A solar finder made from a film canister

A telescope is no place for an optical finder if you want to observe the Sun during the day. This trick provides you with the perfect replacement.

If you still have an old 35mm film lying around in a corner somewhere, then the canister rather than the film inside is all you need to create a small but effective astronomy hack – a solar finder based on the pinhole camera principle. The canister must have a transparent cup with an opaque lid.

Start by making a small hole in the middle of the lid with a pin. In most cases, an injection moulding mark indicates the centre of the lid. In the middle of the bottom of the cup there is another similar mark. When the film canister is held to the sunlight, a small point of light is projected onto the bottom. Now, what's really smart is that the film canister fits almost exactly into a typical 6×30 finderscope adapter, and can usually also be clamped and even adjusted using the adjusting screws.

However, the projected point is quite small. With a 50mm long film canister, the Sun is barely more than 0.4mm diameter. With a 1¼ inch eyepiece and a 1,200mm focal length, the telescope's almost 1.3° field of view is just over 1mm across on the bottom of the film canister. With a two-inch eyepiece, it is just under 2mm. This can be adjusted quite successfully. If the point is too dark, the hole can also be increased to 1mm in size. It is easy to see if the light point is exactly centred around the centre point.

Author: Sven Wienstein / License: Oculum Verlag GmbH