Astrophotography is a specialised type of photography that entails taking photographs of items in the night sky such as planets, stars, and deep sky objects.
Astrophotography is used to reveal objects which are too faint to observe with the naked eye, as both film and digital cameras can accumulate and sum photons over long periods of time.
Astrophotography poses challenges which are distinct from normal photography in that the subject is usually extremely faint, and often small in angular size. Effective astrophotography requires the use of many of the following techniques:
- Mounting the camera at the focal point of a large telescope
- Emulsions designed for low light sensitivity
- Very long exposure times (one hour or more is common)
- Tracking the subject to compensate for the rotation of the Earth during the exposure
- Gas hypersensitizing of emulsions
- Use of filters to reduce background fogging due to light pollution of the night sky.
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.
Since the early 1990s, most professional observatories switched to using CCD devices for astronomical imaging. They have numerous advantages, with increased sensitivity and more linear response. The principle disadvantage is a significant reduction in the field of view. CCDs require specific modifications for best effect in the low light conditions of astronomy, such as:
- cryogenic cooling to reduce thermal noise
- compensation for non-uniform pixel sensitivity
- biasing
Emulsion based astrophotography remains popular with amateur astronomers, but even here CCD imaging is becoming relatively cheap and is starting to replace wet photography.
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