This project helps you aim your telescope at a target in the night sky.
It is called a zero-magnification finder because the view is the same size as seen with your naked eye.
The finder projects a red circle onto the background sky and you move your telescope to align the centre of this circle with your target.
Many stargazers use similar devices because they allow you to keep both eyes open and see a much larger area of sky, making it easier to jump to the target.
How it works
Our design has a red LED that illuminates the white interior of the front of the case.
The LED is powered by a coin cell battery, via a switch on the back panel.
The red light is reflected backwards through a clear glass screen, painted matt black except for a small ring in the centre.
The light is then reflected upwards by a mirror mounted at 45° and through a lens, which produces a sharp image.
This is reflected backwards again by a glass viewing screen, mounted at 45° in a hood.
When the distant sky is viewed through this screen, the red circle is superimposed on it.
Getting the materials
We sourced the components for this project imaginatively to keep the cost down.
The LED, coin cell holder and switch are from an educational supplier.
The clear glass was cut from an old picture frame, and we used the frame’s thin MDF backing to make the case.
The front surface mirror is a disposable dentist’s mirror. We found these online, along with the lens, a Fresnel magnifier sheet.
Cutting glass is easy – score with a glass-cutting wheel and snap over a straight edge – but it is advisable to dull the resulting sharp edges by rubbing on a flat stone.
Alternatively use clear plastic, which can be cut from an old CD case.
How to mount it on your telescope
Once you have made your finder, you’ll need to mount it and align it with your telescope.
We used a camera ball mount for this. The top screw passes through a hole in the bottom of the case.
Use a ¼-20 nut to fix it. Alternatively, a blob of hot glue will suffice.
To attach the ball mount to the scope, use a ¼-20 stud (some mounting rings have screw-threaded holes for attaching accessories), or improvise with a large pipe clip or cable ties to hold the hot shoe part of the ball mount.
Point the scope towards an easy-to-find bright star or planet, loosen the ball mount and align the projected red circle with the same target.
Once aligned, you can use the finder to star-hop around the sky with ease.
Tools and materials
- Ruler, square and pencil for marking out.
- Coping saw, junior hacksaw, drill, drill bits, glass cutter, cutting mat, soldering iron.
- Small sheet (approximately A4 size) of thin MDF or plywood; small piece of thin glass or clear plastic; small metal ring (8–15mm or so in diameter).
- Red LED, 3V coin cell battery, battery holder, toggle switch (single pole, single throw), short length of electrical wire.
- Dentist’s mirror, Fresnel credit-card-sized magnifier.
- Camera ball mount.
- Spray paint, glue (we used a hot-glue gun), black paint, white paint.
