The other major atmospheric condition that astronomers worry about is "transparency". "Transparency" is a very descriptive term that simply refers to how clear the sky is. Most observers measure transparency by simply using

Fig. 8.1. The professional observatory of the Central Texas Astronomical Society. Notice the hood on the side that conceals a large exhaust fan. One on each side of the building helps to minimize the buildup of heat inside the dome during the day. (Credit: John W. McAnally)

the stellar magnitude scale, and this is a system of measurement advocated by the A.L.P.O.

A measure of transparency simply states the limiting magnitude that can be seen in the immediate vicinity of the object being observed, absent light pollution. In other words, transparency indicates clear sky or clouds in varying degrees. For example, if during an observation under dark skies sixth magnitude stars can be seen, then we say the transparency was six. If only third magnitude stars could be seen in the vicinity of the object being observed, we would say the transparency was three, and so on. When observing from a light polluted area, you will need to make a judgement as to what the transparency would be if you were under a dark sky. And so again, the use and application of the transparency scale is common sense.

Along with transparency, other conditions that have an affect on our observations would be the presence of haze, water vapor in the air, or wind. Transparency could be quite good under each of these conditions, but our ability to make a good observation could still be degraded. Recording this type of information is important, as we will see later in this book.

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