Imagers

Imagers capture the two-dimensional pattern of light at the telescope focal plane. They consist of a detector array along with the necessary optics, electronics, and cryogenic apparatus to put the light onto the array at an appropriate angular scale and wavelength range, to collect the resulting signal, and to hold the detector at an optimum temperature while the signal is being collected. Imaging is basic to a variety of investigations, but is also the foundation for the use of other instrument types that need to have their target sources located accurately. In this chapter we discuss the basic design requirements for imagers in the optical and the infrared and guidelines for obtaining good data and reducing it well. We finish with a section on astrometry, a particularly demanding and specialized use of images.

Optical imager design

A simple optical imager consists of a CCD in a liquid nitrogen dewar or cryostat with a window through which the telescope focuses light onto the CCD (see Figure 4.1). Broad spectral bands are isolated with filters, mounted in a wheel or slide to allow different ones to be placed conveniently over the window. Although this imager is conceptually simple, good performance requires attention to detail. For example, if the filters are too close to the CCD, any imperfections or dust on them will produce large-amplitude artifacts in the image.