Astrophotography

Man has always been fascinated by the heavens. He has gazed in admiration at the stars, planets, nebulae, and commits which twinkle in the night sky. And with his camera he has also captured the breath taking images of these brilliant treasures. Read the article and learn to click at the jewels that adorn the night sky…

So, how do we carry on with it? All that you need is camera, film, tripod and a release cable. A camera having arrangement for different shutter speed and aperture is perfect for our need. For this, a SLR (Single lens reflector) camera is on optimum choice. Films are classified according to their sensitivity to light usually referred as speed of the film .It is given in the form of ASA or ISO number (30,60,100,125,200,400,800,1600,3200). More the number higher the speed. But high-speed films are grainers. An ISO 400 film will be the best choice for beginners. A black & white film is prepared over a coloured film while photographing the star. A tripod is needed f or long exposure photographs and a release cable helping clicking photographs without actually touching the camera. Thus preventing the vibration set up by our hands.

Our basic aim in astrophotography is to let the film be exposed to as much light as possible since the objects of interest are very faint. This depends on the aperture of the camera. It is indicated by the numbers viz.1.7, 2,2.8,4,5.6,8,11 and 16. The number 1.7 indicates the lens to be wide open whereas the number 16 indicates the least. Actually the numbers signify a property called the "f" ratio. It shows the ratio of the aperture to the focal length of the lens. The focusing system of the camera must be kept at infinity.

Now regarding the exposure time i.e. speed of the shutter, numbers viz. 2,4,8,15,30,60,125,250,and 500 on the camera indicate this. Suppose we set the camera speed at 125, it means that the shutter of the camera will close within 125th of a second. But these speeds are of no use in astrophotography especially while photographing the sky without any optical instrument. There is a separate setting indicated by B. this allows to keep the shutter open for any desired time. Exposures of more than 15-20 seconds will cause star trails. This is due to the rotation of the earth. This exposure time is for a 50mm lens. For a 300mm lens the exposure time will be 6 times less i.e. 3 seconds for preventing star trails.

A tripod is necessary because while photographing the sky we need to give exposures of at least few seconds. Holding the camera in our hand will never give sharp images however cautiously you do it.

For beginners photographing the star trails is a very good exercise. Fix the camera on the tripod, set the focus to infinity, shutter speed to B., the aperture to 2 or 2.8, point it towards some area near the celestial equator and click. Keep the shutter open for 5-10 minutes and your first photograph is taken. The disadvantage with B setting is that the release cable is to be held pressed for the desired time. So it is recommended to buy a release cable with a locking arrangement this problem is also solved if your camera has an additional setting called T, which allows you to keep the shutter open till the release cable is pressed again. Beware of touching the camera. This may set vibrations and spoil the photograph.

With the same arrangement and keeping the shutter open for about 15 seconds, sharp images of the stars can be obtained. By comparing photographs of the same part of the sky taken on two different nights one can easily see the movement of planets if any, against the background of the stars. One can keep the shutter open for a few minutes on a night of an active meteor shower and hope for catching bright one.

You can also try photographing the crescent moon with a few bright objects like Venus or Jupiter around it. In this case the exposure time will have to be less. It is always good to bracket your photographs by giving different exposure time. You can also try to photograph the magnified image of the moon if you own a small telescope. Remove the lens of camera and also the eyepiece of telescope. Hold the camera in place of eyepiece. Focus the moon by moving the camera up or down the focusing length and click. The exposure in this case will have to be much less, say about 1/30th or may be slightly more. Again bracket your photograph with different exposures. This kind of photography is called prime focus photography. For photographing the planets you will have to use the eyepiece. You will notice the difficulty in focusing and the loss of clarity due to introduction of another lens.

If you desire to photograph a region of the sky without causing star trails then you will have to use a mount which rotates at the same speed as the stars round the pole (i.e. 1 degree in 4 minute). This mount can be either hand or motor driven. Just the words of caution expect more failures than success.