By Forrest Mims
Aerosols such as dust, smoke and smog cause a bright glow around the sun known as the solar aureole. Higher concentrations of aerosols cause larger aureoles. Dust tends to cause the aureole to have a sharper border than smoke and smog. The aureole shown here was photographed at Geronimo Creek Observatory at solar noon on 16 June 2005.
Aureole Photography Techniques
The light of the direct Sun is so intense that photographs of the Sun do not show the solar aureole unless the aureole extends across a substantial portion of the cameraâ€™s field of view (FOV). Proper aureole photography requires that the solar disk be occluded.
The solar aureoles presented in popular books are often photographed with an object such as a street light or flag pole interposed before the solar disk. This method works well when an appropriate object is available, which is often not the case. Moreover, it can be hazardous for oneâ€™s eyes to make aureole photographs in this fashion. That is because it is difficult to frame the photograph without inadvertently looking at a portion of the solar disk.
For serious aureole photography, a simple handheld occlusion device is the preferred way to block the direct solar disk. This method has the added safety advantage of not requiring the photographer to peer through the cameraâ€™s viewfinder when the camera is being pointed toward the Sun. A very simple but effective occlusion device can be made by mounting a spherical ball or disk on the end of a thin rod or stiff piano wire. Both the ball or disk and the support rod should be painted flat black. The ball or disk should have a diameter slightly larger than that of the cameraâ€™s lens.
To make a solar aureole photograph, the camera is placed on a flat surface and tilted toward the Sun until sunlight passing through the viewfinder forms a bright spot of light on the surface below and behind the camera. The spot of light will be surrounded by the cameraâ€™s shadow.
[CAUTION: NEVER VIEW A POLLEN CORONA DIRECTLY OR BY LOOKING THROUGH THE VIEWFINDER OF A CAMERA.]
The camera is then aligned until the spot of light is approximately coaxial with that portion of the shadow that approximates its actual location. The camera is then held in place by one hand with a finger over the shutter button. The occluding device is then held 10 cm or more from the camera until its shadow falls directly over the cameraâ€™s lens. The shutter button is then pressed.
After some practice, this method of aureole photography will yield excellent aureole photographs. The photographs will be even more desirable if the photographerâ€™s fingers and hat do not appear in the finished images. While a literature search did not disclose a description of this method, the principle is so obvious that it is probably known to other aureole photographers.
Manually holding the shadow disk or ball in place while also making sure the camera does not move can be somewhat tedious. An alternative method of blocking the Sun is to mount both the camera and the occluding device on a common platform with a backboard on which the cameraâ€™s shadow will fall when the apparatus is pointed toward the Sun. The shadow device should be mounted coaxially with respect to the lens and at least 10 cm away from the lens. The apparatus is aligned with respect to the Sun by tilting it until the spot of sunlight is aligned as described above.
These methods work well with fixed focus cameras and automatic cameras with a lens that can be set for infinity. Cameras with autofocus lenses should be set up without the shadow device in place. This is done by pressing the shutter button half way until the lens focuses at infinity. The shadow device is then placed between the Sun and the lens and the shutter button is pressed all the way.
Â For more information on photographing and analyzing solar aureole, visit www.sunandsky.org.