Dave's Astronomy Pictures - Home-made IR-pass filter

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The moon, and some planets can be usefully imaged using infra-red light. Webcams (with their built-in filter removed) are very sensitive to IR, and the narrower bandwidth (when compared with "normal" visible light) makes a sharp image easier to obtain.
A useful filter can be made from exposed photographic film, which in two layers is opaque to visible light, but transparent to infra-red.
Here are the details of how I made my filter. Click each picture to see it full-sized..

Raw Materials

1. A piece of exposed 35mm film, cut from the end of a roll where the film is darkest.
2. An unwanted colour 1.25 inch astronomical observing filter

Parts before assembly

1. Unscrew the retaining ring and remove the colour filter from its holder.
2. Cut two pieces of film the same size as the unused filter element. The film isn't quite big enough but the film sprocket holes at the edge will be covered by the retaining ring when you assemble the filter.

Assembled filter

1. Layer the two pieces of film together and place them in the filter holder
2. Fix them with the retaining ring. The filter can now be used attached to the webcam's nosepiece to allow only IR light to the sensor.

Example Photo

The first lunar image I captured using this filter was the crater Humboldt.

Daves Astronomy Pictures

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