The detectors at the foci of telescopes may be position-insensitive such as the classic photomultiplier and the simple proportional counter. Position-sensitive detectors at the focus of a telescope provide an overall field of view that includes many beams (resolution elements). The charge-coupled device is widely used in optical and x-ray astronomy for this purpose. Its internal structure and operation reveal its strengths and weaknesses. Gamma-ray astronomers use plastic and crystal scintillators and spark chambers or their equivalent. Examples are the EGRET and BATSE instruments that were in orbit during the 1990s. The precision of a detected signal is limited by statistical and systematic errors. Knowledge of basic statistical theory enables one to assess the significance and meaning of one's data. Aspects of this are the character of statistical fluctuations (the Poisson and normal distributions), background subtraction with error propagation, and comparison of data to a model with a least squares fit and the chi square test.