Abstract

Using simple inexpensive optics, anyone can make a projection viewer for the safe observation of the partial phases of a solar eclipse. This paper reports on the various safe solar viewers (SSV) constructed at the College of Charleston, Charleston, SC and the Collier County Public Schools, Naples, FL over the last twenty years. A 1-lens SSV using reading glass lenses can project a bright 1-2 cm image of the Sun and is a dramatic improvement on the pinhole projector. A 2-lens SSV using surplus optics can produce a solar image 10 cm across. The most recent design uses $12.50 in optics, is constructed of cardboard, and has image quality equal to the solar projection viewers sold to schools for hundreds of dollars.

1. Introduction

For years prior to the 2017 Total Solar Eclipse, one of us (Richardson) had been making various simple, inexpensive solar projection viewers for observing the partial phases of a solar eclipse. His goal was an inexpensive device producing an image significantly brighter than one made by a pinhole projector yet simple enough for children to make.

These viewers used lenses from reading glasses (or other similar optics such as low diopter close-up camera filters) to project an image onto a rear projection screen made from a white reflective surface or on wax paper. A +½ diopter lens produced a fine 2.0 cm image that many could view simultaneously (Figure 1) but the viewer was long and cumbersome to work with. Still, children showed delight in this instrument when seeing the lens, still in the glasses frame, at one end of a long carpet tube. A +1 or +1.25 diopter lens from reading glasses purchased from various discount stores is a practical choice for personal viewers children can make themselves with some adult help.

2. The First 2-Lens Safe Solar Viewer (SSV)

Two years prior to the 2017 solar eclipse, Richardson designed a solar projection device in a compact package less than a meter in length that projected a solar image 7-10 cm across using surplus optics. This effort resulted in the 2-lens safe solar viewer (SSV) made with $5 in optics from an online retailer (www.surplusshed.com). The SSV had a positive lens (50 mm in diameter by 500 mm in focal length) collecting light and a Barlow (–20 mm in focal length) projecting the image onto white paper (Figure 2).

Galilean optics, such as those in Galileo’s telescopes and also found in opera glasses, were used over Keplerian ones found in conventional astronomical refractors having a positive lens as the objective and an eyepiece with a positive focal length. That choice allowed the SSV to be significantly shorter in overall length for the same image size thus making the viewer easier to construct and operate.

A Julena Steinheider Duncombe Mini-Grant from the American Astronomical Society (AAS) and the National Science Foundation (NSF) provided funds for workshops in which young STEM minority children assembled 100 of these 2-lens SSVs from precut wooden parts (Figure 3) and earned an Eclipse Science Ambassador pin ordered from online button maker, Just Buttons. (Figure 4)

Additionally, physics and astronomy majors from the College of Charleston visited area high schools and middle schools prior to solar eclipse day and left twenty of the 2-lens SSVs with the school’s science teachers.

With funding from the College of Charleston Department of Physics and Astronomy and in collaboration with Visiting Professor Bobbie Lyon and Cultivate SC, a workshop for constructing 1-lens SSVs was held at Mosquito Beach, an area with a significant minority population. Almost 100 of the 1-lens viewers were constructed in a long afternoon session that ran until dark. The children at this workshop used paints and crayons to personalize their viewers and were given instructions on eclipse safety.

Well before the 2017 eclipse, information on constructing an SSV was posted on a web site hosted by the College of Charleston. Individuals from across the country used that information to make SSVs. They sent reports and pictures and also suggested improvements. Others contacted us with questions. Indeed the SSV has birthed a community of inventive folks willing to share their ideas for making and improving the device. Several people have 3-D printed the parts and shared their code. One member sent pictures of a polar axis to more easily track the progress of the eclipse. Another designed and built a foldable SSV for travel and his plans are on the community web site. Several individuals enlisted our help in selecting optics for large-image SSVs. We advised a Los Angeles-based artist, Tristan Duke, in building a giant SSV camera that was guided on the Sun during eclipse day by a regular SSV serving as the giant camera’s viewfinder.

3. An Achromatic 2-Lens SSV

Prior to the 2023 US annular solar eclipse, the authors began to collaborate on improvements to the SSV. Their goal was two-fold. First, they sought a means of making the construction more accessible by removing the need for a woodworking shop. Secondly, they wanted to improve the image quality so that the SSV could show sunspots clearly. Their aim was to reach these goals without a significant increase in cost.

One of us (Thomas) developed a design for the SSV using a standard shipping box (40 x 6 x 6 inches) made by Uline (www.uline.com) (Figure 5).

There is nothing unique about the Uline box since anyone with cardboard and glue can make a box with the same dimensions; however, a standard box is helpful since the SSV size is determined without any measurement and such a box is essential when making multiple viewers in a workshop. In the new SSV, the length is fixed and focusing is accomplished by having the Barlow on a movable support. We have found, that with care in the construction to keep all of the upright surfaces parallel, high image quality is achievable.

By upgrading the SSV objective to an achromat (52 mm diameter by 600 mm focal length), the device now shows all but the very smallest sunspots seen in the NASA Solar Dynamics Observatory daily HMI Intensitygram images. (Figure 6)

Not only are the sunspots revealed, but structure is visible and the penumbra can be seen in the larger regions! Surplus Shed sells this objective with a Barlow (-27 mm) for $12.50 (# L14940).  Information on constructing the SSV is on the web site listed previously and many reported making the new SSV for the October 14, 2023 Annular Solar Eclipse. (Figure 7)

The image quality of the new 2-lens SSV is on par with commercial projection viewers sold to schools. Recently, a cardboard SSV at an outreach event had image quality that appeared the equal of a Sunspotter solar telescope that was also present.

4. Conclusion

With the simplicity of its design and the quality of the image produced, the 2-lens SSV will be an easily made and useful tool for solar observing in middle and elementary school science classes and for safe eclipse viewing by the public for some time to come, as long as affordable surplus lenses remain available.