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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.

The Moon taken through a vintage Celestron C8 reflecting telescope with a digital SLR camera

Contents

Overview

Astrophotography ranges from simple images of bright objects to very complex exposures designed to reveal objects that are too faint to observe with the naked eye. With only a few exceptions, almost all astrophotography employs long exposures since both film and digital cameras can accumulate and sum light photons over long periods of time. This is just one of many distinct aspects of astrophotography that sets it apart from conventional photography.

Astrophotography poses challenges that are distinct from normal photography, because most subjects are usually quite faint, and are 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
  • 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, possibly automated using an autoguider, of the subject to compensate for the rotation of the Earth as well as the possible movement of the subject 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. Commercial astrophotography equipment is easy to find, and modern digital cameras are increasing in popularity due to lower cost and ease of use. However, skill and technique are extremely important, and the hobby can become a life-time passion or short-term frustration.

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 high 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, or off-the-shelf webcams.

It used to be that all astrophotographs had to be taken far away from the light-polluted skies of major cities or towns. This ensured that the sky is dark enough so that the photograph will not be completely washed out and ruined by bright urban light pollution. Simple wide-angle astrophotographs of constellations containing familiar star patterns (such as Ursa Major, Orion, Sagittarius and others) are still made in such a manner. However, with the advent of digital cameras, special light-pollution filters and advanced techniques of computer processing, photographers are now able to capture beautiful astrophotographs even from light-polluted, suburban skies. At the same time, bright targets like the moon and planets can be acquired in a highly-light-polluted environment.

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.

Amateur film astrophotography

Film astrophoto of the Andromeda Galaxy.

Astrophotography using conventional over-the-counter film is still being done to capture the cosmos. Film exposures range from 10 minutes to over an hour, and require constant and accurate guiding to prevent tracking errors from blurring the image. Since film is much slower than digital sensors, tiny errors in tracking can be corrected without much noticeable effect on the final image.

Based on a survey of on-line astrophotography, most film astrophotography is accomplished using a standard 35mm Single Lens Reflex (SLR) camera and a regular camera lens. While a tracking system is used to compensate for Earth's rotation, a telescope is often not needed. Longer focal length work (such as the photograph of the Andromeda Galaxy to the right) is done by connecting a film camera directly to a telescope instead of the camera's usual lens. This provides a long focal length lens with a necessary wide aperture to collect as much light as possible.

Film astrophotography is fast becoming less popular due to the growing popularity of low-cost digital cameras and a diminishing supply of suitable film emulsions. Also, film requires continuous on-going costs (film, processing, printing or scanning) while digital imaging none and also provides instant feedback, which is a major advantage to beginners compared to film based photography's extended development time.

Digital astrophotography

Since the early 1990s most professional observatories have switched from film to digital CCD devices for astronomical imaging. CCDs are more sensitive and have a linear response to light. Their principal disadvantage is a significant reduction in the field of view. Professional CCDs often require specific modifications for best results in the low light conditions of astronomy, such as:

Amateurs are producing spectacular results with standard photographic CCD cameras and inexpensive dedicated astro CCD cameras. With the advent of consumer digital cameras featuring CCD chips more sensitive than film, much astrophotography no longer requires extremely long exposure times, tracking equipment or non-light-polluted skies. Nothing more is required than a tripod, and a camera with manual exposure control and self-timer or cable release. The earth rotation can be compensated with a home made barn door tracker.

Digital images can be brightened and manipulated in a computer to adjust color and increase the contrast. More sophisticated techniques involve capturing multiple images to composite together in an additive process (negating tracking issues and bringing out dim objects), as well as using image processing to filter out light pollution and subtracting a “dark frame” to remove thermal noise (some digital cameras subtract the dark frames automatically).

Unlike terrestrial digital photography where instant results are displayed, digital astrophotography often requires computer post-processing before the results can be viewed (for example, for faint objects with a poor signal-to-noise ratio). This makes it advisable to bracket exposures as is usual with film.

References

See also

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