This week, I conclude my three-part series on filters by discussing specialty filters. Everyone's welcome to their opinion, but, to me, any filter that's not a color filter is a specialty filter. Most specialty filters fall into the category of "light-pollution-reduction" (LPR) filter. Two exceptions are neutral density and polarizing filters.
A neutral density (ND) filter reduces the amount of light (by absorbing it) but doesn't filter any of the colors. ND filters have transmissions as high as 80 percent and as low as 1 percent. You can obtain even darker values by stacking filters. In general, use higher-transmission ND filters for the planets and denser ones for the Moon.
Polarizing filters reduce glare by transmitting only light of a specific orientation. Double-star observers often use single polarizing filters to reduce the glare from the brighter star in a binary system.
Some observers use a polarizing filter to view deep-sky objects when the Moon is bright. They report significant contrast gain 60° from the Moon and astounding results when the object lies between 80° and 90° from the Moon. This gain works best at low magnifications.
LPR filters come in two varieties: broadband and narrowband. "Band" refers to the range of wavelengths the filter will transmit. Broadband filters, usually marketed with "light pollution" somewhere in their names, improve the view a bit from cities or brightly lit suburbs. In fact, they tend to work better from a dark site or one with only mild light pollution. Narrowband filters are a different story entirely. I'll describe three.
A UHC (ultra-high contrast) filter has a wider bandpass (22 to 26 nanometers [nm]) than other narrowband filters but a much narrower one than any broadband filter. Through a UHC filter, the background sky appears darker and stars take on a blue color. Emission nebulae benefit most from this filter.
A Hydrogen-beta (Hβ) filter has the narrowest bandpass of all, only 8nm. Its transmission centers on the Hβ line at 486nm. Amateur astronomers usually purchase this filter to view one object — the elusive Horsehead Nebula in Orion. And it works. Using an Hβ filter, I've seen the Horsehead Nebula through telescopes as small as 6 inches. An Hβ filter benefits few other nebulae. One that does is the California Nebula (NGC 1499) in Perseus.
The Oxygen III (OIII) filter transmits light from doubly ionized oxygen. (The "3" in its name indicates double-ionization &mdash OI represents neutral oxygen and OII is singly ionized oxygen.) OIII filters have bandwidths of about 10nm centered on a pair of spectral lines with wavelengths of 496nm and 501nm. Planetary nebulae especially benefit from this filter, as do supernova remnants. An OIII filter also performs well on bright emission nebulae.
Can't afford all of these filters? Suggest one, you say? Ok. Hands-down, you'll get the most gain with an OIII filter. Even from severely light-polluted locations, observers have gotten good views of planetary and emission nebulae while using an OIII filter. It's not like observing from a dark site, but it is a good compromise in less-than-ideal conditions. Use an OIII filter under a dark sky, however, and the results will make you gasp.