Some characteristics of the pritchard photometer

Using the Pritchard photometer as a specific example, this paper introduces a radial weighting function to describe both the in-focus and out-of-focus operation of a photometer. In both cases the central region in the instrument field of view contributes more to the instrument response than equal regions near the edge of the field. However, for out-of- ocus operation the central region of essentially uniform response will be reduced and may vanish. Further, three different fields of view can enter into the operation of a photometer even for in-focus operation. The size of these fields may vary by nearly 2:1. An experimental method of measuring the weighting function is described.