Astrophotography

Astrophotography is a specialised type of photography that entails making photographs of astronomical objects in the night sky such as planets, stars, and deep sky objects such as star clusters and galaxies.

Astrophotography is used to reveal objects that 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 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
• Emulsions designed for low light sensitivity
• Very long exposure times and/or multiple exposures (often more than 20 per image).
• Tracking the subject to compensate for the rotation of the Earth during the exposure
• Gas hypersensitizing of emulsions to make them more sensitive (not common anymore)
• Use of filters to reduce background fogging due to light pollution of the night sky.

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.

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.

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. 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:

• cryogenic cooling to reduce thermal noise
• compensation for non-uniform pixel sensitivity (Flat-field_correction)
• biasing

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 camera with manual exposure control and self-timer.

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.