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Chapter 5:
Transmittance, Throughput, and Vignetting

5.1 Introduction

Prior chapters discussed image location and size as well as geometric image quality. We have already discussed optical power (units of length-1), which describes the wavefront bending capability of a lens. Here we consider the radiant power (units of watts/m2) necessary to provide such sufficient brightness of an image at the focal plane that it can be recorded with the desired SNR. In the following sections we will discuss those parameters of the design that affect the ability of the optical system to transmit power most efficiently. Without radiant power at the focal plane image, the location of the image and its quality are not meaningful.

Most astronomical objects are very faint, and it is very important that most of the radiation falling on the entrance aperture is passed unattenuated to the image plane. Thus, in Sections 5.2 and 5.3 we will discuss details of system transmittance, define the area-solid-angle product, and gain an understanding of the conservation of the area-solid-angle product throughout the optical system. Two factors are involved in the calculation of power to the focal plane from object space. One of these is the system transmittance, and the other is system throughput (or étendue). These are different and should not be confused.

In Chapter 6 we will provide the analytical tools to determine (1) the radiative power transferred from object to image space, (2) the thermal background radiation incident onto the focal plane, and (3) the temperature distribution of optical elements and their support structures throughout the optical system.

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