Bringing the Sun Indoors (Sep, 1938)

I’m not sure if they still do this at the new Hayden, or if they do elsewhere, but it’s really cool. Basically using a set of mirrors they project an image of the sun onto the roof of the planetarium, so you get 26 foot wide image that’s safe to stare at.

Bringing the Sun Indoors
AT the Hayden Planetarium in New York a huge 26-foot image of the sun is being projected on the interior of the dome every day that the sun shines. This is accomplished by means of a first system of moving and fixed flat mirrors for bringing the sun’s image indoors and a second system of mirrors and lenses for enlarging and projecting it.

The actual sun is shown at the top of the drawing. Its rays are caught by an eight-inch flat mirror mounted on an axis parallel with the earth’s axis. A clock-like mechanism slowly turns this mirror as the earth’s turning “moves” the sun. This image, after passing through an opening in the building, is kept constantly spotted on a second flat mirror which is permanently fixed in position. It in turn passes the image downward to the third element of the Jong optical train, a flat mirror fixed at a 45-degree angle which turns it horizontally. The sun’s image is now where it can be used but as yet it is neither magnified nor projected.

Magnification is done in an ordinary eight-inch reflecting telescope, just as it would be if that telescope were directed at the sun out of doors; and since it is possible with any telescope to view the image not alone by looking into the eyepiece but also by projecting it on a screen at some distance from the eyepiece, the same is done at the planetarium. Here the distance is long, hence the image is very large—larger, in fact, than any solar image previously projected by similar methods. All this apparatus—the coelostat, fixed flats, and telescope—is entirely separate from the regular planetarium apparatus and could be similarly used with any ordinary house or building.

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