UHC and OIII filters are
designed to provide higher contrast views of deep sky objects such as emission
nebula. Some manufacturers will run to describing the views as 'near
Now everyone who
has ever seen a deep sky object in a small telescope knows that the sobriquet
of 'faint fuzzy' is rather fitting so any filter which can boost the quality
has to be considered by observers as a positive boon. The question is which one
do you get ? An O-III or a UHC. These filters are relatively expensive, and
indeed, can equal the cost of some midrange eyepieces. As a keen observer,
albeit one with little time available to indulge, I decided to go ahead and
take the plunge and buy an O-III filter - after all anything that could improve
the view would be worth it I felt.
Having acquired an O-III filter many people kept asking me how it was
working out. At the time the weather and other pressing priorities hadn't given
me much chance to use the filter and give any opinion. Recently, wonder of
wonders the sky has been clear with good transparency and stable atmospherics,
which, combined with a pair of decent dark sky sites has allowed me to get the
O-III in use and, as importantly, write up my impressions.
In the interim I acquired
first refusal on a UHC filter, which was also taken with me to compare with the
O-III. Both filters are Sky-Watcher supplied ones. I have now been able to try
the two filters out against some deep sky targets; The Ring, the Dumbbell
Nebula and the Veil Nebula.
All tests were with a
Sky-Watcher Ultrawide 26mm EP and a Baader Hyperion 13mm EP fitted to a
Sky-Watcher 200P F5 reflector.
Both filters are supplied in
a durable hard plastic case for protection. The filters themselves look very
similar to each other with only the writing on the side to tell them apart. The
filters show as an opaque silver color to the eye with a slight violet tinge
when seen close up.
Overall quality of
both filters was standard for photographic and astronomical hardware. Both
filters were supplied as 2" fit. The eyepiece rings and threads are metal and
worked with all of my 2" eyepieces and adapters with no trouble.
Here's my verdict on the filters in use against some of the targets
for which they are designed.
- The Ring Nebula
filter the ring can be a bit tough to locate. Nestled in among other bright
stars I found without GoTo it was a tough target. Once found the ring shows as
a faint greyish smoke ring.
O-III filter the ring shows up better against the background sky. Other stars
are muted down and the sky goes jet black. M57 shows through the O-III as a
greenish a hue. Prolonged observation hinted at some red colouring in the outer
section of the ring but I wouldn't swear that this wasn't wishful thinking or
Would the Ring be easier
to locate ? My verdict would be no. It will show better once your there but I
can't say it would be easier to find. Focus suffers with the O-III and it's
hard to get accurate focus I found as the few stars that shine through the
filter are so dulled down.
filters generally work better (I am told by those who know these things) in
larger scopes and perhaps 8" is pushing the limits.
The UHC filter did better. Fewer colours were evident in
the ring with ring retaining a greyish colour, perhaps a small tinge of green.
The background sky was jet black again but surrounding stars were less
affected. This made focusing a bit easier. I doubt finding the ring would be
easier with the UHC and the view was very similar I felt in overall aesthetics
to the non-filtered view.
Verdict - the
O-III to produced some colours that hinted at photographic images but at the
cost of clarity. Rather like an out of focus colour picture versus a sharper
black and white one.
M27 - The Dumbbell
With no filter in use
the Dumbbell resembles a grey puff of smoke. Larger to the eye than M57 the
Dumbbell appears as a rather diffuse cotton wool ball. With the UHC filter in
place the filaments within the nebula were more readily observable and the
nebula exhibited more of its dumbbell shape (personally I always think it looks
more like an hour glass) and exhibited a pronounced greenish hue tending to
yellow in some places after a while observing. Contrast gain was very high with
the background turned jet-black and the few stars in the background dulled
The O-III filter produced a
darker more contrasted image but the contrast was so high I personally found
that there was too much loss of detail for the O-III to be acceptable for this
object. A larger scope may have fared better.
Against a more diffuse target the UHC appeared to perform better than the O-III
filter. The O-III produced such a high contrast image that it was rather like
viewing a television with the colour and contrast turned to maximum.
The hard one.
The Veil is notoriously difficult to find and often cited as hard to see. With
a precise GoTo fix I found the Veil easy enough under a different sky to the
tests above (sky conditions were perfect with good transparency and stable
atmosphere and VERY dark) I used both filters on the Veil for both the Western
and Eastern Veil and found surprisingly that neither filter helped very much
and, in fact actually hindered. The best view was obtained with no filter at
Observers around me, uncommitted
to either filter type, were somewhat taken aback that the filters actually
seemed to be disguising the veil altogether and blanking it out. This is a
target I shall be returning to and, if necessary, rewriting this portion of the
review so come back later on and see what my view is after
Verdict - No
score draw, neither filter produced better views than the unfiltered
I personally found on balance the UHC to be the better
choice between the filters. My view may be slanted because of the relatively
small size of the scope and the limited targets. I have a sneaking suspicion
that against a target like the Orion Nebula the O-III would provide better
views owing to its more aggressive cut-off and the brighter general background
that may swamp the UHC filter.
As ever with astronomy
equipment its horses for courses. I'll keep both filters personally but if I
had to choose one it would be the UHC for its wider application. As I get more
time to use the other filters I will add to this review with my findings.