Astrophotography

From Wikipedia, the free encyclopedia

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 at 1/250th of a second, f/11 with a focal length of 800mm

1 Overview
2 History
3 Amateur Astrophotography
4 Difficulties
5 Film astrophotography
6 Digital astrophotography
7 Gallery
8 References
9 See also
10 External links
11 Categories

[edit] 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 time 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 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.

[edit] 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.

[edit] 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 webcam like the Philips ToUCam Pro.

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.

[edit] 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 apears to drop off as the exposure time increases, which also leads to color shift in the image.

[edit] Film astrophotography

Film astrophoto of the Andromeda Galaxy.

Astrophotography using conventional film is still being done to capture the cosmos, but is becoming less popular due to the growing popularity of digital cameras, lower on-going costs, and the instant feedback provided by digital media. However, film cameras are inexpensive, durable, and provide exceptionally fine resolution with a huge field-of-view. A comparative digital setup would entail a significantly more initial investment to gain similar results, but a much lower on-going cost.^{[citations needed]}

Most film astrophotography (including the photo on the right) is accomplished using a standard 35mm Single Lens Reflex (SLR) camera, connected to a telescope. For wide-field astrophotography, medium format film offers exceptional quality. ^{[citations needed]}

[edit] Digital astrophotography

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 principal disadvantage is a significant reduction in the field of view. Professional CCDs often 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 (Flat-field_correction)
biasing

Raw digital photo of Cassiopeia by Randal J.

However, amateurs are now producing spectacular results with off-the-shelf CCD cameras and inexpensive dedicated astro CCD cameras. With the advent of consumer digital cameras featuring CCD chips more sensitive than film, astrophotography no longer requires such long exposure times, special tracking equipment or non-light-polluted skies. Nothing more is required than a tripod, self-timer function and camera with manual exposure control.

The photograph of Cassiopeia to the right was taken at 1/16th of a second, aperture of 3.4 and ISO equivalent film speed of 200, with no additional processing. With digital images however, it is a simple matter to brighten pictures 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.)

Ironically, unlike typical digital photography where instant results are displayed, digital astrophotography often incorporates enough computer post-processing that the final results won't be known until later (such as the case of an object only as bright as the background noise). Therefore it becomes useful to bracket exposures as per traditional film photography.