Astrophotography

Encyclopedia : A : AS : AST : Astrophotography

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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.

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 film photography.

Simple astrophotography

Although the description above suggests that astrophotographs can be made only with expensive equipment by photographers with extensive experience, in fact surprisingly good quality, wide-angle photographs of the night sky can be made by almost anyone using readily available single lens reflex 35 mm traditional film or digital cameras. What is beneficial, however, is that the photographer travel far away from the bright, light-polluted skies of major cities or towns. This will ensure 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) can be made as follows:

1. Mount the camera on a sturdy tripod. Load the camera with high-speed (at least ISO 200) film, or set the digital camera speed to between 200 and 800. (400 film speed and higher will be noticably grainier.)
2. Turn off the autofocus mechanism, and manually set the focus ring of the lens to infinity (or landscape/distance autofocus setting).
3. Set the shutter speed to either the bulb setting, or a range of settings between about 2 and 30 seconds.
4. Point the camera at the desired area of the sky, trip the shutter (preferably using a cable release, or the camera's self-timer in order to minimize vibration), and keep the shutter open for the desired length of time.
5. For best results, stop down the lens from its maximum aperture (often f/1.4, 1.8 or 2.0 for film cameras) at least 1/2 to one full stop. This will (i) reduce the uneven lighting across the image known as uneven field illumination, and (ii) reduce aberrations in the star images that can make them appear bloated or misshapen.

Earth's rotation will cause the night sky to appear to turn overhead. The result is that, using a stationary camera, within a short period of time the stars will appear as streaks of light on the photograph, rather than the sharp, point sources of light that we see with our eyes. Using a standard 50 mm lens on a 35 mm camera, stars will begin to streak across the film plane after about 30 seconds. Although very striking photographs can be made using exposures of minutes or hours - so as to deliberately show long star trails - most astrophographers either use short exposures on a stationary mounted camera, or else use a motor-driven telescope mount, in order to keep the stars as points of light in the final photograph.

Digital astrophotography

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, aperature 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. (The last some digital cameras do automatically for long exposures.)

Ironically, unlike typical digital photography where instant results are displayed, digital astrophotography often incorporates enough 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.

A few ideas

In addition to leaving the shutter open for many minutes to hours, there are other things that can create stunning astrophotos using only amateur equipment.

1. Don't limit your framing and composition to just the sky. Include some foreground objects (you and your binoculars, trees, an observatory). If there is some stray light about (which you should be trying to avoid), it may be enough to illuminate your subject. Otherwise, use a flashlight and literally "paint" your subject with it, or use car headlights as a flash source. The best thing is, your exposure will be at minimum 30 sec, and probably much longer, so you have time to start your picture, apply your flash or torch, then wait for the exposure to finish. This may take some practice (so record everything you do) but you should be able to get bright stars/star trails AND bright foreground subjects.
2. Try to catch a meteor shower and leave your shutter open for a good half hour. You might catch many meteors in one frame (but even just one is a thrill to see when you get your shots developed). You can also look for airplanes and satellites in your exposures.
3. This one is a little more tricky. Start a 5-10 min exposure in perfect focus (focus at infinity). At increments during the shot defocus your camera by small amounts. The shorter the increments and smaller the focus change the better the result, but this also gets increasingly difficult. The result should be cone shapes (instead of points of light) because the stars will move through the frame as they become larger (de-focused) blobs of light. The reason for doing this: unfocused images show their colour much better than tiny points of light. You will be able to compare the beautiful red/yellow/white/blue etc. colors of the different stars. Also, the cone shapes look slightly three-dimensional and very interesting.
4. The faster the film the brighter your stars will look, but also the more grainy your images. As a minimum, use a 400 speed film, but be very tempted to experiment with specialty films (such as 3200 speed).

Always record your shots in the order you take them. It's almost certain that as many as half your shots will not work. Recording your exposure times and other settings is the only way to improve. Photographers using digital cameras should consult the EXIF data.

See also

• David Malin
• Maksutov-Cassegrain_telescope
• National Geographic Society - Palomar Observatory Sky Survey
• Photographic plate
• Photography

External links

• [The Early History of Astrophotography]
• [Amateur Astrophotographies of The Messier Objects]

 

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