Solar Projection

The projection method is preferred by many solar observers (see "Observing the Sun By Projection"). For a quick first look by this technique, hold a white card or paper a foot or two behind the eyepiece of a telescope or a pair of binoculars on a tripod. The card must be shaded from direct sunlight. Focus until the Sun’s limb appears sharpest. Almost as much detail will be visible in the projected image as when the Sun is viewed directly through an aperture filter.

When viewing the Sun via the projection method, follow these precautions for safe viewing:

Cover the finderscope.

Don’t aim the telescope at the Sun by sighting along the tube. Rather, move the tube until its shadow on the ground is smallest, while watching at a distance for light to come blazing out of the eyepiece.

Use a low-power eyepiece. If it has cemented lenses, as almost all modern ones do, stop the telescope aperture down to 2 or 3 inches by cutting a round hole in a piece of cardboard and fitting it over the front of the tube. More than a few inches of aperture will allow too much heat and light into the telescope. This heat can damage the clear cement between eyepiece lens elements. The best eyepiece for solar projection is an old fashioned Huygens or Ramsden, which has no cemented lenses.

Schmidt-Cassegrain telescopes in particular must be stopped down. Excess heat can damage the adhesive that holds the secondary mirror to its mounting in most Schmidt-Cassegrains.

Small telescopes are especially suited to the solar-projection method. Seen here is a simple projection system made from a cardboard box with a piece of white paper as a projection surface. A surprising amount of detail on the solar surface can be seen.

S&T: Craig Michael Utter

One advantage of the projection method is that sunspot positions can be marked right on the paper. A circle is drawn in advance, the Sun’s image is fitted to it and focused, and a few touches of the pencil does the rest. To record the orientation of such a drawing, mark the position of a sunspot, turn off the telescope’s drive and let the image drift, then mark the spot again; a line from the first dot to the second runs east to west. Nudge the instrument north; the Sun’s north edge is the trailing edge. Once a spot’s position is recorded, its solar latitude and longitude (heliographic coordinates) can be calculated using data from an astronomical almanac.

To do any of this, you’ll need a projection screen attached to the telescope to hold the paper. Such screens are sometimes supplied with small refractors. You can make your own (or better yet a projection box, to improve contrast by keeping daylight off the paper) by experimenting with cardboard, wooden dowels, bolts, and plywood. Plan the distance from eyepiece to screen to be 107d/(m – 1), where m is the telescope’s magnification and d is the diameter you want the Sun’s image to be.

When examining the image, try flicking the paper rapidly back and forth. low-contrast detail that was lost in the paper’s surface irregularities will pop into view.

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