Astrophotography
Astrophotography is a specialized type of photography that entails making photographs of astronomical objects in the sky such as the Moon, Sun, planets, stars, and deep sky objects such as star clusters and galaxies.
Effective astrophotography requires the use of many of the following techniques:
- Mounting the camera at the focal point of a large telescope
- Film emulsions with low light sensitivity or specialized CCD cameras
- Very long exposure times and/or multiple exposures (often more than 20 per image).
- Accurate tracking of the subject to compensate for the rotation of the Earth during the exposure
- Use of filters to reduce background fogging due to light pollution of the night sky.
History
The first astrophoto is attributed to John William Draper, who took a photo of the moon in 1840. His son, Henry Draper, later became the first person to photograph the Orion Nebula in 1880, which was essentially the first deep sky astrophoto.
Today, astrophotography is a fast growing hobby that is popular among photographers, amateur astronomers, and hobbyists of all ages.
Amateur Astrophotography
Although the description above suggests that astrophotographs can be made only with expensive equipment by observatories or photographers with extensive experience; in fact, surprisingly good quality photographs of the night sky can be made by almost anyone using readily available single lens reflex 35 mm film cameras, digital cameras, inexpensive dedicated astro cameras to off-the-shelf webcams.
Photographs using exposures lasting several minutes or even hours will show long star trails (because of the Earth’s rotation). Some astrophographers do this on purpose for the desired effect. Most astrophotographers avoid this blurring by either using a short exposures on a stationary mounted camera, or by using a motor-driven telescope mount, in order to keep the stars as points of light in the final photograph.
Difficulties
There are several unique problems with photographing very faint objects, and taking such photographs from a moving platform (Earth) adds to the complexity.
* Astro subjects can be extremely faint - much fainter than the naked eye can see. In many cases the photographer can not see the subject being photographed.
* The spectra (color) of many astro subjects are difficult to record. Some are near the infrared, or require special filters to be separated from the background light.
* The Earth is constantly rotating. When imaging through a telescope or long focal length lens, the effect of Earth’s rotation will ruin an image within a fraction of a second, if nothing is done to compensate for it.
* Camera tracking platforms and/or telescope mounts are expensive, take time to set up, and can be difficult for a beginner. Special guiding techniques and error correction programs are required to ensure that the camera tracks the sky perfectly.
* Long exposure will lead to excessive noise for non-cooled digital cameras; active cooling and stacking can help to reduce this problem. For film based imaging, film will show reciprocal failure, that means the sensitivity to light of different wave lengths appears to drop off as the exposure time increases, which also leads to color shift in the image.
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