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Useful Filters for Viewing Deep-Sky Objects
There are a number of different filters available on the market today for improving the views of various Deep-Sky objects, with most coming in one of three classes:
1. Broad-Band "light pollution" filters
2. Narrow-Band "Nebula" filters
3: Line filters.
Broad-Band "Light-Pollution" Filters
The broad-band "Light-pollution Reduction (LPR) filters are designed to improve the visibility of a variety of Deep-Sky objects by blocking out the common Mercury vapor, Sodium, and some other emission lines from man-made or natural sources which contribute to light pollution, while letting through a broad range of other more useful wavelengths. Since the eye is mainly a "contrast detector", this selective screening out of some of the background skyglow increases the contrast and helps Deep-sky objects stand out more noticably. While these broad-band filters do not eliminate the effects of light pollution or make the objects brighter, in many cases, these filters can improve the visibility of some deep-sky objects to at least some degree. The greatest improvement in the overall view is often found with emission nebulae, but broadband filters can also give a slight contrast boost to some reflection nebulae and a few of the larger more diffuse galaxies. In addition, larger versions of these broadband filters which fit over camera lenses can be somewhat useful for photography of wide star fields when some skyglow is present.
Some available broad-band filters are the Lumicon Deep-Sky, the Meade Series 4000 Broadband, the Celestron LPR, the Thousand Oaks Type 1, and the Orion SkyGlow. The Lumicon Deep-Sky can offer a noticable boost in contrast and visibility of the fainter outer detail in emisson nebulae over non-filtered views for objects like the Orion Nebula (M42), the Lagoon Nebula (M8), the Merope Nebula, the Trifid (M20), and a number of others. However, the improvement is not as noticable on star clusters or galaxies. I have found that using the filter on larger and more diffuse galaxies like M33, M81, M101, NGC 253 and NGC 2403 in my 10 inch when weak skyglow is present will help boost the visibility of the detail, but the effect is fairly mild. On star clusters, there is even less of an effect, since some of their emission falls in the portions of the spectrum blocked by these filters. In that case it may be better to use slightly higher power on some of the smaller objects to dilute the light pollution effect a bit. Since some light is blocked by the filters, there can be times when a few objects may even look fainter from a dark sky site when using a broad-band filter than without one. Severe levels of light pollution may also be too much for the broad-band filters to handle effectively, so you still want to find as dark an observing site as you can. The Lumicon Deep-Sky filter has an additional bonus, as it does work fairly well as a blue filter for observing Jupiter and for bringing out the white clouds and polar caps of Mars. In summary, the broad band "light pollution" filter can be useful in compensating for some light pollution, but may not be the most impressive filter intended for deep-sky use.
Narrow-Band "Nebula" Filters
Narrow-band "Nebula" filters, as the name implies, are mainly designed for viewing many emission nebulae. These filters allow only the bright pair of emission lines of Oxygen III, the Hydrogen Beta emission line, and wavelengths between H-beta and the OIII lines to get through. Narrow-band filters darken the background skyglow significantly without hurting the nebula, and are often of considerable help when observing in mild to moderate light pollution. The filter's improvement of the view of emission nebulae is usually superior to that of the broadband filters, as many faint nebular objects become much easier 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 units. However, you still need to use proper dark adaptation, averted vision and low to moderate powers (4x to 14x per inch of aperture) to get the most out of these filters.
Some available narrow-band filters are the Lumicon UHC, Meade Series 4000 Narrowband, Thousand Oaks Type 2, and Orion Ultrablock. The UHC and Meade Narrowband also have a deep-red passband for the Hydrogen Alpha line. Both the UHC and Ultrablock will, for example, often show the Rosette Nebula TO THE UNAIDED EYE when you look through them. Even under a really dark sky, the contrast and detail improvements are impressive, and most observers continue to use their narrow-band filters at such dark-sky sites. One neat trick for finding tiny planetary nebulae is to "blink" the objects by holding a narrow-band 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, the UHC and Ultrablock have very similar characteristics, although the UHC has a slightly higher light transmission factor in its primary passband than the Ultrablock, which may be helpful for viewing faint nebulae. Spectroscopic comparison of the two filters reveals that the Ultrablock's passband is more rounded and slightly narrower than the more flat-topped UHC, with falloffs in light transmission towards the passband edges, especially towards the H-beta side. The UHC also shows a red "leak" passband including the H-alpha line (the Ultrablock doesn't have one), which may contribute to the image brightness with larger apertures. The Ultrablock's more rounded and slightly narrower passband may be reasons why some observers have reported a bit darker field and slightly higher contrast under light pollution with some objects using the Ultrablock. At times the Ultrablock has also been slightly less expensive than the UHC, but when not sale priced, the two filters are of similar cost. Both will perform very well, and the overall difference between them is very slight. However, these "nebula" filters usually slightly reduce the brightness of most star clusters, reflection nebulae, and galaxies, although in moderate light pollution, a narrowband filter may still be of some use on these objects with larger apertures. Photographic use of these narrow band filters is also not recommended.
Line Filters are very narrow passband specialty units which are designed to let in only one or two spectral lines from emission nebulae, such as the close pair of Oxygen III lines or the Hydrogen-Beta line. In the line filter category, the Lumicon Oxygen III (OIII) filter is the real standout. Its very narrow bandwidth allows only the pair of emission lines of Oxygen to get to the eye, and for many planetary and diffuse emission nebulae, the boost in contrast has to be seen to be believed! The Veil and Helix Nebulae look like photographs in a 10" with the OIII filter, and some 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 some atlases. This filter is often the best one for many planetary nebulae, with the "blinking" technique becoming 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 nebulae with significant H-beta emission somewhat, like the nebulae around Gamma Cygni or the Horsehead. Differences between this filter and Broad-band filters like the Lumicon UHC are mainly in nebula visibility and contrast. Many nebulae show a slightly larger area of nebulosity in the UHC filter with slightly higher brightness, but in the OIII filter, they will often have more contrast and dark detail. However, the OIII filter really dims the view of star clusters and galaxies even more than the narrow band filters do, although observers with large telescopes may find the OIII useful for bringing out a few emission nebulae in other galaxies, like the HII regions in M33. The Lumicon OIII also has a substantial red passband, and on bright emission nebulae like M42 and M8, weak red color in parts of the nebulae have been reported visually using moderate to large apertures. Recently, Thousand Oaks has produced its "Type-3" Oxygen III filter. It doesn't seems to have the tiny "red-ghosting" secondary star images that the Lumicon model does, yielding more point-like star images, although its overall performance in enhancing nebulae is quite similar to the Lumicon model. Meade has also introduced its own OIII filter.
Another somewhat less-used line filter is Lumicon's H-Beta. As the name indicates, the filter only lets through the H-Beta emission line of Hydrogen, and is best known for its effect on the Horsehead Nebula, the California Nebula, the Coccoon Nebula, and a few others. On an 8" to 10" scope, the Horsehead Nebula goes from near invisibility to visibility, and the California Nebula becomes fairly easy, gaining a great deal of contrast and filamentary detail. An improvement over non-filter use for additional objects like M42/43, the North America Nebula, and a few others can also be noted, but in many cases, these other objects can appear somewhat better overall in the UHC or OIII filters. The H-beta can also be used to observe some of the structural details of some brighter nebulae by comparing the H-beta view with that in other filters. However, the H-beta does not usually work well on most planetary nebulae, as it nearly wipes out some of them and greatly dims most of the rest. The total number of emission nebulae which the H-beta will significantly improve is somewhat limited. Many of these "H-beta" objects tend to be fairly faint to begin with (like the Horsehead) and require larger apertures for decent views even with the filter. Unless you REALLY like looking at these faint H-beta targets, you may be able to do without the H-Beta filter. Thousand Oaks also makes their "Type-4" version of the H-beta filter.
For recommendations, if you can afford only one filter, get the Lumicon UHC or Orion Ultrablock (whichever is less expensive at the time). If you can afford to get two filters, the Lumicon OIII makes a good companion filter to a narrowband one like the UHC, but remember to use them with an eye that is properly dark adapted and employ averted vision. Filters won't make the objects brighter, but in many cases, they can make many of them a lot easier to see. Have fun!
David Knisely, Prairie Astronomy Club