Recording astronomical images with a digital camera.

Fig. 1 Moon, 2003 Dec. 01 17h 34m, 100mm refractor (Pic: J C Vetterlein )

These notes are intended for those wishing to make a modest start on astronomical photography without necessarily having recourse to specialized equipment.

A digital camera offers a convenient means of recording astronomical images without a great deal of expense and trouble. Cameras with a small barrel diameter are at a distinct advantage for this work since they fit more snugly over the eyepiece when used with a telescope or binocular. Because digital techniques enable faster effective exposure times than are possible with conventional film cameras, reasonable images may be obtained using static telescopes or binoculars. Nevertheless, the instrument must be mounted on a sturdy tripod.

Under no circumstances attempt to photograph the Sun. This requires special filters and only experienced observers should attempt the task. Even when using filters great care has to be exercised.

The Moon.

Fig. 2a Moon, 2003 Sep. 19 02h 50m, 175mm Maksutov. Click for an enlargement (Pic: J C Vetterlein )

The Moon is without question the most rewarding subject for beginners. Spotter telescopes of between 60mm and 100mm aperture may be used to photograph the Moon simply by holding the camera lens to the eyepiece. It is better to select the speed manually rather than to rely on automatic mode. Since the aim is to use the fastest exposure possible for the subject a high ISO value should be selected (normally 400).

There is only any point in increasing scale and quality on the camera where the resolution of the telescope is adequate to justify it. As a rough guide, 2 mega pixels will do for a 20x80 scope.

The camera should be used in auto focus mode. This will ensure a sharp image provided you first focus the telescope on the selected object (much the same as for terrestrial work - birds etc.).

The Moon photographed through a spotter telescope of say 30x100 specification can give interesting results. (See fig. 1)

A good 150mm instrument working at a power of x100, and a camera at 3 mega pixels, should give images capable of considerable enlargement (a linear Moon diameter of 15 inches at least - see fig. 2).

The correct exposure will have to be assessed by trial and error. (Again, this is one of the virtues of using a digital camera.) The full Moon is not a particularly interesting subject but an exposure of 1/1000th (ISO 400) should be possible on a 20x80 scope. At half phase the speed will be considerably slower.

A half Moon may work well at 1/500th second, but remember the image is not simply half as bright as a full Moon; it is considerably fainter than this for reasons we shall not go into at present. A thin crescent Moon will require significantly slower speeds.

fig. 2b Moon showing Rupes Recta (straight wall) and the crater Birt on the terminator, 2003 Sep. 19 02h 52m, 175mm Maksutov Click for enlargement (Pic: J C Vetterlein )

Optical zoom may be employed to some effect (say, up to x3) but the exposure time will be longer and the risk of camera and telescope shake will come increasingly into play. It is a question of experiment to determine the optimum magnification for a combination of telescope, camera and conditions.

Since the aim is to have as fast an exposure as possible it is pointless increasing power if there is insufficient light to enable exposures faster than 1/100th sec. to be used.

A good deal can be done with the computer to recover an underexposed image. However, a dark image will always lose something in definition if it has to be processed too far.

In the case of the Moon, it is better to convert the captured image to grayscale for maximum contrast.

[This feature to be continued.]

J.V. 18/12/03