The purpose of an astronomical instrument is to extract information from the photons collected by a telescope. In principle, all of the following photon properties are measurable:

1. the direction of the photon;
2. the time of arrival of the photon;
3. the energy of the photon;
4. the polarization of the photon;
5. the bunching properties of the stream of photons.

Imagers are used to study the spatial distribution of the photons from a source, i.e. they primarily measure property (i). Photometers are used to study the brightness of sources, and how they vary with time, i.e. they primarily measure property (ii). Spectrographs look at the wavelength distribution of light, i.e. they primarily measure property (iii). In this part of the course, we shall look at all three of these instruments. We shall not cover polarimeters, which exploit property (iv), as this is a relatively niche area of research. Instruments which exploit property (v), the realm of quantum optics, are even more niche and will not be discussed here either.