Astronomical Observations: Types of Objects It is now appropriate to consider the kinds of objects most sensitive to increased sky brightness. Objects of astronomical observations fall into two categories: point sources and extended sources. Returning to equation (1), we note the term A x N, the brightness of the sky. This brightness is a product of N, the number of photons coming from a unit of area of the sky, and A, the amount of sky being observed. For point sources, every attempt usually is made to make A as small as possible. Although there are limits as a result of the turbulence of the atmosphere, a small aperture does represent a practical method of reducing the effect of sky brightness for objects that appear as point sources, such as stars. However, this strategy will not work for extended objects, such as galaxies. In order to reduce the amount of sky being observed, one must correspondingly reduce the amount of the object to be studied. The case of galaxies is an important one since the average surface brightness of a galaxy (the number of photons emitted per unit area) is considerably less than the average brightness of the dark night sky. Galaxies are brighter toward their nuclei; therefore, the net effect of increasing the sky brightness is to reduce the amount of a galaxy that can be observed. This effect is qualitatively illustrated in Figure 1. The research consequences of not being able to observe the outer regions of galaxies are noted, along with other research programs, after the following section. Astronomical Observations: Organization and Conduct of Programs An observing program is the set of observations that astronomers will make in order to address a particular astronomical problem. The organization of typical observing programs is important because the increase in diffuse sky brightness as a result of SPS is a function of position on the sky, which implies that in particular regions of the sky it will not be possible to observe particular kinds of objects. Observing programs consist of surveys and type studies. Most observing programs include elements of both kinds of studies. A survey entails the
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