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Astrophotography PDF Print Email
Written by James Link & David Carton   
Tuesday, 01 September 2009 16:00

Astrophotography is the science and art of photographing the night sky. Astrophotography allows people to share the wonders of the night sky. The Society has its own equipment for this purpose. You can view our collection in the Astrophotography Gallery. Or if you want to show off your own best snaps, why not upload your own to the Members' Gallery?

There are many techniques in astrophotography and when used correctly they can vastly improve on what a human eye can see. There are two basic principles in astronomy. One is to collect as much light as possible and the other is to capture it undistorted.

The first principle can be dealt with by increasing the exposure time. Simply, the longer the exposure the more light that can be collected and then the fainter objects become visible.

The greatest advantage a camera has over the human eye is the longer exposure time. To achieve this a camera needs to track the rotating sky, which can achieved by mounting the camera to a motorized telescope. The mount must be precisely aligned with the celestial pole, either the north celestial pole (NCP) in the northern hemisphere, or the south celestial pole (SCP) in the southern.
Andromeda Galaxy

 

Not tracking the sky can also produce spectacular photos. The sky rotates about the poles, which produces star trails. The stars and planets produce concentric arcs, but other bright objects that cross the field of view in short periods of time will appear as straight lines. These objects could be satellites or even meteors. Indeed this is the best way of capturing images of the latter during meteor showers, such as the Perseids in August. Star Trails


Long exposures are not always required, indeed the brightest celestial objects such as the Moon and planets would quickly become overexposed. However, on these celestial objects high magnification is important to reveal detail. High magnification brings with it the problems of atmospheric distortion (see below), and requires a longer exposure due to the reduced brightness.

The atmosphere is turbulent and causes distortion known as "seeing" . Seeing is what causes the stars to twinkle and under high magnification they appear to jiggle about. It causes a rippling effect when viewing the surface of the moon or a planet such as Jupiter.

With seeing there are split seconds where the image is undistorted. Capturing these good images is achieved with high frame rate devices, such as web cams. These images can be aligned with each other and stacked to reduce the "noise". They can also be streamed to a laptop in situ.

Stacked webcam image of Jupiter

Noise, mentioned above, is another form of image distortion that is introduced by the detector. There are several causes for this noise, the most common is random thermal noise in the image sensor. This can be reduced by cooling the detector or stacking and averaging the images. If the noise is random, averaging will reduce the effects of noise. This technique can be used on both short and long exposures.

The best way to overcome noise is to reduce its occurrence at the source. This is why choosing a suitable camera is very important. The best equipment available are specialised cooled CCD cameras, but these can cost hundreds or thousands of pounds. After these, the most suitable for planetary imaging are webcams, as explained above, while SLRs (single lens reflex) are better for deep sky imaging. These are cameras for which interchangeable lenses are available, and have the "bulb mode" exposure setting, which allows the shutter to be held open indefinitely, controlled preferably by a cable release. Ideally the body of the camera will be connected directly to the telescope with an adaptor, and no lens in between. This is for reasons explained above.

Last Updated on Thursday, 26 July 2012 13:12