July 1, 2008

In Today's One Minute Astronomer...

Light-Pollution Filters

Light pollution filters help you transform a washed-out city skyscape into reasonably dark location, at least when observing some types of celestial objects. Also called deep-sky or nebula filters, these high-tech wonders are a huge help for backyard astronomers. One look at the Veil Nebula or Barnard's Loop will have you hooked.

The Basics

• Most light pollution comes from mercury and sodium streetlamps, which radiate blue and yellow light that shows up as bright background sky glow. But many nebulae comprise hydrogen and oxygen atoms that emit green and red light. Light-pollution filters work by blocking the bad stuff in the blue and yellow, and letting through the good stuff in the red and green.

• Deep-sky filters block the least amount of light pollution but give the brightest image, making them good for photography. They block blue and yellow light while passing red and blue.

• Ultra-high contrast (UHC) filters pass only light in the blue-green part of the spectrum, from 482 nm to 506 nm. These filters give you a much darker background sky and dramatically better contrast between the sky and nebula.

A Deeper Look

• Oxygen-III (or OIII) filters pass only light from ionized oxygen in planetary nebula and supernova remnants. These filters give amazing contrast with such objects.

• Hydrogen-beta filters only pass 486 nm emission from hydrogen atoms. This is a highly specialized filter, useful for seeing the Horsehead and California nebulae.

Good To Know

These filters work only for diffuse nebula such as the Orion and Lagoon, planetary nebula like the Ring and Cat's Eye, and supernova remnants like the Veil Nebula. Unfortunately, with the exception of the “Deep-Sky” filters, these optics degrade your view of stars and galaxies that emit light over a wide range of wavelengths. So to see stars and galaxies without the ravages of light pollutions, you need dark skies… no filter will help.

Personal View

If you can afford only one of these filters, a UHC is your best bet. But if you can afford it, get an OIII filter as well for really faint planetary and emission nebula. I've never tried an H-beta filter, though I'd love to glimpse the Horsehead Nebula one day.

--- Highly Recommended ---

A superb star atlas for amateur astronomers, The Cambridge Star Atlas features 24 monthly sky maps (12 for each hemisphere) and 20 detailed charts of the entire sky showing stars to magnitude 6.5. Some 900 deep-sky objects are identified in color-coded symbols. Learn more...

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