The passbands of line filters are narrower still. The best known of this class is the OIII ("oh three"), which is a very-high-contrast filter. By the judicious application of many reflective layers, the manufacturers produce a narrow (10-nanometer) passband centered on two Oxygen III nebula emission lines at 496 and 501 nm. What is this Oxygen III? Why is it desirable? Oxygen III is the light of doubly ionized oxygen. It is often referred to in astronomy texts as the "forbidden lines." What is important for the SCT user to know is that this wavelength of light predominates in many nebulae, especially planetary nebulae.
The OIII filter is truly amazing. My Lumicon OIII, when used with my 8-inch SCT, for example, has been able to turn the dim Owl Nebula (M97) from an almost invisible smudge to an easily observed showpiece object in the city. It can improve the appearance of almost any nebula, and not only from bright suburban skies but also from the darkest of dark sites. One of my fondest observing memories is of the Bridal Veil Nebula in Cygnus (NGC 6960) as seen in an OIII filter from dark skies. The filter made this already interesting object into a thing of unending wonder. I spent at least an hour panning my CAT up and down the Veil's wispy tendrils!
But there is always that piper to be paid. The OIII is not a filter for everyone. The OIII actually extinguishes dimmer field stars. There is no doubt that this makes many fields unattractive. Another disappointment with this filter is that it does not "work" on every nebula. Most nebulas, diffuse and planetary, do enjoy a boost from an OIII, but those that lack substantial OIII emission are not helped very much—if at all. Sadly, the greatest nebula in the northern skies, M42, is one of these. It always looks poorer with an OIII than without. Also, some observers think the OIII imparts too much contrast to objects, that nebulas tend to look "cartoonish" and "not real" in the OIII. Finally, because of its density, the OIII works best with telescopes of 8-inches aperture and up.
The OIII is not the only line filter out there. Another is the hbeta, the "Horsehead filter." This one has its passband centered on the red light of hydrogen. This emission predominates in the very dimmest of dim nebulas, such as the faint, legendary clouds like the Horsehead Nebula in Orion (B33/IC 434) and the California Nebula in Perseus (IC 1499). Although the hbeta filter can do a surprising job on these nebulas and a few others like them, it does not work on much else. An hbeta is not a filter to use every night. The Horse will not be visible from a light-polluted backyard with a C8 hbeta or no hbeta. The Celestial Nag did show with this filter and a C11, but only with extreme difficulty and only from the superbly dark skies of the Chiefland star party.
Should novices consider a line filter? A beginner should probably acquire an OIII as a second filter, after a medium-strength filter such as the UHC, and an hbeta as a "third-if-ever" buy.
How much do these things cost, anyway? This depends on the brand and type but expect to pay about $75 to $100 for a 1.25-inch and $150 for a 2-inch. As is the case with all other astrogear, Chinese imports are beginning to drive LPR prices down.
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