Solar projection

Jeffery Sandel uses a 10-inch f/6.5 Newtonian reflector to project a 3-foot-diameter image of the Sun onto the wall of his garden shed. With a clipboard, paper, and pencil, Sandel sketches each sunspot freehand, carefully noting its relative shape, size, and position.

Jeffery Sandel

Of all the stars in the sky, our Sun is the only one near enough to show a sizable disk. Every clear day professional observatories worldwide and in space monitor its dynamic surface features at various wavelengths. One activity that almost any amateur can participate in is tracking sunspots on the Sun's visible surface (or photosphere). Sunspots often change rapidly and unpredictably from day to day; you never quite know what to expect. Long-term, consistent observations of their number, morphology (form), and distribution are valuable to researchers.

Of course, observing the Sun can be dangerous without the proper precautions. Looking directly at the Sun without adequate protective filters can result in irreversible eye damage. (See "Solar Filter Safety.") Of the many ways solar viewing can be conducted safely, I have chosen the screen-projection method. You don't look at the Sun directly; instead a telescope casts a magnified image of the Sun onto a screen.

Any flat, white, smooth screen works well, provided the surface is not glossy, which can result in glary reflections. For my projection setup I use poster paper (2-ply, 50 pounds) fastened to a wall near the doorway of my large garden shed. The telescope — a 10-inch f/6.5 Newtonian reflector with a 16-millimeter wide-angle Erfle eyepiece — is positioned about 4 feet from the screen. This setup projects an image of the solar disk 3 feet in diameter. (You can vary the size of the image by experimenting with various magnifications and projection distances.)

Sketch of a sunspot group

This sketch by Jeffery Sandel shows a complex sunspot group. Unlike the traditional method wherein the sunspots' positions are plotted and their outlines traced directly on the projected solar image, he draws them freehand on a blank 8-inch-diameter circle. This results in exaggerated sunspot sizes.

The detail seen this way can be quite extraordinary. With a clipboard, paper, and graphite pencil, I draw the sunspots freehand on an 8-inch-diameter circle inscribed on the paper, carefully noting their relative shapes, sizes, and positions based on the projected view.

From March 1987 to February 1991 I made daily white-light (unfiltered) solar observations using this projection method. I was able to record the rise of Solar Cycle 22 to maximum activity with 706 full-disk drawings of the Sun. They required about 414½ hours of drawing time at the screen.

You don't have to be an artist to become skilled in making accurate, detailed drawings of solar activity. But practice is the key for distinguishing the great diversity of sunspot forms and noting subtle changes in them. Observe until you feel confident and enjoy your solar viewing. Using sunspots as markers, you can follow the Sun's 27-day rotation period. Some sunspot groups reorganize themselves as they cross the solar disk, and these changes can become quite spectacular.

Observing Tips

Development of a sunspot group during 4 days

Development of large sunspot groups. As they move across the disk, sunspots exhibit complex forms and structure. Large groups tend to be more active, showing marked changes in just a few hours. Jeffery Sandel made these drawings using a 10-inch f/6.5 Newtonian reflector to project a 3-foot-diameter image of the Sun onto the wall of his garden shed. North is up and east to the right. Click on the image to see the full series.

Aiming your instrument at the Sun is as simple as finding the shortest shadow of the telescope tube. Thin clouds present no problem, and while it's true that atmospheric seeing is generally best in early morning, I have witnessed superb resolution in the afternoon! It all depends on your particular setup, location, and prevailing weather.

Use low powers only, because small, high-magnification eyepieces can be ruined by overheating in a very short time. You can also stop down the aperture to reduce the heat and light passing through the eyepiece.

Make sure the screen is exactly perpendicular to the eyepiece and the image is sharply focused on the screen. Wait until the solar disk is roughly centered on the screen before starting your drawing. A large disk will show the most detail, but it will appear faint since the light is spread out over a large area. You can increase contrast (and cut down on the heat buildup) by keeping the screen and the rest of the scope in shade.

A good way to tell the east-west direction is to leave the telescope stationary and let the image drift across the screen due to the Earth's rotation. Make a dot on a small sunspot, let it drift, dot it again, and draw a line between the two dots to record the exact east-west direction.

The west limb of the Sun is the leading or preceding one, and the east limb is following — two terms frequently used to describe celestial west and east on the Sun and planets. The north and south limbs can be identified by nudging the telescope slightly north; the Sun's image will move away from its north limb.

Drawing Sunspots

Some sunspots, especially the very small ones called pores, are quiescent. They form, remain basically unchanged, and die out quickly and uneventfully. Large groups, however, can be much more active and may exhibit remarkable changes in a matter of hours. At these times, large-scale (close-up) drawings of the details of the group are desirable, provided the atmosphere is steady enough. After a group reaches its maximum development, its remnants can persist for several weeks.

Sunspots across the solar face

This rendition by Jeffery Sandel shows several large sunspot groups, as well as faculae regions (bright patches) scattered across the entire solar disk. Unlike the traditional method wherein the sunspots' positions are plotted and their outlines traced directly on the projected solar image, Sandel draws them freehand on a blank 8-inch-diameter circle. This results in exaggerated sunspot sizes.

For many observers, noting the sunspots' exact positions is more important than details. But since I'm interested more in morphology, my attempt to make the renditions "realistic" has resulted in slightly exaggerated sunspot sizes, as shown in the full-disk drawing here.

I try to limit each full-disk drawing to about an hour or less because image rotation becomes apparent after an hour's drift across the sky. With a 2B graphite pencil, I first draw the outline of the umbra (dark central core) and the lighter penumbra of the largest sunspot groups. The smaller spots are then added in. Depending on the level of sunspot activity, I usually forgo drawing the bright patches or faculae around a spot group and just concentrate on the sunspots themselves, especially if the latter are very active or complex. I always check the realism of my sketches with that of the projected image. Finally, the shading of the umbrae and penumbrae are done with SB and EE pencils.

As far as details go, sunspots stand up well under close scrutiny. Noting their fine structure and features can be as exciting as following their evolution and decay across the Sun's disk. There is also a wealth of detail to be drawn in the faculae near the limbs. These sites of intense magnetic fields are as varied in morphology as sunspots, and if time permits the brightest regions of faculae are drawn in after the sunspots have been recorded.

Wilson effect

The Wilson effect, which is greatly exaggerated in this diagram, shows how the umbra and penumbra of a typical sunspot become more and more symmetric as they near the center of the Sun's disk.

Sky & Telescope illustration.

Sunspots near the limb are almost invariably engulfed in faculae, giving superb contrast and interesting, almost three-dimensional perspectives. The Sun then becomes alive with details of the most powerful magnetic fields in the solar system. The Wilson effect also adds to the 3-D impression near the limb, making sunspots look like shallow bowls, as illustrated here.

When sunspots are few I record as much detail in the faculae as distinctly visible, along with sometimes highly variable dark filaments that are often sinuous in shape. Solar pores can be so numerous as to be visible across the entire disk, giving the Sun a spectacular appearance. Some of them are quite persistent; others last only several minutes.

When pores are numerous, the solar granulation (convection cells) filling the Sun's blank areas takes on added interest. Granulation, while best recorded on film, is nonetheless incredibly wonderful to observe visually by projection. Seeing conditions can, at rare times, be so steady that I can see individual granules between the fibrils that make up the penumbrae of sunspots!

Recording and Submitting Observations

How you record your data determines how valuable and useful your observations will likely be. I also keep track of the total observing time I've logged. The essential information I note down are the date, beginning and ending times of each drawing to the nearest minute in Universal Time, any unusual activity observed, and the seeing conditions. The times of your drawings are very important because of the rapid, continuous changes occurring on the Sun.

If you want your data to be archived and possibly used by fellow amateurs and professional researchers, you can send them to the Solar Section of the Association of Lunar and Planetary Observers. See ALPO's website for details on the submission process.

Solar observing is challenging. It's best to practice now so that you'll be ready when the next solar maximum arrives. With careful planning and preparation, you too can enjoy many hours of observing pleasure and surprises as each solar cycle rises to its greatest sunspot activity.


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