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Telescope Eyepiece Filters

Eyepiece filters play an important role in your viewing experience. Filters act like sunglasses, they filter light to produce a better image. If you’ve worn quality polarized sunglasses, then you know what a difference they can make on how you view the world. Grass looks greener, the sky looks bluer than it actually is, and this is achieved by blocking certain light rays from entering your eyes. Telescope eyepiece filters work the same way.

There’s a broad range of filters that you can use for your telescope and they have different applications. For example, a moon filter cuts down the brightness of the moon significantly. Looking directly at the moon in a telescope without a moon filter is hard on the eye! You won’t go blind, but it’s just not comfortable. But filters aren’t just used to reduce light intensity. They can filter out colors or specific bands of light which allows you to see things you normally don’t see without the filter, or at least not as clearly.

Eyepiece filters for your telescope are very easy to use. They simply screw on your eyepiece. Today’s eyepieces have a standard threading made specifically for this purpose. So you simply twist the filter on, the slide the eyepiece in the focuser as usual.

Types of Eyepiece Filter

The main types of filters for your telescope include:

Moon Filter

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We talked about this one already, but it is the probably the first telescope lens filter most people will use. It’s not unusual to see a moon filter included with the telescope. In addition to “dimming the light” a moon filter brings out contrast which defines crater details by making them sharper.

Polarizing Lens Filter

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This type of filter works exactly like polarized sunglasses and it can be used on the moon and on planets because it significantly reduces glare without altering color too much. This filter also works well for observing binary stars and the reduced glare makes it easier to separate each star. An interesting product to look into is the Orion Variable Polarizing Telescope Filters which allows you to “dim” what you’re looking at. The variable polarizing filter is a must have for any backyard astronomer because it allows you to control the amount of light transmission from 1% to 40%

Broadband Light Pollution Filter

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These are perfect for observing nebulas as they get rid of light pollution. The job of this filter is to eliminate (well… reduce) the wavelengths of lights that are interfering with your viewing, like the neighbor’s patio light, or a nearby streetlight. This lens filter also diminishes the light from stars and star clusters so observing those is not advised. However, light from many nebulae are at a different wavelength and therefore are able to pass through the filter. Even if you are observing from an area relatively free of light pollution, a light pollution filter can still produce better deep space object (DSO) imaging than with no filters.

Narrow Band Ligh Pollution Filters

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These are an excellent choice for deep-sky observers located at highly light-polluted sites. Blocks all forms of light pollution including mercury vapor and sodium emission bands while passing the critical hydrogen-beta and ionized oxygen wavelengths. With Orion’s UltraBlock telescope filter, emission and planetary nebulas “surface” from the washed out background sky. In dark skies, these light pollution filters also enhances the sky presence of a significant number of fainter deep-sky celestial objects over unfiltered and wideband-filtered views.

Oxygen III Telescope filters

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The Oxygen III filter is well suited for viewing planetary nebula and some emission nebulas. It is the best filter for looking at these objects from light polluted areas. They allow 85-95% of the light of these nebulas emission while reject almost all of the light from all other sources including street lights, starlight, moonlight, etc. Oxygen III filters will even reveal faint nebulas that are otherwise not visible without a filter. The Oxygen III filters aren’t really used for astrophotography.

Hydrogen-Beta Eyepiece Filters

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Hydrogen-Beta filters, also called H-Beta filters, have very specific applications as they enhance only a few emissions and work well for very faint emission nebula. It’s a great telescope lens filter to use with larger aperture telescopes, and for looking at specific objects. Popular targets for the Hydrogen-Beta filter includes the Horsehead Nebula (IC 434), the California Nebula (NGC 1499), a section in the Orion Nebula (M43), the Cocoon Nebula (IC 5146) and others.