A.1 Imaging the Sun with a Ball and Mirror Projector:

   

This is by far the simplest method to project the Sun's image, which works because the sunlight intensity is so strong. This experiment can be done in any room which has a window or door opening outside, and which can be sufficiently darkened by putting dark cloth over the places from where light enters the room. Complete darkness is not necessary. Just how much darkness is needed you can discover for yourself by trial and error. However, the darker the room the better the results.

You can make a powerful sun telescope at very low cost with a plastic ball and mirror. The ball provides a sensitive but steady mount. As in any good telescope the mount is as important as the optics. The optics is provided by the pocket mirror, If you don't have a hollow plastic ball, a watermelon can also provide a good mount.

Make a small hole in the plastic ball and fill the hollow with sand as fully as possible. Seal the hole with tape.

 

Now cover the mirror with an opaque paper screen except for a circular disc of diameter 2 cm at its center. Your optics is ready. Fix the mirror on the ball with adhesive tape.

A cylindrical ring forms the base for the ball and mirror. The ball can be set at any angle in its base. The sand inside adds to its weight and makes it stable. Your solar telescope is now ready for use.

Place the ball and mirror on a stool outside the room in the sunshine. Adjust the angle of the mirror so that it projects the sun into the darkroom on a white screen . Increase the distance of the mirror from the screen to around 30-40 metres. At this distance you will get a nice big image of the sun around 35 cm in diameter.

Believe it or not, but with this simple projector you can actually see sunspots on the sun. There are two ways to improve the resolution of the image. The first is to adjust the distance between the projector and the screen till you get the sharpest image of the sunspots.

The second is to hold a card with aperture hole punched out between the mirror and the screen, and experiment with different apertures. (Further refinement can be obtained by using elliptical apertures on the mirror depending on the sun's position in the sky. But this is only for the perfectionists).

                     

For best results use trial and error, which always works better than theory. Practice now during
April and May and you will be ready for the 8th of June 2004.

This amazing little optical device is really a powerful telescope with which one can get sun images as large as one wishes, limited only by the size of your dark room. For very large image ( one meter diameter, and even larger ) You will have to also increase the size of the mirror aperture. Use trial and error for best results.

A.2 Imaging the Sun with a Very Long Focal Length (VLFL) Convex Lens:

 

         It is commonly believed that a convex lens concentrates the light from the Sun. This however is true only if the focal length is small. As the focal length of the lens increases the size of the Sun's image increases. The relation is the same as for a pinhole projector. Image diameter = focal length of lens / 110. For a very long focal length (VLFL), the diameter of the Sun's image can be quite large, larger than the lens itself. Navnirmiti has developed VLFL lenses with focal length of 4 metres and ten metres. The second lens gives a large image of the sun more than 9 cm in diameter.
        

 

For a 90 mm diameter lens with a focal length of 10 meters, the image of the Sun will be larger than the lens itself, showing that a convex lens doesn't necessarily concentrate light. Using a VLFL convex lens, fixed within a cardboard shade with a hole cut in the center (see fig), you can get a nice big image of the Sun on which sunspots are clearly visible, if the lens is of good quality. Where can you get a good quality VLFL convex lens? These are not easily available and have to be specially ground and polished. They are also available at the Navnirmiti centres in Mumbai and Pune.

You can make a simple telescope with which to project Sun' image. Though it is possible to look directly at the Sun through the telescope with a proper filter, we do not recommend this method as most common filters are not safe, and a telescope concentrates light making eye damage more likely. The safer method is to project the Sun’s image with your telescope. A simple telescope can be made with only two lenses. The front lens (objective) has long focal length (about 1 meter is ideal), and the eyepiece should have a short focal length of around 5 cm or less. You will also require one long tube (1 meter), and another tube which slides inside the longer tube . (see fig.) The tubes can be effectively and easily made in the shape of triangular prisms out of mount board as is shown in the photographs.

The telescope can be used for projecting Sun's image as shown in the figure and photographs. The advantage of this method is that with good lenses you can get excellent resolution and study some details of the Sun's image. A lens kit to make your own simple telescope is available at low cost at Navnirmiti centres.