are immersed in the cold environment of space and require careful management of the thermal environment of the detector, telescope, and instrument. Thermal energy is transferred to telescopes and instruments by three mechanisms: through radiation from internal and external sources, through conduction, and through convection.

First-order optics and diffraction theory provides us with image location, size, orientation, and diffraction image quality, but provide little information about the amount of power that is transmitted from the object to the image. Radiometry provides the analytical tools that are used for three important applications in optics:

1. Determining the radiative power transferred from object space to image space. An image can be of high quality but is of little use unless it is sufficiently bright to be recorded. This calculation provides information on the signal strength at the focal plane.