Imaging the Night Sky

Fig 1 The constellation Orion. Also in the field are Sirius (low down in trees) and Procyon top left. Rigel appears the brightest star in the picture (bottom right). Betelgeuse (Orion's right shoulder) forms an equilateral triangle with Sirius and Procyon. This is known as the Winter Triangle. Imaged with Nikon D70 camera, 36 mm f/2.8 ISO 500 at 5 sec.

Fig 2 The effects of light pollution in presence of thin cloud. Lower part of Orion. The distortion of brighter stars images due to cloud. The light is from Kirkwall, 12 miles distant. Under such conditions pollution ifs detectable to an altitude of over 50 arc degrees. Nikon D70, 75 mm f/1.4 ISO 500 at 5 sec.. Note the short trails of each star due to diurnal motion.

Fig 3 Looking south at 17h 40m December 10 2004. Constellation of Aquarius with Uranus appearing as a 6th magnitude star. Imaged with Nikon D70 camera, 36 mm f/2.8 ISO 500 at 10 sec.. Light pollution now "normal" at this altitude and prevailing atmospheric conditions.

Fig 4 SWW on Dec. 10 2004 at 17h 42m. The Milky Way in the region Cygnus, Lyra, Aquila. The bright star top right is Vega. Star bottom left, Altair. Nikon D70 camera, 36 mm f/2.8 ISO 500 at 15 sec.

General considerations.

If a stationary camera is set for any period of time on a clear night with the shutter open and pointing to the North Celestial Pole, the stars will appear as a number of small arcs centred on Polaris. The arcs increase in length the farther they are from the pole. The effect is, of course, the result of the Earth's rotation.

The night sky appears vast and confusing to those not familiar with the constellations. A programme of photographing portions of the night sky can be instructive for the purpose of navigating around the stars.

Camera settings.

The aim is to capture the images as quickly as possible in order to minimize the diurnal effect noted above. Unfortunately the majority of digital cameras are not well suited to large field photography of the night sky. One requires a combination of a fast, wide angle lens (24 mm* f/2.8 or better), and a shutter speed of 10 seconds or more. Nevertheless it is often possible to work on the image at the computer in order to extend the number of stars visible.

The camera must have a manual setting facility. Select the widest aperture available. Choose a sensitivity selection of 400 ISO to minimize "noise", a possibility with long exposures. (800 ISO can be a problem from this point of view.)

Experiment on a conspicuous piece of sky such as Orion. Try the longest exposure available to start. This may be 15 seconds. Beyond 30 seconds a camera may well have a "bulb" setting. I would not suggest anything longer than 30 seconds exposure except for the Milky Way.

Some of the sample images demonstrate the effects of excessive light pollution. The colour differences between stars are also quite apparent.

* Standard 35 mm film camera equivalent.

JV 12/12/04

Saturn in decline

Those who have had the good fortune to see the stars and planets this autumn may have noticed Saturn making its way eastward below Castor and Pollux. The three (Saturn is about 2.5 times brighter than Pollux) appear almost in line but the planet is now retrograding (moving westward against the stars) and will do so until March 31 2005.

Dia 1 The path of Saturn from December 1 2004 to 26 Novemeber 2005. Click for enlargement.

Thereafter Saturn will move relatively quickly into Cancer and will be in line with Castor and Pollux during July when, of course, it will be close to the Sun in the sky.

These events signal the decline in Saturn's position for northern observers. Not only this but the aspect of the ring system will also begin to close. (See archives for more details.) Saturn will remain in the constellation Cancer throughout 2005, its declination (and hence altitude) reduced by some 5° compared to the present year.