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CN Report: DGM Optics OIII Filter
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The DGM Optics OIII Filter
The use of the narrowband "Oxygen III" (OIII) nebula filter has become fairly common with deep-sky enthusiasts who are attempting to observe some faint elusive nebulae. For many years, the Lumicon OIII had become a sort of "standard", but there are other OIIIs on the market today which claim to be as good as the Lumicon model. Enter the DGM Optics OIII, which provides a nice effective view of some nebulae at a slightly lower cost than the Lumicon unit.
OK, what exactly *IS* an "OIII" Filter?
An Oxygen III (OIII, or "Oh-three") is an interference "line" filter designed for use on certain types of emission nebulae. These filters pass the prominent "forbidden" emission lines of Oxygen while excluding many other wavelengths. The OIII lines exist in the blue-green part of the spectrum, and have wavelengths of 4959 angstroms and 5007 angstroms, with the 5007 angstrom line being the stronger of the two (generally nearly three times as strong as the 4959 angstrom line). These two lines are what give some brighter planetary nebulae their bluish-green coloration when observed visually. OIII filters do not diminish the nebula's brightness very much, but they will significantly reduce the residual sky glow from light pollution, airglow emission, and other non-nebular light sources. They do this by having a very high transmission at the OIII wavelengths and a very narrow passband width, which is typically less than 200 angstroms Full Width at Half Maxima (FWHM). This can make some faint nebulae a lot easier to see than without filtration, although in some cases, a narrowband filter or H-Beta line filter may occasionally be more appropriate depending on the object. The contrast enhancement is particularly striking on a few notable diffuse nebulae like the Veil, as well as on a large number of planetary nebulae, although these OIII filters are not recommended for use on things like reflection nebulae, star clusters, or galaxies. They tend to work the best under dark or semi-dark sky conditions, as the eye must be well dark-adapted to get the most out of the filter's performance. However, with some shielding and precautions, they have been used for some urban viewing of nebulae.
The DGM OIII:
The filter is a standard 1.25" model with common filter threads (a 2" model is also available for about $170). It is the usual black color with gray lettering, and I had little trouble threading it into all of my eyepieces and my filter slide. The coatings that make this filter possible are "first surface hard oxide" coatings, which tend to be somewhat more durable than the older laminated filters. I took a look at the filter's passband using a hand-held spectroscope and was a little surprised at what I saw. The bandwidth appeared to be somewhat wider than that of my new 2" Lumicon OIII filter, which is counter to the passband graph on the DGM/Omega web page. In consulting with Dan McShane of DGM, he informed me that it is indeed wider (nearly 1.4 times). Still, my new 2" Lumicon OIII is narrower and has a higher transmission than my 1990's vintage Lumicon OIII. This difference between old and new Lumicons would have a real impact in the comparative tests that followed. Unlike my new 2" Lumicon model, the DGM OIII had a substantial red passband, although it was not as strong as the old "red leak" of my 1990 Lumicon OIII, and may not be able to pass H-alpha as the Lumicon filter did. The rejection of the filter outside of the primary Oxygen III passband was quite good, with nearly zero transmission almost everywhere except for near the OIII lines.
I did some extensive testing of the DGM OIII from both my home site, my local dark sky site, and under the pristine skies of the high Sandhills at the Nebraska Star Party. I used my 100mm f/6 refractor and my Celestron NexStar 9.25 inch SCT, along with the Lumicon Multi-filter selector to get more instant comparisons. I compared the DGM OIII against my two Lumicon OIII models (old and new), as well as against my current "favorite" narrowband filter, the DGM Optics NPB. When I first put the DGM-OIII in, I was a little surprised, as the view was somewhat similar to the NPB! The sky background was darker in the DGM OIII, but the NPB was showing nearly as much detail as the DGM OIII did, which I suppose is due to the fine design and production quality used for the NPB! Still, the contrast on the Veil Nebula was somewhat higher and the stars a bit fainter in the DGM-OIII, so it was working like a true Oxygen III filter should. On planetary nebulae, the difference was a bit more pronounced, although again, the NPB wasn't doing too bad here either. For the common "blinking" technique used for isolating small planetary nebulae (holding the filter temporarily between the eye and the eyepiece), the DGM-OIII was a little more effective, as the stars dimmed somewhat more in the OIII than in the NPB, making the planetary stand out more.
I next compared the two Lumicon Filters to the DGM-OIII. Here, there was a big surprise. My "old" 1.25" Lumicon OIII I had "venerated" for so many years just plain got SPANKED! It is clear that filter design has come a long way in nearly 20 years. The objects I looked at *all* looked somewhat better in the DGM-OIII than in that old Lumicon filter. Granted, the coatings on that old filter were laminated and there was some minor deterioration at the edges, but even so, it should have performed better than this (this has made me think seriously about replacing that filter). The stars were a bit brighter and noticeably sharper using the DGM-OIII, while the contrast and brightness of the objects was a bit better as well. Now, against the "New" 2" Lumicon OIII, the difference was a lot more subtle. Here, the Lumicon 2" OIII made the stars and sky background just a tiny bit fainter than with the DGM model, but the DGM-OIII's nebulosity contrast and brightness were nearly identical to that of the "new" Lumicon 2" OIII. Clearly, the "new" Lumicon OIII is a lot better than the old one. The DGM unit might have had a *very* slight edge on nebula brightness over the "new" Lumicon OIII, but it was small enough to be difficult to tell at a glance. One complaint that some people have about OIIIs in general is that they dim the stars a little too much. Well, while the DGM does dim the stars, the dimming isn't quite as much as with the Lumicon OIII, so it is a little less "aggressive" an OIII than some. This lesser star dimming might thus be one minor plus for some people who want to see them as well as the nebula. The DGM OIII's passband width is also less than that found in the Astronomik filters, so I would think that the DGM unit might provide somewhat more contrast than the Astronomik OIII would.
The DGM Optics OIII filter is a very good Oxygen III line filter quite comparable in performance to the newer Lumicon OIII model (as well as being slightly less expensive). It is not quite as aggressive as the narrower Lumicon filter, but still provides very good contrast and overall performance. For those looking for their first OIII purchase, I would have no trouble recommending the DGM-OIII, as it makes for a good buy.