There are a number of different filters available on the market today, with most coming in one of three classes: 1. Broad-Band "light pollution" filters, 2. Narrow-Band "nebular" filters, and 3: Line filters.

 In the Broad band range are the Lumicon Deep-Sky, and the Orion SkyGlow. They enhance many deep-sky objects by blocking out the common Mercury vapor and some other emission lines which contribute to light pollution, while letting through a broad range of wavelengths. While these broadband filters do help increase the contrast of some objects somewhat, their overall effect is not terribly striking. A similar effect can sometimes be achieved by increasing the magnification slightly, since the light pollution effect is then diluted. The Lumicon Deep-Sky does help enhance the visiblity of things like the Merope Nebula, the Trifid, and M42, but not much of an effect is noted with galaxies and star clusters. I have noted that using the filter on M33 and NGC 253 in my 10" when skyglow is higher than usual will help boost the visibility of the detail, but the effect is only moderate. Since some light is blocked by the Deep-Sky filter, there will actually be a light loss, and some objects may even look fainter with the filter than without. The Deep-Sky also works as a passable blue filter for observing Jupiter. Thus, the broad band filter, while somewhat useful, is not the most effective filter intended for deep-sky use.

 In the narrow-band "nebular" filter range, the best filters seem to be the Orion Ultrablock, and the Lumicon UHC. These filters allow only the emission lines of Hydrogen (H-alpha and H-beta) and the bright Oxygen III lines (plus wavelengths between H-beta and the OIII lines), to get through. Their effect can be quite striking, with many faint nebular objects becoming easy to see (without the filter, some may not be visible at all!) Even the more prominent nebulae which are visible without filters gain considerable detail and contrast with the narrow band filters. Both the UHC and Ultrablock will, for example, often show the Rosette Nebula TO THE NAKED EYE when you look through them. Even under a really dark sky, the contrast and detail improvement are impressive, and most observers continue to use their nebular filters at such dark-sky sites. One neat trick for finding tiny planetary nebulae is to "blink" the objects by holding the filter between the eyepiece and the eye. The stars in the field will dim somewhat, but the planetary nebula will remain undimmed, thus standing out from the background stars. In comparison, both the UHC and Ultrablock have very similar characteristics, but some people have reported a slight edge in performance with the Ultrablock. At times the Ultrablock has also been somewhat less expensive than the UHC. Both will perform very well, and should be used at moderate to low powers for best results. However, these "nebular" filters do not usually work very well on star clusters, reflection nebulae, or galaxies. In the "line" filter category, the Lumicon Oxygen III (OIII) filter is the real standout. It allows only the narrow pair of emission lines of Oxygen to get to the eye, and for many planetary and diffuse emission nebulae, the boost in visibility has to be seen to be believed! The Veil and North American Nebulae look like photographs in a 10" with the OIII filter, and many of the "green box" emission nebulae in SKY ATLAS 2000.0 jump out at you. You may even see some nebulae which are not shown on many atlases. Planetary nebulae become easy, and the "blinking" technique becomes vastly more effective, as the stars nearly vanish, leaving the planetary nebula standing out like a sore thumb. However, since the bandwidth of the OIII filter is so narrow, it may hurt some objects slightly, like the nebulae around Gamma Cygni. This filter also hurts the view of clusters and galaxies even more than the narrow band filters do.

 One line filter of note is Lumicon's H-Beta filter. As the name indicates, the filter only lets through the H-Beta emission line of Hydrogen, and is mainly useful on a limited number of objects, like the Horsehead Nebula, the California Nebula, the Coccoon Nebula, and M43. On an 8" to 10" scope, the Horsehead Nebula goes from invisibility to visibility, as does the California Nebula. However, these objects remain fairly faint (especially the Horsehead), and unless you REALLY like looking at them, you can probably forget about buying the H-Beta!

 For recommendations, I like the Orion Ultrablock and the Lumicon OIII, but remember to use them with an eye that is properly dark adapted. Filters won't make the objects brighter, but they will make many of them a lot easier to see. Have fun!

 David Knisely, Prairie Astronomy Club http://www.infoanalytic.com/pac/