Abstract

The ‘Great North American Total Solar Eclipse’ of April 8^th, 2024, stretched across the USA from Texas to Maine. This event allowed those close to the line of totality to experience an event lasting around 2 ½ hours from First contact (beginning of the eclipse) to Fourth contact (official end of the eclipse). Except for the approximate 3 - 4 minutes of totality, where the Moon totally obscured the Sun, the possibility of permanent eye damage to direct visual observers of the eclipse presented a significant health hazard. Here we discuss the concept, design, construction, deployment, and community impact of a series of ‘safe eclipse viewing windows’. Large temporary outdoor viewing filters allow multiple people to view the eclipse together ‘side by side’ as a community without the need for personal protective eyewear. These devices allowed educators to connect with students and members of the public more easily during the eclipse and fostered a community atmosphere. These windows were an essential tool for local educators as well as serving public health and safety during the 2024 eclipse.  

1. Introduction

In the weeks and months approaching a solar eclipse, it is a common and essential practice for educators in the field of physics and astronomy to provide warnings and reinforce the dangers of directly viewing the Sun without the use of protective filters (Chou, 2023).

The Sun emits radiation across the electromagnetic spectrum, including visible, infrared, and ultraviolet wavelengths, which can be damaging to the eye when viewed without appropriate protection. Exposure to UV radiation, especially below approximately 315nm, can cause damage to corneal epithelial cells and solar retinopathy (burning of the retina) (Chou, 2018). Filters used for direct solar observation must not only protect observers’ eyes from visible and infrared radiation but also ultraviolet wavelengths, specifically UVB (290 to 320nm) and UVC (100 to 290nm). The most common form of protection are ‘eclipse glasses,’ small mylar or sometimes black polymer optical filters generally mounted in basic cardboard frames.  

Standard eclipse glasses – certified as ISO 12312 – block more than 99% of incoming light, including most UV radiation, protecting the eye from the harmful effects of solar radiation (Chou, Dain, & Fienberg, 2021). However, visible light is so drastically attenuated that wearers are effectively blind while wearing them. This can prove disorienting and stifle basic visual interactions with others in the vicinity, such as pointing, gesturing, or even facial expressions, thus posing a barrier to communication. Some members of the community may also have difficulty wearing protective eyewear for health reasons, such as those requiring prescription eyeglasses, while other individuals simply may not feel comfortable wearing relatively close-fitting eyewear. Children are especially likely to misuse or damage their eclipse glasses through rough treatment, rendering them ineffective, possibly without the knowledge of supervising adults.

There is also the potential of eclipse glass supplies running short as some areas may see thousands of extra visitors to their location; in this eventuality, these windows could be used in place of solar filter glasses.

To counteract these issues, a collaborative effort between astronomers and physicists, and engineering students and faculty created a solution to allow observers to witness the spectacle of the partial phases of the eclipse with colleagues and friends free of encumbering protective glasses. Our group successfully conceptualized, designed, and fabricated two models of large optical filter ‘Eclipse Windows’ measuring 3 x 6 feet (0.91 x 1.8m) and 6 x 6 feet (1.8 x1.8m) in size, supported by rigid but portable structures.

The development of Eclipse Windows required pairing a lightweight optical filter material capable of meeting or exceeding the ISO 12312-2 safety standard (ISO, 2015) with a rigid yet portable framework that could hold the filters at a comfortable height for easy public viewing.

 2. Methods

The challenge of designing the eclipse windows was initially passed to a team of four student engineers. The student design process for the first model of the eclipse window involved 5 basic stages: 1. Research, 2. 3D modeling, 3. Prototyping, 4. Testing, 5. Finalization and Deployment. Phase 1 focused mainly on the type of solar filter to be used. Phase 2 involved modeling the frame design in Creo 10.0 CAD software. Phase 3 saw the creation of a 1/6 scale prototype to test the functionality of the structure, including the leg extension mechanisms. Finally, Phase 4 culminated in the production of a full-scale prototype to be tested and complete any final checks before full production could begin.

2.1 Optical properties

Our main goal was to preserve the health and safety of eclipse window users by producing an optically safe product. As such, our system was designed to meet or exceed the accepted safe transmittance levels of UVA and UVB radiation (https://eclipse.aas.org/eye-safety/iso12312-2) according to the ISO 12312-2 standards applied to commercial mylar eclipse glasses (https://www.iso.org/standard/59289.html). This optical standard ensures that the maximum luminous transmittance (ratio of transmitted to visible light) is no higher than 0.0032% in order to avoid eye damage due to solar retinopathy (Chou, 2018) or phototoxic maculopathy (Yang et al., 2012). This required the acquisition or fabrication of a very large solar filter or solar filter-like material. The choice of optical filter also informed how the support structure would need to be constructed. Initial design concepts intended to use large sheets of lightweight ISO-12312-compliant mylar, such as those used in smaller-scale eclipse glasses; however, obtaining very large sheets of optical-quality mylar proved difficult and prohibitively expensive.

Another option is Shade 14 welders’ glass. For the August 1999 total eclipse in Romania and the March 2006 Libyan eclipse, the author built and deployed custom optical filters for solar observation and astrophotography utilizing small panels of Shade 14 welders’ glass. While appropriately selected welders’ glass provides excellent optical protection, large plates of shade 14 welders' glass proved too expensive to obtain. Additionally, the weight proved to be another disadvantage; the glass would have required strong, heavy structural components to house and support the large glass panels, and the placement of large, heavy glass panels over observers’ heads, posed a potential safety hazard.

Commercial PVC welding curtain provided the only viable, cost-effective, lightweight option. This material is employed in many workplaces with potentially damaging visual/IR/UV radiation emissions (Tenkate, 1998), is resilient, and adheres to rigorous industrial optical safety standards.

Welding filters – both glass and curtains – are available in a variety of shades. Shade 14, the darkest shade on the commercial scale, is sufficient for solar eclipse viewing and is designed for carbon arc welding, which emits similar UV radiation to the Sun (Kim et al., 2019).

 Unfortunately, the darkest shade curtain commercially available to us was the equivalent of shade 8, which is insufficient for safe, direct solar observation. Welding curtain shade numbers are linearly proportional (OSHA.gov) and, as such, may be physically stacked on top of one another to achieve a higher shade number (Chou, 2018). In our design, we employed two layers of heavy-duty 6x6’ shade 8 ULINE PVC welding curtain (product number S-20233S8) as our filter material. These two layers provide excellent optical protection.  By combining 2 layers of shade 8 green welding curtain, we created an optical filter approaching an equivalent welding shade of around 16. This exceeds the safe-solar viewing protection level afforded by shade 14 (the equivalent optical protection of ISO12312-2 mylar).

Caution is required when selecting the appropriate shade of welding curtain for use in an eclipse window. Many shades and colors of commercial welding filters are available; it should be stated that only the proven ULINE S-20233S8, Shade 8 Green PVC welding curtain of the type shown in Figure 1 should be used in this type of solar filter apparatus. Other available colors of shade 8 welding curtain, such as blue, yellow, red, or any other color, are only effective in industrial welding scenarios, and even double layered are insufficient for safe direct solar observation.

Testing was conducted using commercial UV index sensor systems and light meters to ensure our welding curtains were within tolerance for direct optical use at multiple wavelengths. The PVC optical filter material could then easily be made into the desired 6 x 6’ eclipse windows for naked-eye visual use. Our welding curtains were extensively tested by the lead author personally before being authorized for use by any other individual. As noted by Chou (2018) stacking of welding filters does result in slight image degradation; however, visually, the image is more than adequate to observe the partial phases of a solar eclipse effectively.

We caution that binoculars and telescopes should not be used beneath the window as it is too easy to chase the Sun to the edge of the filter and accidentally harm eyes or equipment.

2.2 Frame Modeling and Design

With visual filters selected, a structure was required to hold the PVC welding curtain taut and at a sufficient angle for solar observation. During the 2024 Total Solar Eclipse, the altitude of the Sun in Erie would vary from around 54° at the eclipse's start to 36° degrees at its conclusion. This would require the welding curtain’s supporting structure to be held at the correct angle for both observer comfort and an optimal view of the Sun.  A large viewing area was also needed to accommodate crowds beneath the optical surface and provide a sufficient field of view for observers while eliminating the need for any major adjustments to be made due to the Sun’s changing position.

3-D models created in the commercially available Creo Parametric CAD software were used to alter and refine the support structure’s design in real-time. Figure 2 shows the final design of the Mark I eclipse window, incorporating four legs fully adjustable for height and viewing angle, along with an exploded view detailing the individual components of the system. This CAD model assisted in the selection of fasteners to hold the optical filter, with both commercial adhesives and Velcro initially considered and rejected.   

The structure was comprised of commercially available PVC pipe, which is both easily machined and lightweight for portability. The revised design incorporated an optical filter consisting of two stacked welding curtains secured to the edges of the frame with zip ties running through the existing mounting grommets manufactured into the curtains, removing the need for cutting or joining of the PVC surfaces.

Legs were joined using eye bolts and wing nuts inserted through drilled holes in both the inner and outer pipes comprising the legs, as shown in Figure 3. This type of fastener enabled easy on-site height adjustment without the use of tools by simply placing bolts through the desired pre-drilled holes in the outer pipes and inner sleeves.  

In order to maintain stability in the adjustable structure, 3D printed sleeves were inserted between the inner and outer pipe legs to minimize any spacing or wobbling of the frame. These sleeves also ensured smooth sliding action as the inner pipes were extended. (see Figure 2 & Figure 3).

 When placed on site, each structure would be secured to the ground, as a 6x6 foot area of PVC sheet functions as an excellent and undesired sail in a brisk wind. As the windows were intended to be distributed across 3 observing sites with differing terrain – astroturf, grass, and concrete – the same method of securing the legs could not be used at each site.

Our engineering team developed two differing interchangeable foot designs to slide onto each leg depending on the surface it was placed on. The foot for grassed areas incorporating holes through which stakes could be inserted as shown in Figure 4 (left). A second design for use on brick/concrete or astroturf areas featured an extended flange upon which weights or sandbags could be placed for added stability (see Figure 4 right). Both designs were 3D printed using and distributed to the appropriate destination.

2.3 Prototype- Scale Model

Prior to the construction of the first full-scale prototype, the student engineering team constructed a 1/6 scale model of the eclipse window, as shown in Figure 5. This model proved invaluable in the finalization of the telescoping leg system design and could function as a small-scale viewing window if fitted with the appropriate S-20233S8 filter material.                     

2.4. Construction and Testing

Following these modeling procedures our team of four student engineers cut PVC pipes to the desired lengths with holes drilled at 2-inch intervals to accept the eye bolts and nuts to allow adjustment of the window. Sleeves were adhered to the inner legs, and the 3D-printed feet were attached. A 6 x 6’ (1.8 x1.8m) top frame was then built to accommodate the PVC welding curtain.

Testing took place from December 2023 to March 2024, taking advantage of any available clear days during the Erie winter to spring season. Figure 6 shows initial test deployments of the Eclipse Window system.  Preliminary tests revealed that the weight of the PVC welding curtain produced bowing in the window perimeter pipes and sagging of the optical surface. A cross brace of PVC pipe was placed underneath the curtains to provide additional structural support for both. Figure 7 shows the underside of the structure and the added supporting cross pattern.

 Optically, the device performed as required; however, the welding curtain does produce a green image of the Sun as opposed to the orange coloration produced through typical mylar filters, a point often commented upon by younger users. Figure 8 shows an image of the Sun taken through a Gannon eclipse window with a standard unfiltered SLR camera.

 A final issue identified during testing was the production of a slightly distracting double image resulting from a separation of the two layers of the welding filter. This separation was eliminated by increasing the tension of the two optical surfaces.

2.5 Production

The Mark II Eclipse Window

The Mk I PVC eclipse window structure discussed above proved optically capable, stable, and fully adjustable. They were also insufficient to serve anticipated crowds. We were responsible for operating three geographically separate observing sites during the April 8^th eclipse. Local institutions and government bodies anticipated crowds of up to 50,000 people attending the city’s viewing events.

To meet this potential demand, we decided to produce a greater number of windows. Consequently, a series of smaller, simpler Mark II eclipse windows were quickly fabricated by Gannon University’s in-house Makerspace (https://makerspace.gannon.engineering/)  manufacturing facilities for deployment alongside the larger, more adjustable Mk I PVC units.

The Mk II window were smaller and less adjustable than the Mk I device, and their design utilized welded steel to create a window frame. The same commercial double welding curtain served as an optical filter, now doubled over to reduce costs by using only a single welding curtain per unit. This produced a final Mk II window with reduced dimensions of 3 x 6’ feet in size. Existing grommets fitted into the curtains were used along with simple hardware store tarp clamps to hold the welding curtain in place and provide appropriate tension to keep the two optical surfaces in contact, preventing the previously mentioned double image problem.

The Mk II window frame was supported either by 2 legs of welded steel construction and 3-D printed grips for basic angle adjustment or by commercially available theatrical rigging stands. The Mk II unit was stabilized with sandbags placed either on the feet, as seen in Figure 9, or suspended from the frame itself. While the Mk II was smaller in overall viewing area, this variant could be produced in greater numbers at short notice, making it the most numerous model deployed on April 8^th.

A constructed Mark II window is shown in Figure 9, demonstrating the relative size and design changes. Being cheaper and simpler, six of the Mk II units were produced to supplement the existing Mk I’s and deployed over each of our 3 observing locations, with one Mk II unit being constructed and shipped offsite for use by a local chapter of the Boy Scouts of America.                                      

 The Mk II windows were set at higher elevations with a more vertical angle to allow adult observers a more comfortable observing experience, while the Mk I PVC pipe units were set at lower heights or with a more horizontal angle optimizing their larger size for use by children or those with disabilities that required the more ‘ceiling’ like viewing angle of the Mk I to more easily observe the Sun.

Both versions were waterproof due to their basic design, as proven during early heavy rain on the morning of April 8th, and were capable of rapid assembly and disassembly in the case of extreme winds. Their excellent reception by the local academic community and the public demonstrated the successful completion of our goal to construct a safe and versatile large-scale viewing window for simultaneous use by multiple observers.

For further details on the design, construction, and 3-D printing of components for eclipse windows or to obtain pre-constructed window readers, you may contact Gannon University’s Makerspace facility (https://makerspace.gannon.engineering/).

 3. Deployment and Public Response

 As discussed above, nine of these devices were produced in total for use on April 8th, three of the Mk I PVC pipe design and six of the Mk II welded construction. Working with city officials and local business partners, these windows were placed at strategic public gathering points within the city, such as landmarks, parks, and city squares.  These locations were advertised in the media and posted on publicly available maps distributed by Erie’s ‘Visit Erie’ initiative.

Five units were deployed in public downtown areas. three more on Gannon University’s campus for use by the college community and another to the Boy Scouts of America French Creek division for use at their own off-site eclipse event. Figure 10 shows the Mk I window shortly after construction and an Eclipse Window in use by observers during the eclipse on April 8^th.

The level of interest in the construction and use of these devices from the public, educational institutions, local government and the media far exceeded expectations. Numerous media outlets featured reports on our window devices leading up to the event. Our windows are featured in numerous online, print, and radio interviews with around 26 television news reports by local, national, and international outlets (https://www.youtube.com/@davidj.hornephd3292). Figure 11 shows Dr. Horne presenting a Mk II window for the first time on the Erie News Now network.

It is estimated that several hundred individuals experienced the partial phases of the eclipse through these windows across all three of our observing sites. The downtown Erie window was a destination for Pennsylvania Governor Josh Shapiro, First Lady Lori Shapiro, and NASA astronaut Warren ‘Woody’ Hoburg to view the eclipse during their official visit to the path of totality shown in Figure 12.

4. Conclusion

Commercial shade 8 green welding curtain can be used to construct large optical filters that allow groups of people to make naked-eye solar observations simultaneously.

The eclipse windows satisfied the design brief ‘to provide a means for individuals to experience the eclipse together safely as a community, without the need for eclipse glasses’. Both models were extremely successful, with hundreds of public users safely experiencing the partial phases of the total eclipse under the supervision of our dedicated volunteers with no injury or discomfort.

The windows proved to be a successful, low-cost, and popular outreach tool in the lead-up to the April 8th eclipse and on the day itself, garnering high levels of media and public attention. The number of media engagements brought about by these windows provided an excellent opportunity to reach a much larger audience with the message of safe solar observing methods and practices than would have been possible without them.

While the 2024 eclipse was the catalyst for the development of these windows, they can also be employed to conduct solar observations on any given sunny day. This affords students enrolled in basic astronomy courses or those participating in public events the opportunity to conveniently and safely observe the Sun without the need for mylar glasses. The light construction and easy disassembly of both versions of the window allow for easy storage when not in use.

A more permanent version of these filters could be fabricated using more robust materials for use at teaching observatories and planetariums, affording visitors at any time a safe view of the Sun. For locations where outdoor maintenance of such a device or extreme weather is an issue, they could potentially be placed indoors under available skylights or large south-facing windows.

This project was a close collaboration between astronomers, physicists, and mechanical engineering faculty and students, allowing individuals who would not normally be involved with astronomical research to engage in meaningful work related to the 2024 total solar eclipse.

 Acknowledgements

We gratefully acknowledge funding for this project from NASA Pennsylvania Space Grant #2923 and Gannon University Faculty Research Grant # 21316.

Web links

1. AAS eclipse eyes safety resources

https://eclipse.aas.org/eye-safety/iso12312-2

 2. NASA Eclipse Eye safety information

https://eclipse.gsfc.nasa.gov/SEhelp/safety.html

 3. OSHA regulations for Welding Eye Safety

https://www.osha.gov/sites/default/files/publications/OSHAfactsheet-eyeprotection-during welding.pdf

4. Dr. David Horne’s YouTube Channel

https://www.youtube.com/@davidj.hornephd3292

 5. Gannon University Makerspace

https://www.gannon.edu/academic-offerings/engineering-and-business/institute-for-health-and-cyber-knowledge/center-for-manufacturing-and-technology/

 6. ULINE 6x6’ Shade 8 Welding Curtain:

https://www.uline.com/Product/Detail/S-20233S8/Welding/Replacement-Welding-Curtain-6-x-6-Shade-8