Build a zero-magnification finder

Make an illuminated aiming device you can use on any telescope

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This month's project helps you aim your scope 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.

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.

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.

Mounting matters