Steven Hansen, age 11, of St. Paul, Minnesota, for his question:

How strong a telescope is needed to view Pluto?

Faraway Pluto was not discovered by peering through a telescope. If astronomers had to depend upon this method, the outermost planet most likely would never have been found. It is only barely possible that human eyes could see it, even with help from the best type of telescope.

The year 1930 got off to a splendid start for Clyde W. Tombaugh. This young American astronomer announced to the world his stupendous discovery of the ninth planet of the Solar System. Newspapers showed pictures of its tiny spot of light in the black velvety sky. An arrow was needed to point it out. For the background was spotted with dozens of other dots of light from distant stars, some dimmer, but most of them brighter, than the newly found planet. It was named Pluto for the ancient god of darkness. And naturally, everybody wanted to look up into the skies and see it for themselves, perhaps to wave a welcome to the new addition to the solar family.

But this was impossible. Pluto is not visible to human eyes through the types of telescopes used by most star gazing hobbyists. It might be seen through a near perfect telescope with an aperture, say, 15 inches wide. But even with such a large, flawless lens, a viewer might have to search the sky for months and maybb years to see Pluto with his own eyes. The hardest job would be identifying the small dot of light against the background. of fixed stars.

Even using more powerful telescopes does not solve the problem. These huge telescopes bring in more light, but they tend to blur the most distant dots of light. Also as their power of magnification increases, they are scaled down to cover smaller areas and the field of viewing decreases. The fixed stars needed to place Pluto in the heavens would be outside such a small field. Maybe a few astronomers have tried to see Pluto for themselves and maybe a few have succeeded. But this is not the usual procedure. And even if a trained expert, using a 15 inch aparture telescope did manage to spot Pluto, he would see only a dot of light, perhaps 80 times smaller than Mars, as our eyes see it in the sky.

Astronomers do not spend much time peering through their telescopes. They depend upon photographs taken by cameras fixed to the giant, light gathering lenses. Dozens of photoraphic plates are made of a selected field of sky at different times and studied later. A blink microscope is used to speed up the comparison of the pictures. When two pictures are flashed alternately on a screen, a split second apart, the eye sees them as one and the same.

But the blink microscope accents the slightest variations in a pair of plates taken at different times. If a spot of light is absent from one plate, it blinks on and off. The blink microscope will also reveal the slightest changes in position and brilliance.

Pluto's average distance from the earth is about 40 Astronomical Units. It moves very slowly in relation to the earth. Against this background of fixed stars, Pluto takes more than four months to move a distance equal to the width of the full moon. Its position was suspected and countless telescope plates were taken of the region. But a blink microscope was needed to reveal the slight variations that proved it to be a planet  and not just another one of the teeming stars.