Wednesday, April 18, 2012

A Problem With Projecting The Sun's Image With Binoculars And Its Solution

I tried doing eyepiece projection with a pair of binoculars and found that they didn't focus the Sun's image of the very well. A small positive lens will converge the rays to form a better image. For a projective image the best focus for the binoculars by themselves occurs when the eyepiece is at its farthest extension. Again an aperture stop in front of the objective lens is needed to reduce the brightness of the image for ease of viewing.

I tried both a 2D (diopter) lens and a 1D = 3D + -2D combination from an Edmund Scientifics 38mm Demonstration Lens Set. With the 1D combination one can create an image 1 inch in diameter. The lens holder was a small "envelope" made from 20lb matte ink jet paper that was attached to the binoculars with cellophane tape.

It helps to leave one of the lens caps on the binoculars and add a cardboard shade. An index card can be used as an imaging screen. The farther away the index card is the larger the image one gets. The aperture may need to be increased for larger images. I found it difficult to find the focal point. There were still some thin clouds out today.

The binoculars are a pair of Galilean telescopes and so the eyepieces are diverging. They were designed to work close to the eye and adding a positive lens corrects the focusing problem. I tried taping the 1.50D clip-on reading glasses over both eyepieces and found that will work if one does not have a lens set available.

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