The DGM Optics OIII Filter
MSRP: $85.00
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.
FIELD TESTS:
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.
SUMMARY:
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.
