A Compact, Lightweight Solar Projection Viewer

Each summer I have the privilege to run the Student Telescope Making Program at the Table Mountain Star Party. Over the course of a few hours, students assemble telescopes and use them that night out on the telescope field. With the next total solar eclipse approaching, it seemed like the right time to do something a little different, perhaps even a bit more inclusive for kids of ALL ages!

It occurred to me that a device to allow safe viewing of the eclipse would be a fun project to undertake. I wanted a design that was:

• Lightweight — It needed to weigh less than many premium eyepieces
• Portable — Able to be broken down for easy transport
• Flexible — Could be used with a variety of scopes of various sizes & focal lengths
• Affordable — Including the eyepiece, it could made for around $50 or less
• Re-useable — Could be used for the eclipse, or viewing sunspots
• Easy to Build — Could be built with commonly available materials

The projector uses a plywood base, carbon graphite arrow shafts for screen supports, and an embroidery hoop to hold the screen. Assembled, the unit weighs less than 13 ounces, including the eyepiece! The arrow-shaft support rods can be unscrewed from the screen holder and base, and the screen itself can be removed from the holder, wrapped in a paper towel, and put in a cardboard tube for transport.

Supplies

• ¾” plywood cut to 4" x 4" — Baltic Birch or similar recommended.
• 1-¼” and 1-3/8” Forstner drill bits
• 1/8" countersink bit with a 5/16" to 3/8” " counter-bore
• Drill press (optional, but really helpful!)
• Two (2) carbon fiber arrows — 30” length with 7.8-mm outside diameter (OD)
• Eight (8) 1”- long flat-head machine screws
• Six (6) carbon shaft aluminum inserts 0.244"
• Embroidery hoop (we used “Hoop-La” plastic hoops)
• Four (4) ¼”-long #6 Phillips thread-forming screws
• Four (4) #6 external-tooth lock washers
• Four (4) angle brackets
• Rear screen projection material
• Simple eyepiece
• Protractor or similar device for measuring angles

Instructions for Projection Viewer Construction

Drill the Hole for the Eyepiece

Use a bit that’s slightly larger than your eyepiece lens housing to drill a hole about 5/8” deep (leave just over 1/8" of the wood remaining) in the center of the 4” x 4” piece of plywood.

Next, use the 1-¼” Forstner bit centered in the first hole to drill through the remaining wood.

Drill Holes for Support Rods

Use your countersink bit to drill a hole at a 75° angle about one inch in from each of the four corners of the base. The hole must be facing away from the center of the base.

Make sure the side of the base with the hole for the lens housing is facing up — this is the “top” of the base. If you have a drill press, you can tilt your table by 15°, as this will make getting the proper angle for the support rods much easier.

Drill until the countersink is approximately 1/3 of way through the base (from the top). Once the four holes are drilled, you can turn over the base and use the 1/8” pilot holes to add countersinks to the bottom of the base if desired. If you don’t countersink the bottom of the base, use slightly longer screws to attach the screen support rods.

Make the Support Rods

For 8” hoops, use 8 ½” long support rods

For 10” hoops, use 13” long support rods

We used a “Mini Tubing Cutter” to cut the arrow shafts to length. Once cut, use Super Glue to add an insert into the “unfinished” ends of the rod.

The Screen Support Hoop

Using the 1/8” bit, drill four pilot holes 90° apart into the side of the inner ring. The holes are best when drilled a bit closer to the bottom of ring.

Angle Brackets

We used 3/8”-wide pieces of ¾” aluminum angle, altering the 90° bend by opening it up to 105°. Drill two holes in each bracket for the 8-32 machine screws.

Increase the inside angle by 15 degrees to 105 degrees.
Jack Day

The holes should be close to the ends of the bracket, with about 1/8” of aluminum between the ends and sides of the bracket.
Jack Day

Attach the angle brackets to the inside of the inner hoop ring, with the bracket pointed below and to the outside of the ring.

Eyepiece Clamping Mechanism

Use a saw to cut from the side of the base to the center of the base.

Drill and countersink a pilot hole in the side of the base that passes through the cut you just made. Insert a wood screw into the pilot hole that is long enough to pass through the saw cut and continue into the other side of the cut.

Drill and countersink a pilot hole in the side of the base that passes through the cut.
Jack Day

Use a slightly larger drill bit to open up the initial section of the pilot hole until the point where the pilot hole reaches the saw cut in the base.
Jack Day

It should travel at least another ½” into the other side of the cut. When tightened, this will allow the base to clamp down onto the barrel of the eyepiece, providing good support for the projection screen.

Put It All Together

Attach the arrow shafts or dowels to your base.

Attach the inner hoop to the support rods.

Now you can add an eyepiece and the viewing screen!

Safety Recommendations

Since this design does not use a solar filter, use an eyepiece with an all-metal lens housing and barrel and make sure no one puts their hands in between the eyepiece and the projection screen. A simple optical design with as few cemented elements as possible is also recommended.

For telescopes with an objective larger than 70mm in diameter “stop down” the telescope to — at most — around 60 to 70 mm (40 to 50 mm for extended viewing times).

Choose the Right Focal Length Eyepiece

For focal lengths:

• Up to 500-mm focal length, use a 9-mm or 10-mm eyepiece
• From 500-mm to ~ 750-mm, use a 12-mm to 20-mm eyepiece
• From 1000-mm to ~ 1500-mm, use a 25-mm or ~ 26-mm eyepiece
• 1500mm+ 30mm or longer focal length eyepiece.

The Projection Screen

We used Rose Brand rear projection screen material, and felt that the standard “grey” screen was ideal. One yard (36” x 55”) provides enough material to build around 10 projectors.

“Hoop-La” hoops are available in many colors, and both the 8” and 10” versions were easy to find at several local stores. We also suggest painting or sealing the projector base to protect it from moisture. We would love to see pictures of your screens in action on Eclipse Day!

Final Thoughts

This project would not have been possible without the support of Table Mountain Star Party (TMSP), and my telescope building crew: Zachary Day and Richard McDonald. Thanks to TMSP for the financial and moral support, and thanks to my crew for making them a reality! Oh, and be sure to join us for TMSP next year! You never know what the telescope building crew will come up with next!