Using a Camera to Record the Sky

Forrest M. Mims III

Since 1988 I have used a wide variety of homemade and professional instruments to measure sunlight direct from the sun and scattered by the sky and clouds. This research has permitted high quality retrievals of data about the ozone layer, solar ultraviolet, the water vapor layer, photosynthetic radiation, and the optical depth caused by such aerosols as smoke, dust and even pollen.

Even an inexpensive digital camera can also provide important information about the atmosphere. For example, the collage above shows the suite of images I make every day that the sun is open enough to make measurements of direct sunlight. The two upper images are made with a Nikon 990 3.3-megapixel camera equipped with a fisheye lens. The upper left image shows the north horizon. The circular image is a 186-degree fisheye view of the entire sky.

The lower three images are made with an older Fuji 1.3- megapixel camera. The lower left image shows the sky over the north horizon. The center image shows the zenith sky straight over Geronimo Creek Observatory. The lower right image shows the solar aureole, the glow around the solar disk caused by aerosols. The aureole images are made with the help of a black ball having a diameter slightly larger than that of the cameras lens. The ball is mounted on the end of a rigid wire and held so that its shadow falls directly over the camera's lens when the camera is pointed directly at the sun.

Caution: It is extremely dangerous to look through a camera viewfinder pointed at the sun! Do not do this! Instead, lean the camera against a flat surface and align it so that the beam of sunlight from the sun falls at the proper location on the shadow of the camera.

For details on how to analyze data from sun and sky images, see F. M. Mims III, Solar aureoles caused by dust, smoke and haze, Applied Optics 42, 492-496, 2003.

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