Deep Sky Filters
by Paul Markov, Dec. 1, 2000

Despite having been an avid deep sky observer for the past 18 years, I have never owned a deep sky filter until last month. The misconception I had about deep sky filters is that they are very expensive and that they are not very useful under relatively dark skies. It was one of our members, Stephen Keefer, at one of our usual observing sessions who set me straight on deep sky filters. In this installment I will describe what kind of deep sky filters are available on the market and what you can expect from each, as well as tell you about some personal observations with the OIII filter from a fairly dark sky and from within the city.

There are three categories of deep sky filters available; broad-band "light pollution" filters, narrow-band "nebula" filters, and the specialized "line" filters. Broad-band filters let the majority of light through, only filtering out a few wavelengths, narrow-band filters block a good portion of incident light, letting through only a few selected wavelengths, while "line" filters only let through one specific wavelength. I want to point out something that is very important before getting into the details - a filter that substantially improves views of star clusters, galaxies, or reflection nebulae does not exist. This is because deep sky objects comprised of stars (or objects which reflect starlight, such as reflection nebulae) shine with a continuous spectrum, meaning that if a filter tried to completely remove any particular wavelength (such as man-made light pollution or natural sky glow) it would also remove light from the actual deep sky object, dimming it considerably, or making it disappear altogether. Filters are most useful for emission and planetary nebulae because they glow at very specific wavelengths. For more details on the physics behind filters, please reference the 2001 Observer’s Handbook, page 49. These filters come in various sizes, with the most common being 1.25-inch, 2-inch, and Schmidt-Cassegrain rear-cell size. The prices noted below are for the 1.25-inch size at local telescope shops. If you are looking for the other two sizes, the cost is about double that of the 1.25-inch size.

Broad-band Filters

Examples of broad-band filters available on the market are the Lumicon Deep Sky, the Meade Broadband, the Celestron LPR, the Orion SkyGlow, and the Thousand Oaks LP-1, and sell for about $90 to $160. These filters are designed to block the most common sources of man-made light pollution such as mercury vapour and sodium lights and are intended to be used on all types of deep sky objects. Although it sounds like this type of filter would really help, in practice it yields just a marginal improvement over unfiltered views. Some gain in contrast can be expected for nearly all deep sky objects, still, I would not recommend broad-band filters unless you are restricted to urban skies at all times. You will enjoy much nicer views under darker skies.

Narrow-band Filters

Examples of narrow-band filters on the market are the Lumicon UHC, the Meade Narrowband, the Orion Ultrablock, and the Thousand Oaks LP-2. These filters only pass the two Oxygen III emission lines, the Hydrogen Beta emission line, and the wavelengths between these two, making them most useful for observing emission or planetary nebulae. Because this type of filter blocks all other wavelengths, the background field of view becomes rather dim, but target objects still stand out well and actually appear brighter because of the added contrast. Brighter nebulae which are visible without a filter still benefit from the increased contrast, while nebulae which are altogether invisible without a filter may become visible. A narrow-band filter will help considerably whether you are observing from the city or from a dark sky, therefore it should be seriously considered. Narrow-band filters cost between $130 and $160.

Line Filters

There are three types of "line" filters – Hydrogen beta (H-beta), Swan band (C2), and Oxygen III (OIII). Each of these can be purchased locally for about $130 to $160.

The H-beta filter transmits only the H-beta emission line and is only useful on a handful of objects, with the most popular being the Horsehead nebula, the California nebula, and the Cocoon nebula. Because of its limited application, this is probably the last filter you should consider buying, however if you are determined to view the above mentioned objects and lack aperture or very dark skies, the H-beta filter is a must. This filter is available from Lumicon and Thousand Oaks.

The Swan band filter is used for enhancing the C2 emission lines found in comets, so it has a very limited application, especially because not all comets respond well to a Swan band filter. I believe this filter is only manufactured by Lumicon.

The OIII filter transmits only the two OIII emission lines but due to the abundance of objects that glow in the OIII region of the spectrum, there are hundreds of objects that benefit from this type of filter. Many emission nebulae and most planetary nebulae will look remarkably better through an OIII filter, which is manufactured by Lumicon, Thousand Oaks, and Meade, to name a few. Because the OIII filter discriminates against all other wavelengths, the background image is considerably dimmer, even more than a narrow-band filter, thus it is more suitable for telescopes 6-inches and larger. The OIII filter is the preferred choice for planetary nebulae.

My experience with an OIII filter

As mentioned above, it was Stephen who introduced me to an OIII filter at our dark sky observing site near Newtonville. Our first target was the Veil Nebula in Cygnus. The OIII filtered view was absolutely stunning and appeared very much like a long exposure photograph – I had no idea a filter could improve an object to that degree! Next he showed me M27, the Dumbbell nebula, and a little later M42, the Orion nebula. Both were spectacular and also resembled long exposure photos with the exception of the missing colour. Again, I was quite taken by views that can typically only be had in books and magazines. Removing the filter made these objects appear less prominent and washed out due to the loss in contrast. After only three objects I was already sold on the OIII filter! Well into the early morning hours we looked at the Rosette nebula in Monoceros which was almost invisible in Stephen’s 16-inch, but the filter revealed the entire nebula with ease. I then borrowed the filter for use on my own telescope to see what it could do on NGC 2359, an emission nebula in Canis Major. Without the filter this nebula was just barely visible in my 10-inch, but with the filter it "stuck out like a sore thumb!" Coincidentally, by observing this nebula I completed my Finest NGC Objects list. My observing log entry for this object reads "the OIII filter does wonders!" and all this was from a dark country sky, which I thought would not benefit from the use of filters. The next day I found out that this little "magic" piece of glass was quite affordable, and the following weekend I bought my own Lumicon OIII filter.

I was fortunate to have two clear nights right after my purchase to evaluate the OIII filter. I observed from my Scarborough backyard and targeted some of the better known objects. My 10-inch equipped with the OIII filter showed a much brighter M57 which was also more clearly defined. M27 was considerably brighter, with a little extra nebulosity visible. M76 was much brighter and showed a more clearly defined shape. M1 was barely visible from the city, and the OIII only improved the view marginally. The filtered view of planetary nebula NGC 7662 in Andromeda was only slightly better than the unfiltered view. M42 was considerably brighter with some additional nebulosity visible. M78 and the Merope nebula in M45 (both reflection nebulae) did not improve at all with the OIII filter as expected, and finally the Rosette nebula was invisible from the city, even with the filter, probably due to the overwhelming light pollution.

Which filter is best?

My recommendation, which is just based on extensive research as I only recently acquired a filter myself, is to buy either a narrow-band filter or an OIII filter. If you want the best "all purpose nebular filter" most experts will recommend a narrow-band filter. Keep in mind that regardless of whether you buy a narrow-band or an OIII filter, you will be very pleased with the results. Each has its benefits and drawbacks; a narrow-band filter shows more nebulosity than an OIII filter, but the OIII filter shows more contrast and fine detail, and a narrow-band filter performs slightly better on emission nebulae, whereas an OIII filter performs slightly better on planetary nebulae. In terms of which brand name to choose, it appears that there is not much difference between manufacturers, with the exception of the Meade narrow-band filter, which seems to noticeably outperform other brands when used with refractors. (*)

Footnote - * Sky & Telescope, July 1995, pp. 41

 

Copyright (C) 2000 by Paul Markov


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