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The Determination of K-lambda

of

Cumberland Observatory

Summary

Light from stars, when passing into the atmosphere, is attenuated by the layers of air. In order to obtain accurate photometric data of an object (unless differential photometry is being carried out) an accurate description of the amount of attenuation is necessary. Without this, actual visual magnitude measurements are impossible. Naturally, the amount of interference depends on several factors. These include angle of observation (as greater angles from zenith or zenith angles mean more atmosphere to pass through), the wavelength of light (light is reddened as it passes through our atmosphere), and others such as temperature and moisture content of the atmosphere. In addition, light pollution from surrounding areas can hamper data collection.

The constant K-lambda or extinction variable can be determined through two major means. The first is by observing two stars of the same spectral type at two different angles in the sky (the High-Star Low-Star method) over the course of several hours, over the course of several nights. The second method is the so called Blue-Star Red-Star method where two stars at the same angle or very close of very different spectral types are observed.

From these data and some simple math, data points of observed magnitude as a function of air mass (or zenith angle) can be plotted. The line described by these points describes Bouguer's Law. First order and second order K-lambda coefficients can be obtained quite simply.

State of the Project

Very little data has been taken for this project, as the weather has been a major difficulty in recent months. However, it is hoped that some data will be taken soon and the coefficients can be determined. This in no way interferes with the differential photometry of 2S0114+65/LSI +65 10, where simply variations in the light curve are observed.

For Further Information

1 March 1996