Light Pollution Filters For Amateur Astronomy
How Amateur Astronomers Can Deal With Suburban Skies
Oct 29, 2009
Cities and towns waste a lot of energy lighting up the sky. Lights used this way don't illuminate roads or sidewalks, don't attract late-night shoppers, and don't deter criminals. It's simply wasted energy that, among other things, makes it difficult to see the stars of the night sky. Astronomers call this light pollution.
Light pollution swamps out the light that makes it from a distant galaxy to a telescope, making such objects difficult or impossible to see.
Option 1: Visit A Dark Sky Site
One option is to visit a dark sky site far away from the nearest city or town. That's often easier said than done though; a major city will spill light outwards for dozens of miles, and driving fifty miles just to spend an hour or two under the stars usually isn't practical.
Option 2: Concentrate On Solar System Observing
Another option is to concentrate on objects that aren't affected by light pollution. Light pollution may spoil views of faint deep sky objects such as galaxies and nebulae, but it has no effect on the Moon or the planets.
Option 3: Use Light Pollution Filters
A third option is to invest in deep sky filters. These are quite expensive, costing from $75-$200 depending on the type of filter and its size, but many suburban astronomers consider them well worth the investment.
Deep sky filters only allow through certain wavelengths of light and are divided into two basic types: broadband filters and narrowband filters.
Broadband Filters
Broadband filters let through a fairly broad range of light wavelengths. They usually only block the wavelengths of light emitted by low-pressure sodium vapor streetlamps. In theory at least, the sky appears darker and deep sky objects are easier to see. Because of this, they're sometimes called light pollution reduction (or LPR) filters.
In practice though, their usefulness depends on what sort of streetlamps and other light sources are in your area. It's a very good idea to try one of these filters out before buying one, just to see if it's worth the investment. Your local astronomy club should be able to help here.
Narrowband Filters
Narrowband filters block more light, letting through only specific wavelengths of light. These work well, but be warned: because they block so much light, the resulting image can be dim where small telescopes are concerned. Narrowband filters are also more expensive than the broadband filters.
The two most popular narrowband filters are the nebular filter and the oxygen-3 (or O-III) filter. The differences between the two filters are the ranges of light they let through. Nebular filters let through wavelengths associated with both ionized hydrogen and ionized oxygen, whereas O-III filters let through only the ionized oxygen wavelengths.
Which To Choose?
Both types of filter work very well on planetary and emission nebulae, though some objects look better through one filter than the other. Neither filter does anything to help views of galaxies, open clusters or globular clusters.
Nebular filters are good all-around performers and will deliver good views of popular objects such as the Orion Nebula, the Ring Nebula, the Dumbbell Nebula and the Lagoon Nebula. O-III filters are slightly less good as general purpose filters, but are exceptional on a few objects, most notably the Veil Nebula and the Helix Nebula.
There are other, more obscure filters available but these are the two that suburban astronomers will find most valuable.
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