Spectrum Scientifics Blog

Many filters are available for telescopes these days. Each of them does something different to get a different effect. Some filters are used to reduce light, others to reduce light pollution, others to bring out other colors of The Planets, etc. Filters can help enhance a viewing session to make it more pleasant and enjoyable. The most common kinds of filters Spectrum Scientifics carries are:

Moon Filters: The Moon is very bright and is unaffected by light pollution. As a result, viewing the Moon through a larger telescope can actually be too bright. But you don’t want to give up the added resolution that comes with the larger telescopes, so Moon filters solve the problem by cutting down the light via a Neutral Density filter. Moon filters are available in 25% Transmission and 13% Transmission. The former is for smaller telescopes or when the Moon is only partially visible, the latter is for larger telescopes and for when the Moon is closer to being full.

Variable Polarizing Filters: Similar to Moon Filters in purpose, polarizing filters have two elements with polarizing film. As they are rotated the amount of light that gets transmitted increases or decreases from as low as 1% up to 40%. This allows much more versatility in viewing than Moon Filters allow.

Color Filters: Color filters are used during planetary viewing to bring out details. For example, the bands on Jupiter can be enhanced using a Medium Blue filter, or a Deep Yellow filter can be used to bring contrast to Lunar viewing.

Light Pollution Filters: There are several versions of these, and they work by cutting out the wavelengths of the most common lights polluting the night skies. Using a light pollution filter can help in urban and suburban viewing, but one must remember that these are filters, they do not make deep sky objects any brighter and so are not a substitute for truly dark skies. However, they are better than no light pollution filtering as this image taken in urban skies demonstrates:

Violet-Minus Filters: Violet minus filters are used exclusively with refracting telescopes. The larger refractor lenses can actually break white light up like a prism. When viewing this results in a violet colored halo around brighter objects like the Moon & planets. Violet minus filters remove that violet halo, resulting in a more pleasing image from your refractor.

Nebula & Astrophotography Filters : Nebula filters are specially design just for viewing nebulas. They cut out almost all other visible wavelengths of light, resulting in a high contrast image of the nebula.  Astrophotography filters work on a similar principle, but are designed for work with digital cameras to reduce background light and other incidental light on the produced image. These are generally specialty instruments, and most astronomers won’t use them very much.

When properly used, telescope filters can bring a lot of improvement to a telescope viewing session. Always remember to pick the right tool for the job. There’s no point using a Moon filter when viewing a Galaxy, and a light pollution filter is almost useless when looking at a bright object. Always make certain to remove the filter after viewing the object the filter was meant for…we’ve seen people have trouble with their telescope when trying to view a nebula with the Moon filter still attached!

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Filed under: Astronomy, Urban Astronomy