measurement of a few parameters for a large number of objects. An example is the National Geographic-Palomar Sky Survey. This survey, conducted in the 1950s, consists of photographs taken through two different colored filters of almost the entire sky visible from Mt. Palomar. A type study is the detailed study of a single object that is either unique or thought to be representative of a class of objects. Generally, type examples are usually the nearest members of the class. When planning the details of an astronomical observing program--specifi- cally, what objects are to be observed--several other factors must be considered. The most important is the fact that the atmosphere is not completely transparent so that the flux of photons from an object is attenuated as it passes through the atmosphere. The amount of attenuation is exponentially related to the amount of the atmosphere the radiation passes through, which is measured in airmasses. An airmass is the ratio of the amount of air toward the object to the amount of air toward the zenith, the point straight above the observer. In order to maximize the number of photons observed, observations must be planned to minimize the average airmass through which the object is observed. In practical terms, this usually means that an object will be observed during the time that the object is within 30° of the meridian, an imaginary line on the sky on which astronomical objects are at their minimum airmasses. Objects are in this band for four hours every day. Many astronomers have concluded that four hours is a practical upper limit for a single observation. Finally, because objects more than 60° from the zenith must be observed through more than two airmasses, objects that never come closer than 60° to the zenith are usually not included in an observing program. Astronomical Observations: Scientific Programs Listed here are current areas of active research that would be negatively affected by the SPS program. These research areas are typical, but no effort at completeness is intended.
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