* Astronomy

UV filters are used to reduce haziness created by ultraviolet light, to which photographic film and sensors are sensitive, but not the human eye. A UV filter passes all or most of the visual spectrum, and blocks ultraviolet radiation.
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Achromatic refractor telescopes will bring ultraviolet light to a slightly different focal point relative to other wavelengths; and many colour films are sensitive to Ultraviolet light which will result in a blue cast and image blur or reduced contrast.

A UV filter should pass most of the visual spectrum, and block ultraviolet radiation, lessening the bluish haze seen in distant subjects.
UV filters should look clear and neutral to the naked eye, and can protect the front lens or digital sensor chip from physical damage and dust.

However, digital camera sensors are not as sensitive* to UV light as photographic film, and IR-cut filters are equally good at keeping the digital sensors dust free.
And it is debatable if adding a filter in the light path just to protect the front lens outweighs the slight reduction in image quality caused by the filter itself.

*CCDs are sensitive down to ~250nm, but the protective glass and coating over the digital chip already blocks the ultraviolet light



The bottom line is that with modern digital sensors UV filtration is no longer necessary.

Achromatic refractors form an out-of-focus IR image on webcam sensors, so it is important to filter out those wavelengths (less than 700nm).

Those on a tight budget may wish to reuse the IR filter from the web camera lens. The tiny IR-cut filter is usually mounted on the rear but sometimes can be found on the front of the webcam lens. The fiddly job of removing the delicate filter and mounting it (perhaps between two metal spacers?), and then fitting that into the telescope/webcam adaptor, is not for those with shaky hands.
But it is possible...

A less surgical procedure is to reuse a green/blueish heat-absorbing filter from an old slide or cine projector. Light bluish coloured infrared filtering glass can also be found in photocopier machines. These filters will effectively remove the excessive IR.

Altair-EX 1.25" IR Cut Blocking Filter

The IR cut-off filter blocks the transmission of Infrared light, while passing visible light. Webcam chips are very sensitive to IR. Because it has a longer wavelength than visible light, when IR passes through optics, it comes to focus at a different point. This causes a blurring effect in Monochrome images with loss of fine detail. When you put a webcam adapter (available separately) on your webcam, the factory IR-Blocking filter is removed (it's usually part of the lens assembly and pretty low quality anyway). This filter fits on to the front of your webcam adapter to make sure that no Infrared light lands on the chip. Please note, this filter cannot be used to image the sun on it's own - it should only be used with the appropriate solar filter on the front of the telescope.

£14.99

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Ed ~ this will not block Ultraviolet frequencies.