The objective is direct solar observation without an approved solar filter. In the absence of such equipment—and even in the absence of a telescope—a basic pinhole projection system can be constructed from common materials found at home (notably, a cardboard box). The resulting image will not match the resolution and contrast obtainable with a properly filtered telescope; however, it does allow observation of solar features via projected light. Because the viewing method relies on a projected solar image rather than direct eye viewing, it eliminates the primary safety concern associated with looking at the Sun.
Materials
- A cardboard box at least 18 inches (≈45 cm) long
- A quarter (for tracing)
- A sharp cutting knife
- Aluminum foil (preferably fairly thick)
- Tape
- A pin (preferred) or a sharp pencil
- White paper (for the screen inside the box)
Construction Procedure
- Select and prepare the box. Begin with a cardboard box of sufficient length. If the box is shorter than roughly 18 inches, the projected solar image will be too small to be useful.
- Cut the pinhole aperture. On a shorter end of the box, trace a circle using a quarter and cut the circle out carefully. The aperture does not need to be perfectly circular; however, it should be cleanly cut.
- Seal the aperture with foil. Cut a piece of aluminum foil large enough to cover the opening and tape it securely over the hole. The foil serves as the substrate for the pinhole.
- Create the pinhole. Puncture the foil using a pin. A pin tends to produce cleaner edges than most alternatives.
- As an alternative, a sharp pencil may be used, producing a larger opening.
- Larger holes increase throughput and thus brightness, but if the opening becomes too large, the pinhole projection effect degrades and the image transitions toward an out-of-focus blob rather than a defined solar disk.
- Check geometric integrity. Verify that the hole is unobstructed and reasonably round.
- Prepare the projection screen. Tape a sheet of white paper to the inside surface of the box opposite the pinhole.
- Observe. Point the pinhole toward the Sun and observe the projected image on the paper.
- Outcome: This completes the assembly and enables direct observation of the Sun via a projected image, with no requirement to look directly into the Sun.
Build a Solar Projector (Finder-Scope / Telescope Projection Method)
- A more capable approach is to use optics from a telescope’s finder scope configured for straight-through viewing. In favorable cases, the finder scope can be swapped between the telescope and the solar projector.
Key Concept
- This method uses the finder scope to form an image of the Sun on a screen at a measurable distance. The quality depends on geometry and the optical path.
Initial Measurements
- Define the focusing distance. Measure the distance from the finder scope eyepiece to the location where the Sun focuses to an image.
- Practical approach for measurement. With assistance, hold a white cardboard target facing the Sun and adjust the finder scope until it projects a reasonably stable solar image. Then measure the distance from the eyepiece to the target.
- Let this distance be T (in inches or any consistent unit).
- Measure the solar image diameter. Determine the diameter of the projected solar disk.
- Let this be D .
- Design and Board Dimensions
- The long arm must be T plus enough additional length to accommodate:
- the finder bracket, and
- the thickness of the screen board that carries the projection target.
- A screen board thickness similar to ¾ inch (≈1.9 cm) is recommended because thinner material may crack when fastened with screws.
Example geometry (for one specific configuration):
- T ≈ 29¾ in (75.6 cm)
- D ≈ 4¼ in (10.8 cm)
- Additional length for finder attachment: 7 in (17.8 cm)
- Screen board: ¾ in thick
- Screen board size: 6 × 6 in to accommodate the projected solar disk plus margin
- Long arm total length: ≈ 38 in (96.5 cm)
- Long arm width: ≈ 2¼ in (5.7 cm) to support the finder base without excessive unsupported mass
- Fabrication Steps (Analytical Summary)
- Smooth all contact surfaces to prevent splinters.
- Attach the finder scope base to the long arm using two bolts of appropriate length (long enough to secure the base but not to protrude below the arm).
- Drill clearance holes for nuts and lock washers so they remain concealed below the arm.
- Attach the screen board to the end of the long arm using two screws, ensuring:
- the screen board is perpendicular to the long arm, and
- parallel alignment is maintained relative to the finder’s front lens plane.
- Determine the tripod mounting balance point.
- Do not simply measure the geometric center of the long arm. Instead:
- assemble the long arm with the finder scope and screen board,
- balance it statically using two index fingers placed far apart (about 30 inches),
- slide toward each other until balance is achieved,
- mark the balance point,
- drill, and tap the location for a ¼–20 thread .
Operational Notes
- Use lens caps for the finder optics if available to reduce dirt accumulation.
- If transporting the device, disconnect and protect the finder optics from shocks.
- Avoid over-tightening tripod mounting hardware to prevent thread stripping in wood. If stripping occurs, metal ¼–20 threaded inserts can be installed.
- Description of a Specific Example Build
- One implementation uses an 8 × 50 finder scope mounted to one side of a cherry-made long arm. The screen board is mounted at the opposite end using two screws. The focusing mechanism is adjusted by loosening a knurled knob and changing the position by rotating the front lens.
The final assembly is designed for straightforward disassembly and stable operation, with the tripod mounting hole placed at the balance point rather than the board’s center because the finder assembly is significantly heavier than the screen board.
Build a Solar Projector from Binoculars
- If a finder scope is unavailable, binoculars can be used to construct a similar projection system. The method relies on projecting the focused solar image onto a screen.
Safety and Practical Considerations
- Projection requires appropriate geometric alignment; the binoculars’ optical complexity influences heat retention.
- Since binoculars may have multiple optical elements, the risk is not mitigated by eye viewing but by projection. Therefore, the projector must not be used for direct ocular observation of the Sun.
- General Construction Components
- Wooden base
- Metal L bracket
- Metal plate for tripod bolt attachment
- Screen support backing
- White projection target (thin cardboard or paper)
- Optional contrast shield (e.g., foamcore positioned over the binocular aperture region)
Geometric Determination
- Measure a suitable focus length. With the Sun aligned through the binoculars, measure the distance from the eyepiece to where the solar image reaches a usable size: sufficiently large for observation but not so large that it becomes dim.
- Select base dimensions accordingly. The base length must accommodate the focus distance plus attachment allowance for bracket placement.
Binocular Mounting Geometry
- Mount the L bracket so the binocular front lenses align with the base’s front end and do not extend unsupported beyond the base.
- Ensure bracket height is sufficient such that the projected solar image is not cut off by the base.
- The bracket height can be set using the solar image diameter from the projection screen:
- Use half the diameter plus a margin (e.g., +½ inch) to define the clearance.
- Screen Installation
- Attach a backstop panel behind a sheet of thin white cardboard so the binoculars project onto a stable surface.
- Weight Consciousness and Stability
- Use lightweight base features when feasible (e.g., drilled holes along the base).
- Mount a tripod at the static balance point determined with the full assembly attached.
Optical Assembly Notes
- If binocular images overlap imperfectly, it may be advantageous to use only one optical side by leaving protective caps in place.
- Create a “Sun Crescent” Sign (Pinhole Pattern Projection)
- This method creates patterned crescent images during an eclipse by projecting light through multiple small holes punched in a thin board.
Materials
- White poster board (or comic backing board)
- Hole-punch tool (leather punches recommended for clean perforations)
- Knife/scissors for cutting board into form
- Optional: camera for documentation
- Hole Quality Requirement
- Image sharpness depends directly on hole cleanliness and geometry:\
- Cleaner holes produce more defined projected crescent shapes.
- Testing multiple punch sizes is recommended to empirically identify the best visual result.
Construction and Use
- Cut poster board to approximately 7 in × 3 in .
- Design layout by drawing a dot-based pattern (word/date/place tied to dot groups).
- Punch holes at marked points to match the intended pattern.
- During the eclipse, project onto a larger sheet and photograph the result while the Moon forms crescents.
- Expect multiple phases. Capture multiple frames during partial phases and select those with the best crescent representation.
- Practical constraint: The method is most effective when lunar coverage is substantial (approximately ≥75%), and it will continue to show partially eclipsed solar shapes even when totality is unavailable at the observer location.
