Design and Construction of a Projection Cage for Studying the Sun

by Gerry Atkinson
Reproduced from The StarGazer - Volume 11 - Winter, 1994
Why Study the Sun?

There are good reasons for including our nearest star in your observing schedule. Light pollution has so worsened that faint galaxies cannot be detected even at rural sites whose horizons are tainted with urban skyglow. Arunah Hill in Cummington is an exception and is being developed as an observing center of high quality. But most of us can enjoy it only after a long trip. The Sun can be viewed and photographed from your backyard. Its light source is more than ample, and you can get by with a small refractor. The backyard venue simplifies setup and allows a more rock-steady mount than you would consider transporting.

Observing the Sun is also a chance to do real science. You can be a contributor to the Solar Division of the American Association of Variable Star Observers (AAVSO) and help determine the American Sunspot Number. This article is aimed at the reader with a somewhat advanced interest in sunspots and describes a method of projecting their images onto a screen.

Why Projection?

The Sun can safely be observed through a telescope in two ways: directly with a solar filter, or indirectly by using projection. One may well ask why I choose projection instead of using a full-aperture filter of aluminized mylar or inconel-coated glass, both types being readily available and reasonably priced. I gave both types of filters a lengthy trial but was completely won over to projection, reserving filters for photography. The most obvious reason is absolute eye safety since, unless positively retained, a glass filter may falloff and a mylar filter may blow off. Pinholes in a filter may increase in number but be disregarded until they constitute a retinal hazard. But there are other compelling reasons.

Advantages of Projection

Eyestrain is greatly reduced during extended viewing because both eyes are used and the image is at a comfortable distance for study. Daytime "seeing" is poor, and one must often wait for minutes until faint detail is revealed. Eye comfort is important.

The observer's posture is seated, erect, and relaxed, making it easy alternately to view and sketch. A star diagonal must be used with a refractor if the direct method is chosen. Otherwise the neck is uncomfortably bent when looking steeply upwards.

Coordinates of sunspot groups on the Sun's face can quickly yet accurately be determined by using a lettered and numbered grid drawn on the screen. This facilitates comparison with the results of others, such as NOAA's weekly reports, usually expressed in heliographic longitude and latitude. Group size can easily be measured for doing a Waldmeier classification of type. similar determinations with the direct method are very difficult unless some form of eyepiece micrometer is used. This advantage alone justifies projection to serious observers.

The erratic vibration of the screen caused by a slight wind reveals obscure spots.

Simultaneous observation by more than one person is possible. This is especially useful for public showings and during eclipses.

Disadvantages of Projection

The cage cannot be bought, but instead must be constructed. If original in concept, source books on design will probably have to be consulted. Material costs may exceed the cost of a filter.

The cage presents a large sail area and is susceptible to wind shake, particularly if sized to obtain a large image. High wind can be a more adverse factor than thin haze.

Projection into a large cage is practical only with a small refractor or a larger one stopped down. Apertures greater than four inches are discouraged because of internal overheating. The cemented joint between the corrector plate and secondary mirror of a Schmidt-Cassegrain is especially susceptible. The Newtonian reflector is excluded for balance reasons unless a lightweight, exposed screen is used.

Concentration of heat near the focal point can damage eyepiece coatings and melt cement between the lens elements. Both can be avoided if the image is not vignetted and rays pass only through glass and do not strike eyepiece metal.

Despite my preference, some more experienced observers, including those regularly reporting sunspots to AAVSO and flares to the Association of Lunar and Planetary Observers (ALPO) , prefer the direct method and think that finer detail can be resolved.

Important Design Considerations

The usual shape of the cage is a pyramid truncated at the tip to accept the scope's draw tube and with the screen at the big end. The cage must be easily removable if the scope is to be used at night.

The image of the full Sun must be large to realize the advantages of projection. The standardized Stonyhurst discs used as screens by many sunspot observers have a target circle 6 inches in diameter. I regard this as too small. It is appropriate only in an uncaged screen where a very bright disc is needed to compensate for dilution by stray light. An image diameter 3 to 4 times the scope's aperture is much better. Fitted to a 3.2-inch refractor, my cage provides a 9-inch diameter solar image and falls at the low end of this range. In future versions I will enlarge it to 11 inches. Size the screen to provide a generous area surrounding the Sun. This allows time before the disc is vignetted and recentering is needed.

Darken the inside of the cage as much as possible. Make the viewing port as small as practical. The screen should be thick enough to be completely opaque with no light leaks around its edges. Line the inside walls completely with a textured material, such as black flocking, to keep all wall-reflected light out of the image. This significantly improves image contrast. Overlap the lining material at all seams. If the cage sides are made of translucent foam, the lining will also block light transmission through the foam. Avoid aerosol sprays to provide the blackening. They will dissolve the foam.

Lighten the cage to reduce strain on the scope's draw tube, permit balance around the RA and DEC axes, and reduce the load on the clock-drive gears. Combining large size with light weight is first in Priority. The cage must rotate around the tube's axis to compensate for the Sun's east-west tilt at various seasons, since the full range is a considerable 52. Most of this is done by rotating the scope in its rings, with finer tweaking provided by the cage.

The screen must be an intense white with a smooth surface devoid of blemishes. My choice is heavyweight Bristol board, sold in art stores. Get a smooth or "plate" finish, not the matte. Another candidate is photographic paper which is unexposed, fixed, and washed. Walter Scott Houston recommends a thin, wooden board painted with seven coats of white acrylic fine sanded between coats. The gridded screen is used only for measurement of sunspot position. For ; group study and sketching it is replaced by an unmarked screen.

The best choice of mount is an equatorial under clock drive, preferably with a drive corrector for occasional recentering. In the daytime the mount can be polar aligned by leveling the base, setting altitude to the local latitude, and aiming the RA axis toward true north using an orienteering compass which compensates for local magnetic declination. So aligned, the Sun will remain unvignetted for several minutes under uncorrected clock drive.

For the projection eyepiece select an inexpensive type with few lens elements, since the cement between them is regarded as liable to softening. Huyghens and Ramsden eyepieces seem ideal with their air-separated singlets, but these primitive lenses are rarely available and their images are inferior. I compromise with a Kellner, which consists of a singlet air-separated from a doublet. The singlet is the first element struck by the rays. The caution about softening may be outdated and derive from days when balsam was the lens cement. Casper Hossfield, chairman of AAVSO's Solar Division, used projection for years with a four-element orthoscopic eyepiece and noticed no damage if he avoided vignetting. My Kellner is undamaged after three years of use. Incidentally, the type of cage described is often referred to as a "Hossfield Pyramid."

Construction Details of My Cage

Detailed drawings of my cage are shown in Figures 1-3. Dimensions are such that an 18mm Kellner eyepiece used with an 80mm, f/12 refractor will cast a full image of the Sun which is 9 inches in diameter. Cage weight is 1.5 pounds.

Click on each image for a larger view
Figure 1
Figure 2
Figure 3

The following equation is given for readers with different telescopes or desiring different image diameters:

D = dM/107

where

D = diameter of the projected Sun in inches

d = distance from eyepiece lens to screen in inches

M = visual magnification = f/fe

f = focal length of telescope in mm

fe = focal length of eyepiece in mm

This equation permits dimensioning the length of your cage once the projection eyepiece has been selected. A longer lens means a longer cage and tougher balancing problems. Choose a short enough lens to minimize your difficulties.

To save weight, the cage sides are made from expanded foam. This material is sold at hobby-craft stores as 24" x 24" x 1/2" sheets which weigh only 7.8 ounces. It is easily cut with an Xacto blade. The white surface reflects incident light and reduces heat transfer to the interior. To join the sides I recommend a white, aliphatic-resin glue such as Franklin "Titebond." Acetone-based glues such as Testors, Duco, or Ambroid will eat away the foam, and any other types should be tested first on scrap foam. The long glue joints are reinforced in the interior by aluminum right-angle brackets, painted flat black, primarily to maintain the rectangular cross-section of the cage.

Foam has little surface strength against impact, and the long edges must be protected against deep dents. Drawing on my model- airplane background, I used balsa in the form of 1" x 1/4" trailing edge stock, which is tapered in cross-section. The surface strength of the balsa was increased by covering it with fine-weave silk and clear doping. This also gives the cage a more finished look.

The wooden nosepiece which clamps around the scope's draw tube requires the most effort. Because it had the right dimensions and an attractive walnut finish, I used the base from an old golf trophy! A center hole was drilled large enough to fit around the draw tube. Then the block was sawed in half and fitted with a hinge and tightening bolt. The inside of the hole was lined with felt for a snug but protective fit. The cage is easily removed, freeing the scope for other uses.

A framework of balsa strip was built at the big end of the cage to hold the screen in a light-tight fashion. The frame was white-glued to the cage and clear doped. Its edges are set up above the foam edges to protect the latter from denting. The screen is inserted through a slot in the top and slides down side grooves into a bottom groove. Dimension the slot and grooves for a slip but snug fit on the screen. The entire framework must be light, for it is at the end of a long lever which magnifies its weight.

A foam sunscreen backed with poster paper is attached near the nose of the cage to shadow the observer's head when positioned near the port. size the sunscreen to shadow the entire cage side nearest the observer. This reduces direct and side-reflected glare and lessens stray light entering the port. The sunscreen shown in Figures 1 and 2 is undersized! Be sure to cap off the finder to prevent facial burns.

Hints on Using the Projection Cage

Begin each observing day by lining up the alphanumeric grid used to measure group location. Turn off the clock drive and watch the drift of a distinctive spot across the grid. Rotate the scope in its rings until the spot drifts along a line without deviation. Do the final tweaking by a slight rotation of the cage about the draw tube. The grid is now aligned with the Sun's path through the sky.

Only in January and July will the Sun's equator roughly coincide with this path. In April the Sun's axis will appear to tip 26 to the west and in October 26 to the east. It helps to know the position angle of the axis for the day, because you can better anticipate where the sunspot groups will be. They lie in a band parallel to the equator, both above and below. In a group the leader and follower spots will also parallel the equator, as will the fainter spots between them. It is much easier to recognize groups as such and classify them if you know the Sun's tilt. The slight tip of the Sun's poles toward or away from the earth is far less important and is significant only if you need to know which hemisphere the groups near the equator are in. Two excellent sources of information are the Observer's Handbook published by the Royal Astronomical Society of Canada, and The Astronomical Almanac issued jointly by the U.S. Navy and the Royal Greenwich Observatory. Both are available from Willmann-Bell.

A sunspot cycle is defined to begin at a minimum point. Cycle 23 should begin about 1995. Be on the lookout for an occurrence you might not expect. As a cycle ends the spots are typically grouped within 7 of the Sun's equator, and the observer grows accustomed to looking for them there. However, about 18 months before the end of the old cycle the new spots begin forming at latitudes of about 25. At the start of the new cycle the fresh spots will form even higher, at latitudes of about 30. If you are not aware of this "jump," the accuracy of your sunspot count may temporarily suffer.

If you position the viewing port as shown in Figure 1, the Sun's eastern limb will be at the far side of the cage and its north pole will be up. Spots will first appear at the eastern limb and will o cross the disc in about 14 days. For an accurate count you must search the eastern limb very carefully. The view of the Sun is very foreshortened in this region and groups are highly compressed in area. This same problem exists at the western edge, but here you usually have the advantage of observing on previous days and you are alert for a group's last appearance before "going 'round the bend."

The unmarked screen should be replaced when it yellows slightly or becomes flyspecked. The gridded screen has a longer life, because its usage is restricted to measuring group position and not discovering faint groups. For this reason I do not recommend a double-sided screen serving both functions. The far side of the cage is darkest and gives good image contrast. When studying a group I usually move it to the far side. The best contrast is obtained by draping a black cloth over your head and the port.

I recommend sketching each group for a variety of reasons. Except for very simple groups, it is easier and more accurate to count sunspots from your sketches than directly from the screen. Today's sketches will help tomorrow in locating and classifying groups. On a given day you may be unsure as to whether an assortment of spots is really one group or two. You can mark your sketch as provisional and use appearance on following days to make the decision. Do not mark group positions on a screen with pencil. It is hard to reach through a small port and your paper costs will skyrocket! Write the coordinates on your log sheet.


BIBLIOGRAPHY

The following references are restricted to those describing the projection method in ways useful to the amateur. Sidgwick is the best single source.

Books

SIDGWICK, J.B. Observational Astronomy For Amateurs. NY: Dover Publications, Inc., 3rd ed. 1980

BAXTER, W.M. The Sun and the Amateur Astronomer. Newton Abbot (U.K.) : David & Charles Ltd., 2nd ed. 1973.

TAYLOR, Peter 0. Observing the Sun. NY: Cambridge University Press (Practical Astronomy Handbook #3) 1991.

HILL, Richard E., Editor. The New Observe and Understand the Sun. Washington, DC: The Astronomical League. 1990.

Magazines

TAYLOR, Peter 0. "Watching the Sun", Sky & Telescope, Feb. 1989

HOSSFIELD, Casper H. "Observing the Sun", The Review of Popular Astronomy, Dec. 1967. (Includes excellent photograph of author's projection cage mounted on a refractor.)

HILL, Richard. "Equipped For Safe Solar Viewing", Astronomy, Feb. 1989.

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