How Blue is The Sky Up There: Introduction

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch May 2004

Since the density and the humidity of the air decrease with altitude, one expects the sky to become darker and more blue when one goes upward. With a sounding rocket to a maximum flight height of 7000 m one reaches roughly one scale height of the atmosphere, where the density has dropped to about 36 percent of its value on the Earth's surface, one might already measure this.

The principal aim of the experiment is to measure throughout the flight both the brilliance and the colour of the sky in near zenith direction, and well away from the sun.

This is done by using arrays of simple photometers looking at small regions of the sky (about 10 degrees diametre) in white light as well as through a red and a blue filter. Because no precise imaging is necessary, the optics consists of simple collimator tubes which restrict the field of view of each sensor. There are three sensor array consisting of three photometers in the three colours. Each array is pointing out of the payload cylinder, in directions separated by 120 degrees, so that if one array happened to point into the Sun, the other two would still give valid data.

The main phase of the experiment is during descent on the parachute, as the ascent phase would be too short.

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